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How Forces Affect Motion

NCERT Class 9 Science Chapter 6: How Forces Affect Motion (Pages 95–115)

Class 9 CBSE hub Science chapters Exploration

Summary of How Forces Affect Motion

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How Forces Affect Motion at a Glance

Board

CBSE

Class

Class 9

Subject

Science

Book

Exploration

Chapter

Pages

95–115

Resources

6 study resources

How Forces Affect Motion Summary

In this chapter, we explore the relationship between forces and motion, starting with the concept of force. A force is any push or pull that can change an object's state of rest or motion. We learn that forces can cause an object to move from rest, change its speed, or alter its direction. For instance, a ball at rest moves when you kick it, illustrating how a force initiates motion. The chapter further elaborates on Newton's three laws of motion, which form the foundation of classical mechanics. Newton's first law states that an object at rest stays at rest, and an object in motion continues to move at constant velocity unless acted upon by a net external force. This principle introduces the concept of inertia, highlighting that objects tend to resist changes in their state of motion. The second law relates net force to an object's acceleration, expressed as "force equals mass times acceleration." This relationship shows that larger forces result in greater acceleration. For example, pushing a stationary box requires more force than keeping it moving at a steady pace, emphasizing the role of friction. The third law states that for every action, there is an equal and opposite reaction. This means that when one object exerts a force on another, the second object exerts a force of the same magnitude but in the opposite direction on the first object. Everyday examples include walking, where pushing against the ground allows us to move forward, and the action of a rocket lifting off by expelling gas downwards. Through a series of activities and examples, the chapter encourages students to apply these concepts in practical situations, emphasizing that understanding forces and motion is fundamental in science. Moreover, students will learn about how the force of friction affects motion, which varies based on surface types and contact pressures. Ultimately, these principles underpin various technological applications, from simple machines to complex vehicles, illustrating the relevance of physics in everyday life.

How Forces Affect Motion Revision Guide

Download the How Forces Affect Motion revision guide with key points, summaries, and quick revision notes for CBSE Class 9 Science.

Key Points

Define force and its SI unit.

A force causes an object to move, change speed/direction. SI unit: Newton (N).

Explain balanced forces.

Forces are balanced when opposing forces are equal, resulting in no acceleration.

What are unbalanced forces?

Unbalanced forces result in net force, causing acceleration in the direction of the larger force.

State Newton's First Law.

An object at rest remains at rest; an object in motion continues uniformly unless acted on by a net force.

Describe inertia.

Inertia is the tendency of objects to resist changes to their state of rest or motion.

State Newton's Second Law.

Acceleration is produced when a net force acts on an object, F=ma, where a is acceleration, m is mass.

What formula relates force, mass, and acceleration?

F = ma describes how the force applied to an object causes it to accelerate.

Define gravitational force.

The force attracting masses toward each other; Earth's gravitational force is approx. 9.8 m/s².

What is friction?

Friction opposes motion; its magnitude depends on surface nature and the normal force.

Explain Newton's Third Law.

For every action, there is an equal and opposite reaction; forces always occur in pairs.

Illustrate action-reaction pairs.

When you push against a wall, the wall pushes back with equal force but in the opposite direction.

Example of net force calculation.

When two players pull a rope in opposite directions, calculate net force by subtracting magnitudes.

What is net force?

Net force is the overall force acting on an object, determining its acceleration.

Define motion graphs.

Position-time and velocity-time graphs illustrate changes in motion and can indicate acceleration.

State the relationship between mass and acceleration.

For constant force, lighter objects accelerate faster than heavier ones, showing inverse relationship.

Force of gravity and weight calculation.

Weight (W) = mass (m) × gravitational field strength (g); W = mg.

Explain how air resistance affects motion.

Air resistance acts against motion, slowing down objects as they move through the air.

What is an example of real-world application of Newton's laws?

Airbags in cars reduce impact force during collisions by increasing time of deceleration.

How to calculate friction force?

Friction force can be measured using a spring scale when an object begins to move.

What happens in free fall?

In free fall, only gravitational force acts on the object, resulting in acceleration g (approximately 9.8 m/s²).

How Forces Affect Motion Practice Questions & Answers

Practice important questions and exam-style problems from How Forces Affect Motion. These questions cover key topics from the CBSE Class 9 Science syllabus.

How to practice: Start with the questions below to test your understanding of How Forces Affect Motion. Use the revision guide to review concepts you find difficult, then come back and retry the questions for better retention.

