Physics Momentum Problem?

Question

A 23-g bullet traveling 230 m/s penetrates a 2.0-kg block of wood and emerges cleanly at 170 m/s. If the block is stationary on a frictionless surface when hit, how fast does it move after the bullet emerges?

Answer 1

Consider the motion in one dimension with the positive direction being the direction of motion of the bullet. Let “A” represent the bullet, and “B” represent the block. Since there is no net force outside of the block-bullet system (like frictions with the table), the momentum of the block and bullet combination is conserved.

MaVa+MbVb=MaV'a+MbV'b

Mb=2.0 kg
Vb=0 m/s
Ma=0.023 kg
Va=230 m/s
V'a=170 m/s

V'b=Ma(Va-V'a)/Mb=(.023)(230-170)/2

=0.69 m/s (answer)

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