A 7.80-g bullet moving at 560 m/s penetrates a tree trunk to a depth of 6.40 cm.

Question

A 7.80-g bullet moving at 560 m/s penetrates a tree trunk to a depth of 6.40 cm.
(a) Use work and energy considerations to find the average frictional force that stops the bullet.

________ N

(b) Assuming the frictional force is constant, determine how much time elapses between the moment the bullet enters the tree and the moment it stops moving.

______ s

Explanation / Answer

(a) By work energy theorem:- =>W = delta KE =>Ff x s = 1/2mv^2 =>Ff = mv^2/2s =>Ff = [7.8 x 10^-3 x (530)^2]/[2 x 5.2 x 10^-2] =>Ff = 21067.50 N (b) By v^2 = u^2 - 2as =>0 = (530)^2 - 2 x a x 5.2 x 10^-2 =>a = 5401923.08 m/s^2 By v = u - at =>0 = 530 - 5401923.08 x t =>t = 9.8 x 10^-5 sec

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