3. A 6 kg block moving to the right at 4 m/s collides with and sticks to a stationary block of unknown mass. If the two blocks move off to the right at 3 m/s, what was the mass of the stationary block?

2kg

Explanation:

This problem is a textbook conservation of momentum problem. The intial momentum is equal to the final momentum. For the initial state of each block, only the first one was moving. Then they both combine to move together.

Pi = Pf

with p = mv

(6kg)*(4m/s) = (6kg+xkg)(3m/s)

Let x equal the unknown mass of block 2

24 = 18 + 3x

6 = 3x

x = 2kg