What is the primary effect of friction on an object in motion?

Single Answer MCQ

Q-00171836

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If a box on the floor is pushed and does not move, what factor is primarily at play?

Single Answer MCQ

Q-00171838

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What happens to the frictional force when the surface becomes smoother?

Single Answer MCQ

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What is the role of the normal force when an object is at rest on a surface?

Single Answer MCQ

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If an object is pushed with a force greater than the force of friction, what will happen?

Single Answer MCQ

Q-00171841

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Which of the following factors does NOT influence the force of friction?

Single Answer MCQ

Q-00171842

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When pulling a block with a spring balance, the reading just before it starts moving indicates what?

Single Answer MCQ

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How does the force of static friction differ from kinetic friction?

Single Answer MCQ

Q-00171844

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View all 100 How Forces Affect Motion questions

A block slides down a frictional slope. What will happen to its speed as it moves down?

When an object is moved on a rough surface compared to a smooth surface, what trend is observed in the force of friction?

If the mass of an object doubles, and the normal force also doubles, what happens to the frictional force if the coefficient remains constant?

Why does a bicycle come to a stop after pedaling ceases?

What kind of friction acts when an object is sliding over a surface?

If a box is sliding down a frictional ramp, which force is acting against its motion?

How can friction be reduced in a system?

In a game of tug-of-war, how does friction help?

What is the definition of balanced forces?

When are forces considered unbalanced?

What happens to an object when the net force acting on it is zero?

If you pull a block with 10 N of force and friction applies 4 N in the opposite direction, what is the net force?

In a tug-of-war, if Team A pulls with 15 N and Team B with 10 N, what is the direction of the net force?

Which of the following is an example of balanced forces?

What force opposes the motion of an object sliding across a surface?

In which of the following scenarios are the forces unbalanced?

If a force of 20 N acts in one direction and a force of 15 N acts in the opposite direction, what is the resultant force?

Which situation demonstrates unbalanced forces acting at angles?

In a game of tug-of-war, if each side pulls with 50 N and the rope doesn’t move, which statement is true?

Which force is not a type of contact force?

What will happen to an object at rest if unbalanced forces are applied?

Two children push a wagon towards the right with forces of 8 N and 12 N. What is the net force acting on the wagon?

If a block is being pulled right with 10 N and also has 5 N of friction acting left, what is the net force?

What is the SI unit of force?

Which of the following describes a balanced force?

When a net force acts on an object, it causes the object to:

Which of the following forces acts on a ball in free fall?

Which scenario represents an unbalanced force?

When two objects collide, which of the following is true?

What is the force of friction?

Which force is responsible for keeping planets in orbit around the sun?

In a tug of war, if both teams pull with equal force, the rope will:

What happens to an object if the net force acting on it is zero?

What does Newton's Third Law of Motion state?

What describes the relationship between mass and weight?

If you push a wall with a force of 10 N, what force does the wall exert on you?

A push or pull on an object is known as a:

Which example illustrates Newton's Third Law of Motion?

If a force of 10 N acts on an object and it does not move, what can be concluded?

What role does friction play when walking, according to Newton's Third Law?

What is an example of a non-contact force?

Why do you feel a recoil when shooting a gun?

What can increase the force of friction between two surfaces?

In which scenario is Newton's Third Law NOT applicable?

When a fish swims by pushing water backwards, what principle is demonstrated?

If two people are playing tug-of-war and exert a force of 150 N on each other, what can be said about the forces?

Why is climbing a smooth tree harder than climbing a rough tree?

What happens when you jump off a small boat onto the shore?

In a collision, what can be said about the forces exerted on both objects involved?

When a hammer strikes a nail, which forces are at play according to Newton's Third Law?

According to Newton's first law, what happens to an object at rest when no net force acts upon it?

What is a key term used to describe an object's resistance to change in motion?

If a ball rolls on a frictionless surface, how will its speed change?

An object is moving with a constant velocity of 10 m/s. Which of the following applies according to Newton's first law?

If a car suddenly stops, what happens to the passengers inside due to inertia?

Which of these scenarios illustrates Newton's first law of motion?

In space, a toy rocket is floating with no forces acting on it. What can be said about its motion?

Newton's first law implies that if an object is already in motion, what must be true?

Which of the following best describes inertia?

What does the term 'net force' refer to?

If the net force acting on an object is zero, what can we conclude?

