Class 9 Science Chapter 7: Work, Energy and Simple Machines (Work

What is the scientific definition of work done by a constant force?

In this chapter, work done by a constant force is defined using a simple idea: a force must cause displacement in its own direction. If a constant force F acts on an object and the object is displaced by a distance s in the direction of the force, then the work done W is given by W = F × s. This definition applies whether the motion is horizontal, vertical, or in any other direction, as long as displacement is considered along the force direction. Always specify the agency (force) doing the work and the object on which work is done.

What is the SI unit of work and how is 1 joule defined?

The SI unit of work is the joule, written as J. Using the relation W = F × s, 1 joule is defined as the work done when a constant force of 1 newton displaces an object by 1 metre in the direction of the force. So, 1 J = 1 N × 1 m. Since 1 N = 1 kg m s⁻², the joule can also be written in base units as 1 J = 1 kg m² s⁻². This same unit is also used for energy in the SI system.

How does lifting more mass or lifting to a greater height affect the work done?

The chapter uses the example of lifting wheat bags to show how work changes. If you lift three identical bags to the same height one after another, you do three times the work compared to lifting one bag. If you lift the same bag to three times the height, you again do three times the work. This matches W = F × s: increasing the force (heavier load) for the same distance increases work proportionally, and increasing the distance (greater height) for the same force also increases work proportionally. A machine using fuel would similarly require more fuel for more total work.

How can work be found from a force–displacement graph?

When force is plotted on the y-axis and displacement (in the direction of the force) on the x-axis, the work done equals the area under the force–displacement graph between the initial and final positions. For a constant force, this area is simply a rectangle, so work = force × displacement. The chapter notes that even if force is not constant, work can still be calculated by finding the area under the curve. This method provides a visual way to compute work and connects neatly to the definition W = F × s for constant forces.

When is work done equal to zero according to this chapter?

Work done becomes zero in three key situations described in the chapter. First, if the force is zero (F = 0), then no work is done. Second, if the displacement is zero (s = 0), work is zero even if a force is applied—like pushing a rigid wall that does not move. Third, if the force acts perpendicular to the displacement, there is no displacement in the direction of the force, so work done by that force is zero. An example is carrying a box horizontally while applying an upward force to balance its weight.

Why do you feel tired when pushing a wall even though scientific work is zero?

The chapter explains that when you push a rigid wall, the wall does not move, so displacement s = 0 and the work done on the wall is zero by the scientific definition. However, you still feel tired because your muscles consume internal energy. To maintain the applied force, muscles repeatedly contract and relax, using chemical energy from your body even though the wall’s energy does not change. So, fatigue comes from energy used inside your body, while “work done on the object” remains zero because the object has no displacement.

What is positive work and what is negative work?

Work does not have a direction, but it can be positive or negative depending on the relative directions of force and displacement. Work is positive when displacement is in the same direction as the applied force, such as pushing a wheelchair forward so it moves forward. Work is negative when displacement is opposite to the direction of the force, such as a goalkeeper stopping a football: the force applied by the hand is opposite to the ball’s motion, so the work done on the ball is negative. The sign helps describe whether energy is gained or reduced.

In lifting and lowering a dumbbell, when is the work positive and when is it negative?

The chapter’s example explains this clearly. When a girl lifts a dumbbell upward slowly, she applies an upward force and the dumbbell’s displacement is also upward, so the work done by her on the dumbbell is positive. When she lowers the dumbbell slowly, she still applies an upward force to control its motion, but the dumbbell’s displacement is downward. Since force and displacement are in opposite directions during lowering, the work done by her on the dumbbell is negative. This shows how the sign of work depends on directions, not on effort felt.

How is work calculated when a force acts opposite to motion, like stopping a ball?

When a force acts opposite to displacement, the work done is negative because displacement in the force direction is negative. The chapter’s goalkeeper example uses W = F × s, with displacement taken negative. If the force is 200 N and the hand (and ball) move 0.15 m backward while stopping, then displacement in the direction of the goalkeeper’s force is −0.15 m, so W = 200 × (−0.15) = −30 J. The negative sign shows that the ball’s energy decreases due to the stopping force.

What does the work–energy theorem state?

The work–energy theorem gives a direct link between work and energy changes. It states: work done on an object equals the change in its energy. In the chapter, it is written as work done on an object = change in its energy. This theorem applies not only to a single object but also to a system of objects, and it remains valid even when the forces are not constant. It becomes especially useful in situations where applying Newton’s laws directly is difficult, allowing you to solve problems by tracking energy changes instead.

How does doing positive work affect an object’s energy?

