A0.30 kg ball of putty is thrown at a stationary 1.2 kg cart. the ball of putty is initially moving with a speed of 6.0 m/s. the putty and the cart stick together and are now free to roll across the floor. what is the speed of the cart with the putty attached?

In order to determine the speed of the entire assembly, we employ conservation of momentum. Momentum p = mv where m is the object's mass and v is the velocity.
   The putty ball's initial momentum p1 = 0.3kg*6m/s = 1.8 kg*m/s
   That momentum is conserved, so the momentum of the new system having mass 0.3 kg + 1.2 kg = 1.5 kg is:
   1.8 kg*m/s = 1.5kg*v. Solving for v, we find that the velocity is 1.2 meters/second.