A 20-g bullet moving at 1 000 m/s is fired through a one-kg block of wood emerging at a speed of 100 m/s.If the block had been originally at rest and is free to move,what is its resulting speed?
A)9 m/s
B)18 m/s
C)90 m/s
D)900 m/s

Question

Quizplus · Accepted Answer

By conservation of momentum, we have:
m1v1 = m1v1' + m2v2'
where m1 is the mass of the bullet, v1 is the initial velocity of the bullet, v1' is the final velocity of the bullet (which we don't need to know), m2 is the mass of the block, and v2' is the final velocity of the block.
Solving for v2', we get:
v2' = (m1v1 - m1v1')/m2
Substituting the given values, we get:
v2' = (0.02 kg)(1000 m/s - 100 m/s)/(1 kg) = 180 m/s
Therefore, the resulting speed of the block is 180 m/s, which is equivalent to choice B.

A 20-G Bullet Moving at 1 000 M/s Is Fired