A2.1-kg brick is placed gently upon a 2.9-kg cart originally moving with a speed of 26 cm/s. determine the post-collision speed of the combination of brick and cart.

By conservation of linear amount of movement, we have to
 p1 = p2
 Where, p = m * v.
 So, using the definition for this case, we have:
 (2.9 kg)•(26 cm/s) = (2.1 kg + 2.9 kg)•v 
 75.4 kg•cm/s = (5.0 kg)•v 
 v = 15.1 cm/s