A ball is thrown straight up. Which force will act on it once it is in motion?

A hockey puck slides on a smooth ice surface and comes to rest. Which force is responsible for this?

In which scenario would an object NOT obey Newton's first law of motion?

What is the net external force acting on a system of two connected boxes if only one box is subjected to an applied force?

What does Newton’s first law imply about an astronaut floating in space?

According to Newton’s second law, how do we calculate the acceleration of a system of two boxes with masses m1 and m2 being pulled by an external force F?

What role does the tension in the string play when analyzing the motion of two boxes connected by a string on a frictionless surface?

If box 1 has a mass of 5 kg and box 2 has a mass of 3 kg and they are connected by a string with a force of 40 N applied to box 1, what would be the acceleration of the entire system?

Which of the following best describes Newton's third law as applied in a system of connected objects?

When treating two connected boxes as a single system, what should be taken into account to calculate the system's motion?

If a system of objects experiences an upward normal force equal to the weight of the system, what can be concluded about their motion?

If two boxes connected by a string are on a frictionless surface, which of the following forces could affect the motion of the boxes if modified?

What happens to the acceleration of the system if the applied force remains constant but the mass of box 2 is doubled?

In a system of two boxes, if box 1 pulls box 2 with a tension of 10 N, what is the force that box 2 exerts back on box 1 according to Newton's third law?

How do unbalanced forces affect the motion of a system of objects connected by a string?

When analyzing a system of multiple objects, what can be concluded about the internal forces acting within that system?

What concept allows us to treat multiple connected objects as a single entity in physics?

If box 1 undergoes a downward gravitational force, how does this affect box 2 that is connected to it on a string?

If a net force of 10 N is applied to a 2 kg object, what is its acceleration?

Which of the following best describes the relationship between force, mass, and acceleration?

A car with a mass of 1,000 kg accelerates at 2 m/s². What is the net force acting on the car?

If a force of 20 N is applied to an object with a mass of 4 kg, what will be its acceleration?

A boy pushes a skateboard with a force of 30 N, but friction provides a resistance of 10 N. What is the net force acting on the skateboard?

Which factor will NOT affect the acceleration of an object according to Newton's second law?

What happens to acceleration if the mass of the object is doubled while keeping the force constant?

If two objects of different masses are subjected to the same force, which one accelerates more?

What is the unit of force in the SI system?

Which of the following statements is true about net force?

If a fielder catches a fast-moving ball and pulls his hands back, what is the primary reason for this action?

In a car crash, how do airbags help to protect passengers?

What experimental setup could demonstrate Newton's second law effectively?

Single Answer MCQ

Q-00171964

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How Forces Affect Motion Practice Worksheets

Download and practice How Forces Affect Motion worksheets to improve problem-solving accuracy and speed for CBSE Class 9 Science exams.

How Forces Affect Motion - Practice Worksheet

This worksheet covers essential long-answer questions to help you build confidence in How Forces Affect Motion from Exploration for Class 9 (Science).

Practice

Questions

Define force and explain its significance in motion. Provide real-life examples that illustrate different types of forces.

A force is a vector quantity that causes an object to change its velocity, either by starting motion, stopping motion, or changing direction. For instance, friction acts opposite to motion, while gravity pulls objects downward. Understanding forces is crucial for analyzing motion as they’re the primary cause for changes in velocity.

What are balanced and unbalanced forces? How do they affect the motion of objects? Provide examples.

Balanced forces are equal in magnitude but opposite in direction, resulting in no change in motion. Unbalanced forces lead to movement. For example, when pushing a stationary box with a force equal to friction, it remains stationary (balanced). If pushed harder, the box moves (unbalanced).

Describe Newton’s first law of motion and provide an example of its application in everyday life.

Newton’s first law states that an object at rest stays at rest, and an object in motion continues moving at a constant velocity unless acted upon by a net force. An everyday example is a seatbelt preventing you from moving forward in a car during sudden braking.

Explain Newton’s second law of motion and derive the formula F = ma. Include an example of its application.

Newton’s second law states that the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. This is expressed mathematically as F = ma, where F is force, m is mass, and a is acceleration. For instance, pushing a car involves more force than pushing a bicycle due to the difference in mass.

What is the role of friction in motion? Discuss the different types of friction and their impact on movement.

Friction is a force that opposes motion, acting parallel to the surface in contact. The main types include static (preventing movement), kinetic (opposing moving objects), and rolling friction (resistance to rolling motion). For instance, static friction prevents a parked car from rolling down a hill.

