A small cart mounted on rails is held at one end by an electromagnet. A long coiled spring attached to the cart accelerates the cart when the magnet is released. The spring falls off in such a way that the integral of F ds is constant for all trials. Near the other end of the track, a cylinder on the cart interrupts the light shining on a photocell. The time of the interruption is measured with a Hunter klockcounter so that the velocity of the cart can be computed. Midway on the rail an electromagnet holds a sack containing iron shot. This sack can be dropped onto the cart. The velocity is measured under three conditions: the empty cart, the cart with the sack initially on it, and the cart which is initially empty but has the sack dropped on it from a variable height. By dropping the sack one can show that momentum is conserved and that energy is not conserved. The total energy of the horizontal motion is the same when the cart was accelerated initially with or without the sack in place.