Work-Energy Theorem Class 9 Science | Work, Energy, and Simple Machines

Understand it

When net work is done on a body, its speed changes.
If net work is positive, kinetic energy increases.
If net work is negative, kinetic energy decreases.
This theorem links force-displacement calculations with motion and energy.

Example

If the net work done on a cart is 80 J, its kinetic energy increases by 80 J.

Rule to remember

W_net = ΔKE = KE_final - KE_initial = 1/2 mv² - 1/2 μ².

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

1A 2 kg object speeds up from 3 m/s to 5 m/s.
2Change in KE = 1/2 × 2 × (25 - 9) = 16 J, so net work done is 16 J.

Method you can reuse

Write initial and final speeds, calculate initial and final kinetic energy, subtract initial from final, and match it with net work.

Visual tip

A before-after speed diagram can show a body speeding up or slowing down due to net work.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

If net work done on a body is negative, what happens to its kinetic energy?

Think first, then show answer

Its kinetic energy decreases because negative net work reduces the body's speed or opposes its motion.

Common wrong answer

Using only final kinetic energy as net work is wrong when the object already had initial speed.

Don't mix this up

Students sometimes use total work by one force instead of net work by all forces acting on the object.

Exam tip

Look for the word net. The theorem is about total effective work after considering all forces, not only the applied force.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

Questions may ask for net work from change in speed, or ask students to predict whether kinetic energy increases or decreases.

1
Enough for 1 mark
The work-energy theorem states that the net work done on an object is equal to the change in its kinetic energy.
2
Add for 2 marks
Example: If the net work done on a cart is 80 J, its kinetic energy increases by 80 J.
3
Add for 3 marks
When net work is done on a body, its speed changes.

Use the concept graph around this idea instead of revising it in isolation.

Work Done by a Force

Work is done by a force when the force causes displacement of an object in the direction of the force or with a component along the displacement.

Kinetic Energy

Kinetic energy is the energy possessed by a body due to its motion.

Positive, Negative, and Zero Work

The sign of work depends on the angle between force and displacement: work is positive when both are in the same direction, negative when opposite, and zero when perpendicular or when displacement is absent.

Forms of Energy

Energy appears in different forms such as mechanical, heat, light, sound, electrical, chemical, and nuclear energy, and it can change from one form to another.

Previous: Positive, Negative, and Zero Work Next: Forms of Energy Back to full mind map

Work-Energy Theorem

Understand it

See it worked out

Check yourself

Write it in the exam

Practice this concept with focused MCQs

Help improve this page

Work-Energy Theorem

Understand it

See it worked out

Check yourself

Write it in the exam

Related concepts

Practice this concept with focused MCQs

Help improve this page