posted by Kristi .
A 3.0-kg cart is rolling along a frictionless, horizontal track towards a 1.2-kg cart that is held initially at rest. The carts are loaded with strong magnets that cause them to attract one another. Thus, the speed of each cart increases. At a certain instant before the carts collide, the first cart's velocity is +3.9 m/s, and the second cart's velocity is -2.5 m/s. (a) What is the total momentum of the system of the two carts at this instant? (b) What was the velocity of the first cart when the second cart was still at rest?
(a) 3.0*3.9 - 1.2*2.5 = 8.7 kg*m/s
(b) Since TOTAL linear momentum must be conserved at all times before (and after)collision, when cart 2 is motionless, the speed of cart 1 must be 8.7/3.0 = 2.9 m/s.
Total momentum is conserved because the two carts interact magnetically with EACH OTHER rather than with an external magnet. The two forces on each other cancel.