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Class 9 · Science · Chapter 7 · NCERT Class 9 Science

Work, Energy and Simple Machines Class 9 Notes

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Chapter mind map

The whole chapter at a glance: the big idea, then each branch and what sits under it.

Work, Energy, and Simple Machines

The study of mechanical work, energy transformations, and the mechanical advantage provided by simple machines to make tasks easier.

  1. Scientific Definition of Work

    Work is the product of force and displacement in the direction of that force (W = F × s). It is a scalar quantity measured in Joules (J).

    • Conditions for Work — Work is zero if there is no displacement (pushing a wall) or if force is perpendicular to displacement (carrying a box while walking).
    • Positive vs Negative Work — Positive when force and displacement align; negative when they oppose, such as friction or a goalkeeper stopping a ball.

  2. Mechanical Energy Forms

    Energy is the capacity to do work. Mechanical energy consists of kinetic energy (motion) and potential energy (position/shape).

    • Kinetic Energy (K) — Energy of motion: K = (1/2)mv². Doubling velocity quadruples kinetic energy. Objects at rest have zero kinetic energy.
    • Potential Energy (U) — Stored energy due to height (U = mgh) or configuration (stretched rubber band). Gravitational potential depends on height above ground.

  3. Energy Dynamics

    The relationship between work and energy states, and the laws governing their transformation and conservation.

    • Work-Energy Theorem — The work done on an object equals its change in energy (W = ΔE). Doing work transfers energy to the object.
    • Conservation Law — Total mechanical energy (K + U) remains constant in ideal systems. As a ball falls, U decreases while K increases.
    • Power: Rate of Work — Power (P = W / t) measures how fast work is done. SI unit is the Watt (W); 1 horsepower (hp) equals 746 W.

  4. Simple Machines & Advantage

    Devices that change force magnitude or direction. They do not reduce total work but allow smaller effort over longer distances.

    • Mechanical Advantage (MA) — The ratio of Load to Effort. MA > 1 means less effort is needed to move a larger load.
    • Pulleys and Ramps — Fixed pulleys change force direction (MA=1). Inclined planes provide MA = Length / Height.

  5. Levers and Classes

    Rigid bars rotating around a fulcrum. Balanced when Effort × Effort Arm = Load × Load Arm.

    • Three Classes of Levers — Class I: Fulcrum in middle (seesaw). Class II: Load in middle (wheelbarrow). Class III: Effort in middle (tongs).
    • Lever Advantage — MA is the ratio of effort arm to load arm. Class III levers prioritize speed/precision over force advantage.

Chapter notes

A comprehensive study of the scientific definitions of work, energy, and power, the law of conservation of mechanical energy, and the mechanics of simple machines like pulleys, inclined planes, and levers.

01 · Explore

Scientific Definition of Work

In science, work has a specific meaning that differs from everyday usage. It is not just about effort; it requires a force to cause a displacement of an object.

Work is defined as the product of the force applied to an object and the displacement of that object in the direction of the force. Mathematically, this is expressed as W = F × s. If you push against a rigid wall, you may feel tired because your muscles are contracting, but scientifically, no work is done on the wall because its displacement is zero.

The SI unit of work is the joule (J). One joule is defined as the work done when a constant force of 1 newton (N) displaces an object by 1 metre (m) in the direction of the force. Since 1 N = 1 kg m s⁻², 1 J is also equal to 1 kg m² s⁻².

Work is a scalar quantity, meaning it has magnitude but no direction. However, it can be positive, negative, or zero. Work is positive when the force and displacement are in the same direction. It is negative when the force acts opposite to the direction of motion, such as friction or a goalkeeper stopping a ball. Work is zero if the force is perpendicular to the displacement, like carrying a box while walking horizontally.

