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Potential Energy

Potential energy is the energy possessed by a body due to its position or configuration. In this chapter, gravitational potential energy is mainly used.

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Student-friendly explanation

When an object is raised above a chosen reference level, work is done against gravity and energy is stored as gravitational potential energy. The value depends on mass, gravitational acceleration, and height from the reference level.

How to write this in exams

  1. 1

    Start with the exact idea

    Potential energy is the energy possessed by a body due to its position or configuration. In this chapter, gravitational potential energy is mainly used.

  2. 2

    Then show how to use it

    Identify mass, take g as given or 9.8/10 m/s² as instructed, identify vertical height from reference level, multiply m × g × h, and write joule.

  3. 3

    Add one concrete example

    Water stored in a tank on the roof has gravitational potential energy because it is at a height.

  4. 4

    Avoid this incomplete answer

    Using horizontal distance instead of vertical height is wrong because gravitational potential energy depends on height against gravity.

Definition

Potential energy is the energy possessed by a body due to its position or configuration. In this chapter, gravitational potential energy is mainly used.

Example

Water stored in a tank on the roof has gravitational potential energy because it is at a height.

Rule to remember

PE = mgh, where m is mass in kilogram, g is gravitational acceleration in m/s², and h is vertical height in metre. PE is measured in joule.

Memory hook

Height stores energy against gravity.

Examples and method

Worked example

A 2 kg bag is lifted 3 m. Taking g = 10 m/s², PE = mgh = 2 × 10 × 3 = 60 J.

Method to apply

Identify mass, take g as given or 9.8/10 m/s² as instructed, identify vertical height from reference level, multiply m × g × h, and write joule.

Diagram support

A vertical height diagram from reference level to object can help prevent path-length mistakes.

How CBSE asks it

Students are asked to calculate gravitational PE, compare PE at different heights, or explain stored energy in raised objects.

Avoid common mistakes

Common confusion

Students sometimes use total height from the ground even when the question gives a different reference level.

Common wrong answer

Using horizontal distance instead of vertical height is wrong because gravitational potential energy depends on height against gravity.

Exam tip

Read the reference level carefully. Height in mgh means vertical height from the chosen level, not distance travelled along a path.

Quick check

Why does a book kept on a high shelf have gravitational potential energy?

The book has gravitational potential energy because work has been done to raise it against gravity, and it is at a height above a chosen reference level.

Answer writing and exam use

1-mark answer

Potential energy is the energy possessed by a body due to its position or configuration. In this chapter, gravitational potential energy is mainly used.

2-mark answer

Potential energy is the energy possessed by a body due to its position or configuration. In this chapter, gravitational potential energy is mainly used. PE = mgh, where m is mass in kilogram, g is gravitational acceleration in m/s², and h is vertical height in metre. PE is measured in joule. Water stored in a tank on the roof has gravitational potential energy because it is at a height.

3-mark answer

When an object is raised above a chosen reference level, work is done against gravity and energy is stored as gravitational potential energy. The value depends on mass, gravitational acceleration, and height from the reference level. PE = mgh, where m is mass in kilogram, g is gravitational acceleration in m/s², and h is vertical height in metre. PE is measured in joule. A 2 kg bag is lifted 3 m. Taking g = 10 m/s², PE = mgh = 2 × 10 × 3 = 60 J. Students are asked to calculate gravitational PE, compare PE at different heights, or explain stored energy in raised objects. Using horizontal distance instead of vertical height is wrong because gravitational potential energy depends on height against gravity.
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