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Question
A bullet of mass 50 g moving with an initial velocity 100 m s-1 strikes a wooden block and comes to rest after penetrating a distance 2 cm in it. Calculate: (i) Initial momentum of the bullet, (ii) Final momentum of the bullet, (iii) Retardation caused by the wooden block and (iv) Resistive force exerted by the wooden block.
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Solution
Mass, m = 50 gm = 0.05 kg.
Initial velocity, u = 100 m/s.
Final velocity, v = 0.
Distance, s = 2cm = 0.02 m.
(i) Initial momentum = mu = (0.05) (100) = 5 kg m/s-1
(ii) Final momentum = mv = (0.05) (0) = 0 kg m/s.
(iii) Acceleration, a = (v2 - u2)/2s.
Or, a = (02 - 1002)/ 2(0.02).
Or, a = -2.5 105 ms-2.
Therefore, retardation is 2.5 × 105 ms-2.
(iv) Force, F = ma
Or, F = (0.05 kg) (2.5 × 105 ms-2)
Or, F = 12500 N
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