Discuss Newton’s third law of motion with examples. How does it apply to interactions between different objects?

Newton’s third law posits that for every action, there is an equal and opposite reaction. For instance, when you push a wall, the wall pushes back with an equal force. This principle applies in sports, like when a football player kicks a ball and feels a force from the ball in return.

How do external and internal forces act on a system of objects? Explain with an example involving multiple objects.

External forces influence the motion of an entire system, while internal forces occur between components of that system. For example, in a train, the engine applies an external force to pull multiple cars, while tension in the couplings represents an internal force.

What happens when two forces act in the same direction on an object? Illustrate your answer with a diagram.

When forces act in the same direction, the net force is the sum of the magnitudes of the forces. For example, if two people push a car with forces of 100 N and 150 N, the net force is 250 N in that direction, causing the car to accelerate. Diagrams should show both forces and the resultant vector.

In what situations do you find the force of friction useful? Analyze its effects in various scenarios.

Friction is helpful when it provides necessary traction, such as when walking or driving. It allows brakes to function effectively in vehicles. However, excessive friction can cause wear and energy loss. Balancing friction is crucial for efficiency.

How does the concept of inertia relate to the mass of an object? Discuss its implications in real-world applications.

Inertia is the tendency of an object to resist changes in its state of motion. It is directly related to mass; greater mass means greater inertia. For example, a heavy truck requires more force to accelerate than a bicycle due to its greater inertia.

How Forces Affect Motion - Mastery Worksheet

This worksheet challenges you with deeper, multi-concept long-answer questions from How Forces Affect Motion to prepare for higher-weightage questions in Class 9.

Mastery

Questions

Explain Newton's First Law of Motion and provide a real-life example that illustrates balanced and unbalanced forces.

Newton's First Law states that an object at rest will stay at rest, and an object in motion will maintain its velocity unless acted upon by a net force. For example, a book resting on a table remains in place until someone pushes it (unbalanced force). The forces acting on it are the gravitational force downwards and the normal force from the table upwards, which balance each other.

Using the concept of acceleration, discuss how a vehicle's motion changes when it experiences both friction and thrust.

When a vehicle accelerates, the thrust provided by the engine must overcome friction. If thrust is greater than friction, the vehicle accelerates; if equal, it moves at constant speed; if less, it slows down. This can be illustrated with a force diagram showing these forces in opposition.

Compare the forces acting on an object moving in a straight line with a constant velocity to an object accelerating due to a net force.

An object moving with constant velocity experiences balanced forces (net force = 0), while an accelerating object experiences unbalanced forces (net force > 0), resulting in its acceleration in the direction of the net force.

How does the mass of an object influence the amount of force needed to change its state of motion?

According to Newton's second law (F=ma), a greater mass requires a greater force to achieve the same acceleration. Hence, as mass increases, more force must be applied for the same change in velocity.

Describe an experiment to measure the effect of different surfaces on the force of friction.

One can use a spring scale to pull a block across various surfaces, recording the force required to maintain motion. Analyzing this data allows for comparisons of static and kinetic friction across different materials.

Illustrate Newton's Third Law of Motion with a practical example involving a fireperson holding a hose.

When the hose expels water forward, the water pushes back on the hose with an equal and opposite force, which can cause the fireperson to struggle with controlling the hose. The reaction force is the backward push the fireperson feels.

Evaluate how friction can be both an advantage and a disadvantage in sports.

Friction helps runners get traction, enabling better acceleration (advantage). Conversely, it can hinder speed on poorly maintained tracks (disadvantage). This dual nature of friction can be analyzed in the context of different sports.

What is the relationship between force, mass, and acceleration and how can it be observed in a classroom setting?

According to Newton's second law, force is directly proportional to acceleration and mass (F=ma). A simple classroom experiment involves rolling different masses down a slope, measuring time to observe differences in acceleration.

Discuss how the understanding of inertia applies to everyday situations, such as an abrupt stop in a car.

Inertia is the resistance of an object to change its motion. When a car abruptly stops, the passengers continue moving forward due to inertia, illustrating the importance of seatbelts in preventing injury.

Illustrate the concept of action and reaction forces in the context of two skaters pushing off each other.

When two skaters push off each other, each exerts a force on the other. According to Newton's Third Law, the force one skater applies is equal in magnitude and opposite in direction to the force applied by the other. Thus, both skaters move away from each other.

