When the ball rolls in a circle then reaches the gap in the wall, it will continue in a line. This teaches Netwon's first law, that an object will continue with a constant speed and direction if it isn't pushed by some force. In this case, the wall pushes the ball, keeping it moving in a circle. When the wall is gone, the ball's direction stays constant.

• a circular container, or some cardboard pieces to construct one
• brightly colored golf balls

If you have a round container with a flat base, use that. It helps to have two, and cut a gap in the wall of only one of them. If you don't have such a container, cut a cardboard circle, then cut a strip of carboard to make a wall around the circle, and duct tape it in place.