The model illustrates Newton's third law using two interconnected bricks as an example. A weightless string connects two bricks through a pulley. Gravity acts on one of the bricks. Observe the motion of the entire system by changing the bricks? masses. Pay especial attention to the forces that act on the bricks. Note that the elastic forces exerted on the bricks by the thread are equal in absolute value, and are exerted in opposite directions: F12 = -F21.