Recall that the moving carts in the first demonstration and this third demonstration are of equal mass. Since they are released in the same manner (by magnetic repulsion), and they both travel the same distance, at the point of collision their accelerations and masses are the same; so the force, by Newton's Second Law (F=ma), is the same, and by Newton's Third Law, the force felt by the stationary carts are the same, regardless if their masses are different. (It just means that the distance traveled after the collision by the heavier cart will be less. Since the force is the same, but the cart is heavier, its acceleration must be smaller)