Purpose: To introduce the dynamics of circular motion.

A platform with strings attached at the corners, three plastic cups, container of water.

To introduce the topic, pour water into one plastic cup and ask students who can turn it upside down without anything covering the top of the cup, and not have any water spill. Some will suggest swinging it. Invite one to come up and demonstrate. Then fill the other cups and invite the student to "turn over" all three at once. Offer the use of the platform. Generally the volunteer will be able to swing the platform and cups in a vertical circle, and stop them, without spilling (much) water.

A qualitative explanation can be based on Newton's first law, by making note of the fact that the water at the top of the circle would tend to go in nearly a straight line. The curved path of the cup causes it to pull down on the water, forcing it to stay inside.

Subsequently, estimate the radius of the circle and calculate, using Newton's second law of motion, the minimum speed for the cup at the top of the circle if the water is to remain in the cup.

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