Example 1 · level 1
A 2 kg brick moves at 17 m/s. Find its momentum in kg m/s.
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- p = 2 × 17 = 34 kg m/s.
Answer: 34
CLASS 9 SCIENCE · FORCE AND LAWS OF MOTION
Momentum and its conservation
Momentum p = mv (kg m/s). In a collision or explosion with no external force, total momentum before = total momentum after.
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Worked examples
Example 2 · level 2
A 5 kg trolley moving at 6 m/s hits a 5 kg trolley at rest and they move off together. Find their common velocity in m/s.
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- 30 = (5 + 5) v
- v = 30 ÷ 10 = 3 m/s.
Answer: 3
Example 3 · level 3
A bullet of mass 20 g is fired at 200 m/s from a rifle of mass 5 kg initially at rest. Find the recoil speed of the rifle in m/s (correct to 2 decimal places).
Show solution
- Momentum of bullet = 0.02 × 200 = 4 kg m/s.
- Recoil speed = 4 ÷ 5 = 0.8 m/s (backwards).
Answer: 0.8