© 2025. The Trustees of Indiana University
Copyright Complaints
1229 East Seventh Street, Bloomington, Indiana 47405
News, Arts and Culture from WFIU Public Radio and WTIU Public Television
Play Live Radio
Next Up:
0:00
0:00
0:00 0:00
Available On Air Stations
The Magic Is Ours to Keep. Support Public Media Today
Some web content from Indiana Public Media is unavailable during our transition to a new web publishing platform. We apologize for the inconvenience.

Fun With Physics: Centrifugal Force And Inertia

Here's a little fun you can have to demonstrate centrifugal force. You might want to do it outside.

Take a glass of water. You'll find that if you move it fast enough, you'll be able to swing the filled glass around your head without spilling a single drop of water.

The Secret

The secret lies in centrifugal force. It's no myth, though it is what physicists call a "fictitious force." If you tie a rock to a string and spin it around, it feels as though some force is trying to drive the rock away from the center of the spin.

What you're actually feeling, though, is just the rock's inertia.

The Law Of Inertia

The law of inertia says that something in motion will tend to stay in motion, and what's more, to move in a straight line. The rock is set into motion and is trying at every moment to leave the circle and move away in a straight line.

The string is keeping it from doing so, which is why it feels like a mysterious force is pushing the rock away.

No Spills!

When you spin the glass over your head, the water inside it is trying to move off in a straight line too. But the glass prevents it from doing so, as long as you keep the bottom of the glass pointed away from yourself. Inertia holds the water inside the spinning glass!

Stay Connected