The weight of the counterweight determines how fast the projectile goes. The heavier the counterweight, the faster the projectile. This shows gravity because the counterweight brings the arm down, flinging the projectile. This shows force because gravity is pulling the counterweight, which is exerting a force on the arm to fling the projectile. This shows acceleration because the projectile starts at 0 pixels/second, but accelerates to a certain speed. This shows mass because you can change the mass of the counterweight, increasing the speed of the projectile. Weight is shown because the gravity pulling on the mass of the counterweight makes it weigh a lot, increasing the force as it falls to the ground faster.
