At what speed does the block start to move?

Question

An 8.4g bullet travelling at 240ms^-1 hits a block of wood of mass 1.2kg balanced on a wall. The bullet stops in the block. At what speed does the block start moving?

Answer 1

A typical inelastic collision case where only momentum (P) is conserved.

P_bullet = mV = 8.4 x 10^-3 kg x 240 m/s ≈ 2.02 kgm/s

P_wall = 0

P_total(before)= 2.02 kgm/s = P_total(after) = m_(combined) x V_(after) = (1.2 + 0.008 kg) x V_(after)

V_(after) = 2.02/(1.2 + 0.008 kg) ≅ 1.67 m/s