Answer:
We can use conservation of momentum to solve for the unknown mass of the second box. The total momentum before the collision is equal to the total momentum after the collision.
Before collision:
p = m1v1 = (7 kg)(6 m/s) = 42 kg m/s
After collision:
p = m1v1 + m2v2 = (7 kg)(2 m/s) + m(2 m/s)
Setting the two expressions for momentum equal to each other, we can solve for m2:
42 kg m/s = (7 kg)(2 m/s) + m(2 m/s)
42 kg m/s = 14 kg m/s + 2m kg m/s
28 kg m/s = 2m kg m/s
m = 14 kg
Therefore, the mass of the second box is 14 kg