Class 9 Science Chapter 10: Work and Energy

Work and Energy is Chapter 10 of NCERT Class 9 Science. It explains what "work" means in physics, the different forms of energy, how energy is conserved, and how fast work is done (power). Watch the five video lessons below, then use the notes, formulas and solved examples to revise.

What you will learn

• The scientific meaning of work and when work is positive, negative or zero
• Kinetic energy and potential energy, with their formulas
• The law of conservation of energy and a freely falling body
• Power, its units, and the commercial unit of energy (kWh)

Video Lessons

Work and Energy - Part 1

Work and Energy - Part 2

Work and Energy - Part 3

Work and Energy - Part 4

Work and Energy - Part 5

Key Concepts and Formulas

Work

Work is done when a force acts on an object and the object is displaced in the direction of the force. If there is no displacement, no work is done, however hard you push.

W = F × s
(s = displacement in the direction of the force)

• SI unit: joule (J). 1 J is the work done when a force of 1 N moves an object by 1 m.
• Work is a scalar quantity: it has magnitude but no direction.
• Positive work: force and displacement are in the same direction (e.g. pushing a trolley forward).
• Negative work: force is opposite to displacement (e.g. friction on a sliding block).
• Zero work: force is perpendicular to displacement, or there is no displacement (e.g. carrying a bag on a level road, pushing a wall).

Energy

Energy is the capacity of a body to do work. A body that does work loses energy, and the body on which work is done gains energy. Energy is measured in joules, the same unit as work. Its forms include kinetic, potential, heat, chemical, electrical and light energy.

Kinetic Energy

Kinetic energy is the energy an object has because of its motion. Starting from v² − u² = 2as and W = F × s = ma × s, for an object starting from rest the work done equals ½mv².

KE = ½ mv²
Work done to change speed from u to v:
W = ½ m(v² − u²)

• Doubling the velocity makes the kinetic energy four times larger.

Potential Energy

Potential energy is the energy stored in an object because of its position or shape, for example a raised hammer, a stretched rubber band or a wound-up spring.

Gravitational PE at height h:
PE = mgh

• It depends only on the vertical height, not on the path taken to raise the object.

Law of Conservation of Energy

Energy can neither be created nor destroyed; it can only change from one form to another. The total energy before and after a transformation stays the same.

Freely falling body (ignoring air resistance):
PE + KE = constant

• As a body falls freely, its potential energy changes into kinetic energy, but the total stays the same at every point.

Power

Power is the rate of doing work, or the rate of transfer of energy.

P = W / t
1 W = 1 J/s
1 kW = 1000 W
Average power = total energy used ÷ total time taken
1 kWh = 3.6 × 10⁶ J

• SI unit of power: watt (W).
• The commercial unit of energy, kWh, is the "1 unit" on your electricity bill.

Solved Examples

Example 1: A force of 5 N moves an object 2 m in the direction of the force. Find the work done.

W = F × s
W = 5 × 2
∴ W = 10 J

Example 2: An object of mass 15 kg moves with a speed of 4 m/s. Find its kinetic energy.

KE = ½ mv²
KE = ½ × 15 × 4²
KE = ½ × 15 × 16
∴ KE = 120 J

Example 3: A car of mass 1500 kg speeds up from 30 km/h to 60 km/h. Find the work done.

u = 30 km/h = 25/3 m/s
v = 60 km/h = 50/3 m/s
W = ½ m(v² − u²)
W = ½ × 1500 × [(50/3)² − (25/3)²]
W = 208333.3 − 52083.3
∴ W = 156250 J

Example 4: An object of mass 10 kg is raised to a height of 6 m (g = 9.8 m/s²). Find its potential energy.

PE = mgh
PE = 10 × 9.8 × 6
∴ PE = 588 J

Example 5: An electric bulb uses 1000 J of energy in 10 s. Find its power.

P = W / t
P = 1000 / 10
∴ P = 100 W

Example 6: Four 60 W bulbs are used for 6 hours a day. Find the energy used per day.

Energy = 4 × 60 W × 6 h
Energy = 1440 Wh
∴ Energy = 1.44 kWh (1.44 units)

Quick Revision

W = F × s        (unit: joule, J)
KE = ½ mv²
PE = mgh
P = W / t        (unit: watt, W)
1 kWh = 3.6 × 10⁶ J

• Total mechanical energy of a freely falling body stays constant

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