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This page is designed to help you with your exams - by providing notes and model answers from exam questions.

## Calculations

### Energy

Question: A moving block of mass 5kg is dropped from a height of 4m above a table. When it hits the table it reaches a speed of 8 m/s. What percentage of the gravitational potential energy is lost to heat and sound energy?

To answer this question you need to know that as the block falls the gravitational energy it has is converted into kinetic energy and thermal and sound energy. Note that often exam questions often ask in part (i) to find the g.p.e.; part (ii) find the speed - note here the kinetic energy has to be the gravitational energy you worked out in part (ii).

• g.p.e. = mgh OR gravitaitonal potential energy = mass * g * height (state formula in symbols or words)
• g.p.e. = 5 * 10 * 4
• g.p.e. = 200J (remember symbols)
• k.e. = 0.5mv2
• k.e. = 0.5 * 5 * 82
• k.e. = 160J
• assuming heat and sound energy lost is g.p.e. - k.e., the energy lost is 200J - 160J = 40J
• percentage loss is 40/200 * 100
• = 20%
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