Two carts, cart X and cart Y , are initially at rest and placed next to each other on a horizontal track, as shown in Figure 1. A switch on top of cart Y can be pressed so that a compressed spring inside of the cart expands and pushes a plunger outward, causing the two carts to recoil, as shown in Figure 2. Both carts have identical but unknown masses, M . The carts are designed so that bars of additional but unknown mass can be added and secured to the carts. A group of students are asked to determine the relationship of the momentum of the cart X –bar system to the momentum of the cart Y –bar system immediately after recoil. The students have access to equipment that can be found in a typical school physics laboratory.