Someone fires a 0.04 kg bullet at a block of wood that has a mass of 0.5 kg. (The block of wood is sitting on a frictionless surface, so it moves freely when the bullet hits it). The wood block is initially at rest. The bullet is traveling 300 m/s when it hits the wood block and sticks inside it. Now the bullet and the wood block move together as one object. How fast are they traveling?

We can solve the problem by using the law of conservation of momentum. In fact, the total momentum before and after the collision must be the same, so we can write:

[tex]m u + M U = (m+M)v[/tex]

where:

m = 0.04 kg is the mass of the bullet

u = 300 m/s is the initial velocity of the bullet, before the collision

M = 0.5 kg is the mass of the block

U = 0 is the initial velocity of the block

v is the velocity of the bullet+block after the collision, as they stick together

Solving for v, we find:

[tex]v=\frac{mu}{m+M}=\frac{(0.04)(300)}{0.04+0.5}=22.2 m/s[/tex]