ConditionWork Done (W)Example
Force and displacement in same directionPositivePushing a wheelchair forward
Force and displacement in opposite directionsNegativeA goalkeeper stopping a moving ball
Force is perpendicular to displacementZeroCarrying a box while walking horizontally
No displacement occursZeroPushing against a rigid stationary wall

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Question

A girl lifts a 2 kg book 1.5 m off the floor. Calculate the work done by her. (Take g = 10 m s⁻²)

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NCERT reference: chapter PDF pages 2, 3, 4.

02 · Explore

The Work-Energy Theorem

Work and energy are closely related concepts. Doing work on an object changes its energy state, giving it the capacity to perform further work.

Energy is defined as the capacity to do work. When positive work is done on an object, it gains energy. For example, a moving cricket ball hits a wicket and makes it fall; the ball acquired this energy from the work done by the fielder who threw it.

The work-energy theorem states that the work done on an object is equal to the change in its energy. Mathematically, W = ΔE. This relationship allows us to calculate changes in an object's state by tracking the mechanical work performed by various forces.

The SI unit of energy is the joule (J), the same as work. Energy can exist in many forms, including mechanical, thermal, chemical, electrical, light, sound, and nuclear energy. These forms can be converted into one another, such as chemical energy from food powering muscular work.

Energy Transfer via Work

  1. 1

    Applied Force

    An external agency applies a force over a distance.

  2. 2

    Work Done

    Mechanical work is performed on the object (W = F × s).

  3. 3

    Energy Gain

    The object acquires energy, increasing its capacity to do work.

  4. 4

    Energy Transfer

    The object can now do work on another object, transferring its energy.

The sequential process of energy transfer through mechanical work.

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Question

If 50 J of work is done on a stationary object, what is its change in energy?

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NCERT reference: chapter PDF pages 5, 6.

03 · Explore

Kinetic Energy

Kinetic energy is the energy possessed by an object due to its motion.

Every moving object possesses kinetic energy. An object at rest is defined to have zero kinetic energy. The amount of kinetic energy depends on both the mass of the object and its velocity.

The mathematical expression for kinetic energy (K) is K = (1/2)mv². This formula is derived using the third kinematic equation (v² = u² + 2as) and Newton's second law (F = ma). Substituting s = (v² - u²)/2a into W = F × s gives W = (1/2)m(v² - u²). If the initial velocity u is zero, the work done equals the final kinetic energy.

Kinetic energy is proportional to the mass and the square of the velocity. This means doubling the velocity of a vehicle quadruples its kinetic energy. Like work, kinetic energy is a scalar quantity and has no direction.

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Question

Two objects A and B have masses m and 4m respectively but the same kinetic energy. What is the ratio of their velocities?

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NCERT reference: chapter PDF page 7.

04 · Explore

Potential Energy

Potential energy is the energy stored in an object due to its position or its configuration (shape).

When you stretch a rubber band, pull a bowstring, or compress a spring, you do work to change the object's shape. This work is stored as elastic potential energy. When released, this stored energy is converted into kinetic energy, allowing the object to move or shoot an arrow.

Gravitational Potential Energy (U) is the energy an object possesses due to its height above the ground. To lift an object of mass 'm' to a height 'h', a force equal to its weight (mg) must be applied. The work done is W = force × displacement = mg × h. This work is stored as U = mgh.

Potential energy can also be stored in systems of interacting objects, such as separated magnets or electric charges. In this chapter, potential energy primarily refers to gravitational potential energy near the Earth's surface, where 'g' is assumed to be constant.

Energy of a Raised Hammer

U = 5 kg × 10 m s⁻² × 2 m = 100 J

A 5 kg hammer is lifted 2 m high. Using U = mgh with g = 10 m s⁻², the stored potential energy is 100 Joules.

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Does a ball moving horizontally at a constant height change its potential energy?

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NCERT reference: chapter PDF pages 8, 9, 10, 11.

The rest of this chapter

Keep reading Work, Energy and Simple Machines, free

  1. Locked: 1. Conservation of Mechanical Energy
  2. Locked: 2. Power: The Rate of Doing Work
  3. Locked: 3. Simple Machines: Pulleys and Inclined Planes
  4. Locked: 4. Levers and their Classes

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