Two carts, cartXand cartY, are initially at rest and placed next to each other on a horizontal track, as shown in Figure 1. A switch on top of cartYcan 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 cartX-bar system to the momentum of the cartY-bar system immediately after recoil. The students have access to equipment that can be found in a typical school physics laboratory.

(a)

State a basic physics principle or law that the students could use to determine the relationship of the momentum of the cartX-bar system to the momentum of the cartY-bar system immediately after recoil.