Class 9 Science Chapter 6: How Forces Affect Motion

What 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.

Why 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.

What 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.

How 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.

What 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.

What 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.

Why 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.

Why 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.

What 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.

How 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.

What 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.

What 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.

State 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.

How 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.

What 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.

State 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.

What 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.

How 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.

What 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.

Why 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.

State 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.

How 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.

How 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.

If 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.

How 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.

How 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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