What is the speed

Question

A 20-gram rubber bullet traveling at 150 m/s hits a stationary 8.5 kg concrete block. If the bullet bounces off of the block in the opposite direction at 100 m/s, what is the final speed of the concrete block

Answer 1

Use conservation of linear momentum.

 

mv_i = mv_f + Mu

 

m = 20 g = 0.020 kg

v_i = 150 m/s

v_f = -100 m/s  [minus sign means it is now moving in the opposite direction to v_i]

M = 8.5 kg

u = ?

 

u = m(v_i - v_f)/M = 0.020[150 - (-100)]/8.5 m/s ≅ 0.588 m/s