How Forces Affect Motion - Challenge Worksheet

The final worksheet presents challenging long-answer questions that test your depth of understanding and exam-readiness for How Forces Affect Motion in Class 9.

Challenge

Questions

Evaluate the implications of Newton's First Law of Motion in scenarios involving friction. Consider a perfectly smooth surface vs. a rough surface.

Discuss how the law governs motion in both scenarios, focusing on friction's role in changing object motion. Provide examples to justify your reasoning.

Analyze a situation where constant speed is maintained while pushing a box. What role do balanced forces play?

Explain the dynamics of balanced forces in relation to applied and frictional forces, using a detailed diagram to illustrate your explanation.

Hypothesize the effect of increased mass on the acceleration of an object when a constant force is applied, using Newton's Second Law.

Discuss the inverse relationship between mass and acceleration, followed by examples illustrating real-world applications.

Evaluate an instance of a vehicle braking suddenly. How do Newton's laws explain the motion of passengers inside?

Analyze the forces involved, focusing on inertia, and how it affects passengers during sudden deceleration. Provide a clear example.

Derive the net force acting on a system with multiple forces using Newton's Third Law, specifically in a tug-of-war scenario.

Apply the law to demonstrate how forces influence the system's acceleration, backing your deductions with physical examples and calculations.

Investigate the impact of friction on a moving object transitioning between two surfaces: polished and unpolished.

Discuss how friction alters acceleration and velocity through detailed experiments or thought experiments. Include specific values.

Critically assess the statement: 'For every action, there is an equal and opposite reaction.' Apply this in the context of rocketry.

Outline how rocket propulsion aligns with Newton’s Third Law, providing calculations or diagrams to illustrate the concepts involved.

Compare and contrast the forces acting on an object in free fall versus those acting on an object in uniform motion.

Analyze gravitational forces and net forces in each scenario, ensuring to include conditions for equilibrium and non-equilibrium.

Evaluate real-life examples where air resistance plays a significant role, particularly in high-speed scenarios.

Delve into how Newton’s laws describe the interplay between motion and air resistance, providing detailed examples such as skydiving.

Analyze the principles of levers. How do forces and distances influence the effectiveness of a lever according to Newton's laws?

Discuss how effective a lever can be in adjusting force and the distances involved, using appropriate examples and diagrams.

How Forces Affect Motion Frequently Asked Questions

Learn Class 9 Science Chapter 6 “How Forces Affect Motion” from Exploration: concept of force, balanced and unbalanced forces, friction, and Newton’s 1st, 2nd and 3rd laws. Includes net force, F=ma, g, action–reaction, and systems of objects with solved-style explanations.

QWhat is meant by force in this chapter, and what can a force do to an object?

In this chapter, a force is described as something that can change an object’s motion or shape. A force can make an object move from rest, change the speed of a moving object, change its direction of motion, or change its shape. Examples include kicking a ball to set it moving, a cricket bat changing a ball’s direction, and squeezing a lemon to change its shape. The chapter also emphasizes that force is not just a “push or pull” idea; it is treated as a physical quantity used to explain changes in position, velocity, and acceleration.

QWhy is force called a vector quantity, and what must be specified for a force?

Force is treated as a vector quantity because it requires both magnitude and direction for a complete description, similar to position, displacement, velocity, and acceleration. The chapter highlights that whenever forces are mentioned—friction opposing motion, gravity acting downward, buoyant force acting upward, magnetic or electrostatic forces causing attraction or repulsion—the direction is always specified. The magnitude tells the strength of the force, and the SI unit is newton (N). If the magnitude or direction (or both) changes, the effect of the force on motion also changes.

QWhat is the SI unit of force and how is it written correctly?

The SI unit of force is the newton, written with a small ‘n’, and its symbol is N (capital letter). The chapter notes a convention: when a unit is named after a person, the full name starts with a lowercase letter (newton), but the symbol is capitalised (N). The unit is defined using Newton’s second law: one newton is the force that produces an acceleration of 1 m s⁻² in an object of mass 1 kg. This connects the unit directly to measurable mass and acceleration.

QHow can we measure the magnitude of a force using a spring balance?

A spring balance can measure the magnitude of force by showing how much the internal spring stretches when a force is applied. The chapter reminds that weight is a force (the gravitational force with which Earth pulls an object), and a spring balance is commonly used to measure weight. But it can also measure force in general: if you pull the free end of a spring balance, it measures the pulling force. In activities, the spring balance reading is used to estimate frictional force, especially when a block is just about to start moving.