According to the chapter, when positive work is done on an object, the object gains energy and therefore gains a greater capacity to do work later. For example, a ball thrown by a fielder gains energy due to the work done during the throw, and then it can transfer that energy to the wickets upon collision, causing them to move. Similarly, raising a flowerpot increases its energy due to work done against gravity; when the pot falls, it can do work on objects below. Positive work is closely associated with energy increase and energy transfer.

How is energy transferred besides mechanical work?

The chapter emphasizes that mechanical work is one way to transfer energy, but not the only way. Energy can also be transferred as heat when objects at different temperatures come into contact, flowing from the hotter object to the colder one. Energy can transfer without direct contact too, such as the Sun’s energy reaching Earth through radiation. The chapter also mentions energy transfer in electric circuits, via sound waves, and in nuclear reactions (like those powering the Sun). This broadens the view of energy beyond just forces and motion.

What are the main forms of energy discussed in the chapter?

The chapter presents energy as existing in many forms and shows that it can change from one form to another. It lists mechanical energy (related to motion and position), thermal energy (related to warmth), light energy, sound energy, electrical energy (related to charges), chemical energy (stored in fuels and food), and nuclear energy (stored in atomic nuclei). Examples include electrical energy converting to light in a bulb or to thermal energy in a water heater, chemical energy from food converting to mechanical energy in muscles, and mechanical energy converting to sound in a ringing bell.

What is mechanical energy and what two parts make it up?

Mechanical energy is the energy an object possesses due to its motion or position. The chapter defines mechanical energy as the sum of kinetic energy and potential energy. Kinetic energy is due to motion, while potential energy (in this chapter, usually gravitational potential energy near Earth’s surface) is due to position in a force field like gravity. By tracking how these two energies change—such as during free fall or pendulum motion—you can understand many real situations. Mechanical energy helps connect force, displacement, and motion through energy ideas.

What is kinetic energy and what formula is derived in the chapter?

Kinetic energy is the energy an object has because of its motion. The chapter uses the work–energy theorem and kinematic equations (for constant force and constant acceleration) to derive the expression for kinetic energy. Starting from work W = F × s and using F = ma along with v² = u² + 2as, it arrives at the change in energy as (1/2)m(v² − u²). For an object starting from rest (u = 0), its kinetic energy becomes K = 1/2 mv². The SI unit is joule (J).

If the speed of a vehicle doubles, how does its kinetic energy change?

The chapter highlights the strong dependence of kinetic energy on speed using K = 1/2 mv². If the speed changes from v to 2v, then the new kinetic energy is K' = 1/2 m(2v)² = 1/2 m·4v² = 4(1/2 mv²). So the kinetic energy becomes four times the original value. This is an important concept for understanding why high-speed motion is much more energetic (and harder to stop) than low-speed motion, even if the speed seems “only” doubled.

What is potential energy and how is it explained using deformation and position?

Potential energy is described as stored energy due to configuration or relative positions in a system. The chapter explains two main ideas: (1) Deformation—when an object like a spring, rubber band, or bow is stretched or compressed, work done to deform it is stored, and when released it can do work and give kinetic energy to another object. (2) Relative position—systems interacting through gravitational, electric, or magnetic forces can store energy when their parts are separated or arranged differently, such as two magnets separated or a ball lifted away from Earth. This stored energy is potential energy.

What is gravitational potential energy near Earth’s surface and what is its formula?

Gravitational potential energy (near Earth’s surface) is the stored energy of the Earth–object system due to the object’s height. The chapter often refers to it simply as the potential energy of the object because Earth’s mass is much larger and Earth hardly moves. Taking potential energy as zero at ground level, raising an object of mass m gradually to a height h requires applying a force equal to mg and doing work W = mg × h. By the work–energy theorem, this work becomes the increase in potential energy, giving U = mgh. The unit is joule (J).

How does the sand-depression activity show the idea of potential energy?

In Activity 7.1, a heavy ball is dropped into loose sand from different heights. The chapter notes that the depression is deepest when the ball is dropped from the greatest height. Raising the ball to a greater height requires more work against gravity, so the ball–Earth system stores more gravitational potential energy. When the ball is released, that stored energy converts into kinetic energy during the fall and then is used to do work on the sand, creating a deeper depression. The activity visually connects height, work done in lifting, and the amount of energy available to cause effects on impact.

What does conservation of mechanical energy mean in free fall (ignoring friction)?

Conservation of mechanical energy means the total mechanical energy (kinetic + potential) remains constant when only gravity acts and other external forces like friction are neglected. The chapter analyses an object dropped from height h. Initially, kinetic energy is zero and potential energy is mgh, so mechanical energy is mgh. As it falls, potential energy decreases while kinetic energy increases by the same amount. At a later time t, the chapter shows potential energy becomes mgh − (1/2)mgt² and kinetic energy becomes (1/2)mgt², so their sum is still mgh. This demonstrates that gravity converts potential energy to kinetic energy without changing the total mechanical energy.

