A 7.80-g bullet moving at 650 m/s penetrates a tree trunk to a depth of 5.90 cm.

Question

A 7.80-g bullet moving at 650 m/s penetrates a tree trunk to a depth of 5.90 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

work done by all forces = change in kinetic energy work done by friction = change in ke = .5mv^2 let friction force be F F*d = .5mv^2 => F = .5*7.80*10^-3*650^2/.059 = 27927.96 N B)If the Friction is constant the acceleration is constant and equal to a a = F/m = 27927.96/7.80*10^-3 now v = u +at => t = 650/(27927.96/(7.80*10^-3)) = 1.81*10^-4sec

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