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Fun With Physics: Centrifugal Force And Inertia

Learn about what physicists call a "fictitious force", on this Moment of Science.

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Photo: Oliver Degabriele (Flickr)

You can thank inertia for being able to swing this glass over your head and not worry about spills!

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!

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