How does a pendulum demonstrate conservation of mechanical energy?

In Activity 7.2, a pendulum bob is released from a height and swings to the other side. At the extreme point (like P), the bob has maximum potential energy and almost zero kinetic energy because it momentarily stops. At the lowest point (Q), potential energy is minimum (often taken as zero relative to that point) and kinetic energy is maximum because speed is greatest. At the opposite extreme (R), kinetic energy again becomes nearly zero and potential energy is regained. The bob reaches almost the same height, showing mechanical energy stays nearly constant. The chapter also notes that in real life the pendulum slows down and stops due to friction at the support and air resistance, which cause energy loss.

What determines a child’s speed at the bottom of a frictionless slide of height h?

The chapter uses conservation of mechanical energy to answer this. At the top of a slide of height h, a child has gravitational potential energy mgh. If friction is neglected, this potential energy converts entirely into kinetic energy at the bottom: 1/2 mv² = mgh. Solving gives v = √(2gh). The result shows the speed at the bottom depends only on the vertical height h (and g), not on the mass of the child or the shape of the slide. So two children of different masses would reach the bottom with the same speed if frictional effects are ignored.

What is power and why can two tasks with the same work feel different?

Power describes how fast work is done. The chapter explains that running up stairs in one minute feels different from walking up in five minutes even if the same work is done (same height, same weight). Power is defined as the rate of doing work: average power P = W/t. Doing the same work in less time requires more power, and doing more work in the same time also requires more power. The SI unit of power is the watt (W), where 1 watt = 1 joule per second (1 W = 1 J s⁻¹). This links energy use to time, explaining why faster tasks demand greater power output.

What are simple machines and why don’t they reduce the total work required?

Simple machines are devices that make tasks easier by changing the magnitude or direction of the applied force. The chapter focuses on pulleys, inclined planes, and levers. Although they can reduce the effort needed or make it more convenient to apply, they do not reduce the total work required for a task (ignoring friction). This is because work is the product of force and displacement: if a machine reduces the force, it increases the distance over which you apply that force, keeping work about the same. The chapter summarizes this idea: machines do not create energy; they help us use it more effectively while conserving mechanical energy in ideal conditions.

What is mechanical advantage, and what do ‘effort’ and ‘load’ mean?

To describe how a machine changes the applied force, the chapter defines mechanical advantage (MA) as the ratio of load to effort: mechanical advantage = load/effort. The ‘effort’ is the force you apply to the machine, and the ‘load’ is the force that needs to be overcome (often the weight of the object being lifted). A larger mechanical advantage means the machine allows a smaller effort to overcome a larger load. This concept is used throughout the simple machines section to compare pulleys, inclined planes, and levers. It helps students quantify “how much easier” a machine makes a task, while remembering that total work is not reduced (ignoring friction).

How does a fixed pulley help, and what is its mechanical advantage?

A fixed pulley is a wheel with a groove that guides a rope, fixed at the top. The chapter explains that a fixed pulley does not reduce the magnitude of the force required; instead, it changes the direction of the effort. For example, it allows you to pull downward to lift a load upward, which is more convenient for most people than applying an upward force directly. Because the effort and load are equal in magnitude for an ideal fixed pulley, its mechanical advantage is 1. So, the benefit is convenience in direction rather than a reduction in required force.

How does an inclined plane reduce the required effort, and what is its mechanical advantage?

An inclined plane helps lift a load to a height h using a smaller force by spreading the required work over a larger distance. The chapter’s activity shows that as the plank becomes less steep (longer length L for the same height), the pulling force needed becomes smaller, but the distance moved increases. Ignoring friction and moving at constant speed, the work done along the plane is F′ × L, and the gain in potential energy is mgh. Using the work–energy theorem, F′ × L = mgh, giving mechanical advantage MA = load/effort = mg/F′ = L/h. Since L > h, MA > 1, meaning effort is reduced compared to lifting vertically.

How does a lever lift a heavy load with a smaller effort, and what formula is used?

A lever is a rigid bar that rotates about a fixed point called the fulcrum. The chapter explains that a lever can reduce the effort needed by increasing the distance (arm) over which effort is applied. The key idea is that work input on one end is transferred to the other end: F1 × d1 = F2 × d2. In the beam balance activity, the balance condition becomes effort × effort arm = load × load arm. Mechanical advantage for a lever is load/effort = effort arm/load arm. So, by increasing the effort arm, a smaller applied effort can balance or lift a larger load, though the effort end moves a larger distance and total work is not reduced.

Work, Energy, and Simple Machines

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