Wheels and Whirlwinds: the Coriolis Effect

The Coriolis effect can whip up hurricane winds, help monitor water flow, and even seem to defy gravity! But what powers this mysterious "fictitious force"? Although it usually only plays a role at extremely large scales, you can explore the physics of the Coriolis effect by heading to your nearest playground for a life-size demonstration!

What you'll need:

  • a playground with a "roundabout" or merry-go-round (not to be confused with a carousel)
  • a soccer ball, tennis ball, or other balls of similar size
  • a partner

What to Do

Hop on the roundabout and sit at the edge facing the disc's interior, and seat your partner on the other side. Get the roundabout spinning and, once it's up to speed, try and roll the ball across to your friend. After getting a feel for how the ball behaves, give it a toss instead. You ought to notice after a few tosses that it's very difficult to get the ball to roll straight across; if the roundabout is spinning clockwise, the person throwing the ball will see its motion curve to the left. This is the Coriolis effect.

What's Going On?

The Coriolis effect is a result of your reference frame's rotation. Imagine being the only person on a very fast-spinning roundabout, and tossing the ball straight forward. If you time it right, you might be able to catch the ball! To someone standing next to the roundabout, (in the non-rotating or "inertial" reference frame) it'd be clear that you tossed the ball straight and then swung around to catch it. In the rotating, non-inertial reference frame of the roundabout, the ball seems to be subject to weird forces that cause it to curve around back to you.

While the ball is in your hands, its velocity is the same as yours. Let's say the disc has a circumference of 12 feet and is spinning clockwise, completing one revolution every three seconds. So at any given time, you're moving 4 ft/s to the left, if you're sitting at the edge of the disc facing the center.

But closer to the center of the disc, things are a little different. Imagine a concentric circle drawn on the disc of the roundabout, with a circumference of 9 feet instead of 12. A person sitting at that circle is only moving to the left at 3 ft/s, so if you toss the ball to them, you'll have to aim a little to the right. 

Apply It!

The Coriolis effect is what creates the circular motion of hurricanes. At the equator, a point on Earth's surface is moving close to 25,000 miles every day—but the further from the equator you get, the less quickly a point on the surface has to move to make it all the way around in 24 hours, thanks to the smaller circumference of a sphere far from the middle. As a result, wind blowing north or south from the equator finds itself moving faster in an east-west direction than the air around it, and this can create circular currents when interacting with zones of low pressure.

Hurricane Irma off the coast of Florida

Earth rotates in an eastward direction—which way does the wind get deflected, if it's traveling south from the equator?

-Stephen Skolnick