QWhat are balanced forces, and what happens to motion when forces are balanced?

Balanced forces are forces that are equal in magnitude and opposite in direction, producing a net force of zero. The chapter uses tug of war: if both teams pull equally, the rope does not move. When forces are balanced, an object’s motion does not change due to those forces—an object at rest stays at rest, and an object already moving continues with constant velocity. Balanced forces can still exist even when several forces act, as long as their combined effect (net force) is zero, resulting in zero acceleration.

QWhat are unbalanced forces and how do we find the net force in simple cases?

Unbalanced forces occur when forces acting on an object do not cancel, so the net force is non-zero. The chapter explains two common cases. If forces act in opposite directions but have different magnitudes, the net force equals the difference of magnitudes and points in the direction of the larger force. If forces act in the same direction, the net force equals the sum of magnitudes and points in that common direction. Example cases with 10 N and 6 N show net forces of 16 N (same direction) or 4 N (difference) with direction determined by the larger force.

QWhy does the chapter say motion depends only on the net force, not on all individual forces separately?

The chapter states that multiple forces may act on an object, but its motion depends only on the net force. This is illustrated with pushing a box: your applied force acts forward while friction acts backward. The object’s acceleration (change in velocity) depends on whether the forward force is greater, equal to, or less than friction, i.e., on the net force. Similarly, for a floating ball, gravity acts downward and buoyant force acts upward; whether the ball rises, sinks, or stays depends on the balance between these forces. Net force summarises the combined effect of all forces.

QWhy do you sometimes have to push harder to start moving a box on the floor?

You may need to push harder because friction acts opposite to the direction in which you try to move the box. When the box is at rest, friction between the bottom surface of the box and the floor can balance the applied force up to a certain point. If your applied force is not larger than the frictional force, the net force remains zero and the box does not move. The box starts moving only when the applied force exceeds friction, creating a net force in the direction of motion and producing acceleration.

QWhat forces act on a box being pushed on a horizontal surface, according to the chapter?

For a box being pushed on a horizontal surface, the chapter describes four main forces: the applied force (in the direction you push), the force of friction (opposite to motion), the gravitational force or weight (downward), and the normal force exerted by the surface (upward and perpendicular to the surface). The weight and normal force are balanced in the vertical direction for the situation discussed. Air friction can also act when the box moves through air, but in many everyday cases its magnitude is small enough to be neglected.

QHow does friction affect a moving object when you stop applying force?

When you stop applying force to a moving object, friction continues to act opposite to the direction of motion. Because there is no longer a forward applied force to balance it, friction becomes the main horizontal force, so the net force acts backward. This net force causes the object’s velocity to decrease gradually until it comes to rest. The chapter connects this to daily experience: a bicycle slows down when you stop pedalling, and a pushed ball stops after travelling some distance. Continuous force is needed in many real situations to counter friction and maintain constant velocity.

QWhat do Activities 6.1 and 6.2 show about friction on different surfaces?

Activities 6.1 and 6.2 demonstrate that friction depends on the nature of the surfaces in contact. In Activity 6.1, a rubber band launches a taped stack of coins across different surfaces (wood, laminate, marble/tile). The coins travel farther and slow down more slowly on smoother surfaces, suggesting smaller friction. In Activity 6.2, a spring balance pulls a wooden block; the reading when the block just starts moving gives an approximate measure of friction. Comparing readings across surfaces shows smaller readings on surfaces where the coins travelled farther, confirming different friction magnitudes.

QWhat is the thought experiment about zero friction meant to explain?

The thought experiment asks you to imagine an object and floor so smooth that friction between them is zero. If you repeat the coin-and-rubber-band activity in such a case, once the object is set in motion, its velocity would not decrease due to friction, so it would not come to rest on its own. This supports the idea discussed through Galileo’s argument: if a body moves on a horizontal plane and all impediments are removed, it will continue to move indefinitely. The experiment helps students see why in real life objects stop: friction is almost always present.

QState Newton’s first law of motion as given in the chapter.

Newton’s first law of motion is stated as: an object at rest remains at rest and an object in motion continues to move with a constant velocity, unless a net force acts upon the object. The chapter clarifies that if the net force is zero, the object cannot begin to move or change its velocity, so acceleration is zero. “Constant velocity” means no change in magnitude or direction of velocity; if it is non-zero, the object moves in a straight line with the same speed and direction. The law describes motion when net force is absent.

