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Kinetic Energy

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

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Student-friendly explanation

A moving object can do work because of its motion. Kinetic energy depends on mass and the square of speed. This means doubling mass doubles kinetic energy, but doubling speed makes kinetic energy four times.

How to write this in exams

  1. 1

    Start with the exact idea

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

  2. 2

    Then show how to use it

    Write m and v in SI units, square v, multiply by m, divide by 2, and write joule as unit.

  3. 3

    Add one concrete example

    A moving cricket ball has kinetic energy and can move the stumps when it hits them.

  4. 4

    Avoid this incomplete answer

    Forgetting to square velocity gives 1/2 × 4 × 3 = 6 J, which is wrong because the formula uses v².

Definition

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

Example

A moving cricket ball has kinetic energy and can move the stumps when it hits them.

Rule to remember

KE = 1/2 mv², where m is mass in kilogram, v is speed in metre per second, and KE is measured in joule.

Memory hook

Speed gets squared, so speed matters more.

Examples and method

Worked example

For a 4 kg body moving at 3 m/s, KE = 1/2 × 4 × 3² = 2 × 9 = 18 J.

Method to apply

Write m and v in SI units, square v, multiply by m, divide by 2, and write joule as unit.

Diagram support

A comparison table can show same mass at v and 2v to highlight the square effect of velocity.

How CBSE asks it

Questions often ask for numerical KE, comparison after doubling speed, or why a fast vehicle is more dangerous.

Avoid common mistakes

Common confusion

Students often think mass and speed affect kinetic energy equally, but speed has a stronger effect because it is squared.

Common wrong answer

Forgetting to square velocity gives 1/2 × 4 × 3 = 6 J, which is wrong because the formula uses v².

Exam tip

In numerical questions, square the velocity first and then multiply by 1/2 m.

Quick check

What happens to kinetic energy if the speed of a body is doubled while mass remains constant?

The kinetic energy becomes four times because kinetic energy is proportional to the square of speed.

Answer writing and exam use

1-mark answer

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

2-mark answer

Kinetic energy is the energy possessed by a body due to its motion. KE = 1/2 mv², where m is mass in kilogram, v is speed in metre per second, and KE is measured in joule. A moving cricket ball has kinetic energy and can move the stumps when it hits them.

3-mark answer

A moving object can do work because of its motion. Kinetic energy depends on mass and the square of speed. This means doubling mass doubles kinetic energy, but doubling speed makes kinetic energy four times. KE = 1/2 mv², where m is mass in kilogram, v is speed in metre per second, and KE is measured in joule. For a 4 kg body moving at 3 m/s, KE = 1/2 × 4 × 3² = 2 × 9 = 18 J. Questions often ask for numerical KE, comparison after doubling speed, or why a fast vehicle is more dangerous. Forgetting to square velocity gives 1/2 × 4 × 3 = 6 J, which is wrong because the formula uses v².
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