Example 1 · level 1
Find the kinetic energy (in J) of a 40 kg object moving at 10 m/s.
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- KE = ½ × 40 × 10² = ½ × 40 × 100 = 2,000 J.
Answer: 2,000 J
CLASS 9 SCIENCE · WORK AND ENERGY
Kinetic and potential energy
KE = ½mv², PE = mgh. For a falling body (no air resistance) PE lost = KE gained, so v = √(2gh).
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Worked examples
Example 2 · level 2
A bag of cement of mass 2 kg is kept on a roof 3 m high. Find its potential energy relative to the ground (in J). Take g = 10 m/s².
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- PE = 2 × 10 × 3 = 60 J.
Answer: 60 J
Example 3 · level 3
A ball is dropped from a height of 5 m. Using conservation of energy, find its speed just before it hits the ground (in m/s). Take g = 10 m/s², ignore air resistance.
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- v² = 2 × 10 × 5 = 100
- v = √100 = 10 m/s.
Answer: 10