medium difficulty5 min

Conservation of Mechanical Energy

Conservation of mechanical energy means that the sum of kinetic energy and potential energy remains constant when only conservative forces like gravity act and friction is neglected.

In one line: PE and KE trade, total stays.

Practice This Concept

Understand it

Mechanical energy is KE + PE.
During free fall without air resistance, potential energy decreases while kinetic energy increases by the same amount.
The total mechanical energy remains unchanged.
With friction, some mechanical energy changes into heat and sound, so mechanical energy alone is not conserved.

Example

A freely falling ball loses gravitational potential energy and gains kinetic energy as it comes down.

Rule to remember

Mechanical energy = KE + PE.

If friction is absent, KE_initial + PE_initial = KE_final + PE_final.

See it worked out

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

Worked example

1A ball has 100 J PE at the top and starts from rest.
2If 40 J PE is left at some lower point, KE there is 60 J, ignoring air resistance.

Method you can reuse

Check if friction is neglected, write total mechanical energy, calculate known PE or KE, and use total = KE + PE to find the missing value.

Visual tip

A three-position fall diagram can show maximum PE at top, mixed PE and KE in between, and maximum KE near bottom.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

What happens to potential energy and kinetic energy of a freely falling object if air resistance is ignored?

Think first, then show answer

Its potential energy decreases and its kinetic energy increases, while the total mechanical energy remains constant.

Common wrong answer

Saying total mechanical energy increases during falling.

Why it loses marks: Gravity converts PE into KE without creating extra mechanical energy when losses are ignored.

Don't mix this up

Students often say kinetic energy remains constant during fall, but it actually increases while potential energy decreases.

Exam tip

Mention the condition clearly: mechanical energy is conserved only when friction and air resistance are ignored.

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 ask energy distribution at different heights, conversion between KE and PE, or whether conservation applies with friction.

1
Enough for 1 mark
Conservation of mechanical energy means that the sum of kinetic energy and potential energy remains constant when only conservative forces like gravity act and friction is neglected.
2
Add for 2 marks
Example: A freely falling ball loses gravitational potential energy and gains kinetic energy as it comes down.
3
Add for 3 marks
Mechanical energy is KE + PE.

MCQ Quiz

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