QHow do balanced forces relate to Newton’s first law and constant velocity?

Balanced forces produce a net force of zero, which directly connects to Newton’s first law. When the net force is zero, acceleration is zero, so the object’s velocity does not change. The chapter gives an example: if a person pushes a moving box forward with a force equal to the frictional force backward, the two forces balance. Because the net force is zero, Newton’s first law predicts that the box will continue moving with constant velocity. This helps students understand that constant velocity motion does not require a net force; it requires zero net force.

QWhat position–time and velocity–time graphs represent motion with no net force?

The chapter explains that if no net force acts, there are two possibilities: the object is at rest or it moves with constant velocity. For an object at rest, position does not change with time, so the position–time graph is a horizontal line, and the velocity–time graph is a line at zero velocity. For an object moving with constant velocity, the velocity–time graph is a horizontal line at a non-zero value, and the position–time graph is a straight line with constant slope. Both cases show zero acceleration because net force is zero.

QState Newton’s second law of motion in words as given in the chapter.

Newton’s second law of motion is stated as: when a net force acts on an object, the object accelerates in the direction of the net force. The magnitude of acceleration is proportional to the magnitude of the net force and inversely proportional to the mass of the object. This law explains why stronger pushes cause greater acceleration for the same object, and why heavier objects accelerate less for the same force. The chapter supports this idea with demonstration activities using a cart and pulley system to vary force and mass.

QWhat is the mathematical form of Newton’s second law used in this chapter?

The chapter expresses Newton’s second law mathematically as a = F/m, or equivalently F = ma, where F is the net force, m is mass, and a is acceleration. It also stresses that the direction of acceleration is the same as the direction of the net force. Using SI units, mass is in kg and acceleration in m s⁻², so force becomes kg m s⁻², which is defined as one newton (1 N). This formula is used in numerical examples, such as finding net force on a car from its acceleration or calculating displacement when friction is present.

QHow is one newton (1 N) defined using mass and acceleration?

One newton is defined as the force that produces an acceleration of 1 m s⁻² on an object of mass 1 kg. The chapter derives this using F = ma: if m = 1 kg and a = 1 m s⁻², then F = 1 kg × 1 m s⁻² = 1 kg m s⁻², which is called 1 N. This definition links the unit of force to measurable quantities and helps students interpret what force values mean. The chapter also gives a feel-based reference: holding a 100 g mass in your palm involves about 1 N of upward force.

QWhat is acceleration due to gravity (g) and how is weight written in this chapter?

The chapter defines g as the acceleration due to the gravitational force by the Earth, with unit m s⁻². Near Earth’s surface, g is taken as 9.8 m s⁻² and is nearly constant; for quick estimates, it can be approximated as 10 m s⁻². Using Newton’s second law, the gravitational force on a mass m is written as F = mg. This connects the idea of weight to force and allows calculations like the force needed to hold a barbell steady. The chapter also notes that g does not depend on the mass of the object.

QWhy does catching a fast cricket ball by moving hands backward reduce injury?

The chapter explains this using Newton’s second law. When a fielder catches a fast-moving ball, the ball’s velocity must reduce to zero. If the hands are pulled backward with the ball, the time over which the ball stops increases. For the same change in velocity, a longer stopping time means smaller acceleration magnitude. Since force is related to acceleration (F = ma), reducing acceleration reduces the force required to stop the ball. A smaller force on the hands reduces the chance of injury. The same idea is applied to safety devices like airbags, which increase stopping time and reduce force on passengers.

QState Newton’s third law of motion and explain what is special about the force pair.

Newton’s third law of motion is stated as: whenever one object exerts a force on a second object, the second object simultaneously exerts an equal and opposite force on the first object. The chapter emphasizes that forces always occur in pairs, but the two forces act on two different objects. Because they act on different objects, they do not cancel each other as balanced forces do on a single object. Examples include pushing a table while sitting on a wheeled chair, walking (foot pushes ground backward; friction pushes person forward), rowing a canoe, and rocket motion due to expelled gases.

QHow does walking or running involve Newton’s third law and friction?

