Example 1 · level 1
A ball of mass 0.16 kg moving at 10 m/s is caught and brought to rest. Find the change in its momentum (magnitude, in kg m/s).
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- Δp = 0 − 0.16 × 10 = −1.6 kg m/s; magnitude 1.6 kg m/s.
Answer: 1.6
CLASS 9 SCIENCE · FORCE AND LAWS OF MOTION
Force from change in momentum
Force = change in momentum ÷ time (F = m(v − u)/t). Increasing the time of impact reduces the force — that is why fielders pull their hands back.
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Worked examples
Example 2 · level 2
A fielder stops a 0.15 kg ball moving at 20 m/s in 0.1 s. Find the average force on the hands (in N, correct to 2 decimal places).
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- Δp = 3 kg m/s.
- F = 3 ÷ 0.1 = 30 N.
Answer: 30 N
Example 3 · level 3
A 0.15 kg ball hits a wall at 25 m/s and rebounds at 20 m/s. Contact lasts 0.2 s. Find the average force exerted by the wall (in N, correct to 2 decimal places).
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- Δp = 0.15 × (20 + 25) = 6.75 kg m/s.
- F = 6.75 ÷ 0.2 = 33.75 N.
Answer: 33.75 N