While walking or running, you push the ground backward with your foot. By Newton’s third law, the ground exerts an equal and opposite force on your foot, which pushes you forward. The chapter explains that this forward force from the ground is provided by friction. In this situation, friction helps motion rather than opposing it. Without friction, your foot would slip backward when trying to push the ground, and you could fall. This is why grooves on footwear soles and treads on tyres are important: they increase friction with the ground, improving grip, and why wet polished floors or ice make walking difficult and driving risky.

QHow do two connected spring balances verify Newton’s third law experimentally?

In the activity, two identical spring balances are connected hook-to-hook and pulled in opposite directions while one end is fixed. When the system is stationary, both spring balances show the same reading every time, even if you vary how hard you pull. This observation indicates that the forces they apply on each other are equal in magnitude and opposite in direction, matching Newton’s third law. The key idea is that each balance measures the force exerted through the connection, and the equality of readings demonstrates the equality of action and reaction forces in an interaction.

QIf action and reaction forces are equal, why doesn’t the Earth move noticeably toward a falling fruit?

The chapter answers that the Earth and the fruit do exert equal and opposite gravitational forces on each other, but their accelerations depend on their masses (a = F/m). The Earth’s mass is enormously larger than the fruit’s mass, so the acceleration of the Earth due to the same force is extremely small—too small to notice. The fruit, having a much smaller mass, gets a much larger acceleration and moves toward Earth. This example clarifies an important point: Newton’s third law guarantees equal forces, not equal accelerations, because masses can be different.

QHow can Newton’s second law and third law together explain gun recoil and bullet acceleration?

The chapter’s example uses Newton’s third law to state that the bullet and the gun exert equal and opposite forces on each other during firing. However, using Newton’s second law (a = F/m), the accelerations are not equal because their masses differ. With the same force, the bullet (small mass) gets a large acceleration, while the gun (large mass) gets a small acceleration in the opposite direction, observed as recoil. This shows how the two laws work together: third law gives equal force pairs, and second law explains how those equal forces produce different accelerations on different masses.

QHow do we apply Newton’s laws to a system of connected objects, and why can tension be ignored in system analysis?

For connected objects (like two boxes joined by a string on a frictionless surface), the chapter suggests treating the boxes and string as a single system to simplify analysis. The tension force acts within the system: Box 1 pulls Box 2 via tension, and Box 2 pulls Box 1 with an equal and opposite tension (Newton’s third law). These internal forces do not affect the net external force on the system, so they can be ignored when finding the system’s acceleration. Only external forces matter, such as the pulling force F. The acceleration is then a = F/(m1 + m2), so the system behaves like one object with total mass.

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What is force?

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A force is a push or pull that can change the motion of an object.

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SI unit of force?

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The SI unit of force is the Newton, symbolized as 'N'.

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What are balanced forces?

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Balanced forces are two forces acting in opposite directions on an object, equal in magnitude, resulting in no movement.

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What are unbalanced forces?

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Unbalanced forces result in a net force acting on an object, causing it to accelerate in the direction of the net force.

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Newton's First Law of Motion?

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An object at rest stays at rest, and an object in motion continues in motion at the same speed and direction unless acted upon by a net external force.

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Define friction.

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Friction is the force that opposes the relative motion of two surfaces in contact.

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Newton's Second Law of Motion?

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The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass, expressed as F = ma.

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What is inertia?

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Inertia is the tendency of an object to resist changes to its state of rest or uniform motion.

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What is net force?

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Net force is the vector sum of all the forces acting on an object.

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Formula for calculating weight?

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Weight is calculated using the formula W = mg, where 'm' is mass and 'g' is acceleration due to gravity (9.8 m/s²).

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Newton's Third Law of Motion?

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For every action, there is an equal and opposite reaction.

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What is acceleration?

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Acceleration is the rate of change of velocity of an object with respect to time.

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Effect of friction on motion?

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Friction always acts in the direction opposite to the motion of an object, slowing it down.

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What causes movement in a canoe?

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A canoe moves forward when the canoeist pushes water backwards with the paddle by Newton's third law.

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What is the gravitational force?

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Gravitational force is the attraction between two masses, such as an object and the Earth.

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How to measure force?

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Force can be measured using a spring balance, which displays the force exerted on the spring.

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What is constant velocity?

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Constant velocity means moving at the same speed in a straight line without changing direction.

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What is mass?

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Mass is a measure of the amount of matter in an object, typically measured in kilograms.

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Describe motion graphs.

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Position-time graphs show an object's position relative to time and can indicate if the object is at rest, moving, or accelerating.

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