About this experiment
Slide a puck across a spinning merry-go-round with no friction. Seen from above, the puck goes in a perfectly straight line, but the disc turns underneath it, so a rider on the disc sees the puck curve away from where it was aimed. The Earth is a huge rotating disc too: winds and ocean currents that travel a long way are turned to the right in the Northern Hemisphere and to the left in the Southern. Compare the two views of the same throw, then move to the Earth and see how far a push of air curves at different latitudes.
Learning goals
- Explain that the Coriolis effect is an apparent deflection seen from a rotating frame of reference
- Predict the direction of deflection in the Northern and Southern Hemispheres
- Explain why the effect is strongest near the poles and absent at the equator
- Use the Coriolis parameter f = 2Ω sin φ to estimate the size of inertial circles
- Relate the Coriolis effect to the spin of large storms and to global wind belts
How to use it
- 1Press Throw with the default settings. Compare the path in the two views. Which one is straight?
- 2Watch the dashed aim line in the outside view. Where is the target when the puck reaches the rim?
- 3Double the rotation speed, then halve the throw speed. Which changes make the puck land further from the aim?
- 4Switch on Turn clockwise. Which way does the rider now see the puck curve?
- 5Switch to On the Earth. Push the air at 45° north and watch it for one circle. Then try 10°, 0° and -45°.
- 6At 45° north, double the wind speed. What happens to the size of the circle and to the time it takes?
The model
Assumptions
- The puck slides without friction and does not bounce. It leaves the disc when it reaches the rim.
- The thrower moves with the disc, so the puck starts with the sideways speed of the point it is thrown from.
- On the Earth, the air is pushed once and then has no pressure differences, friction or other forces acting on it, so it moves on an inertial circle in a small flat region around one latitude.
- The change of f with latitude across the circle is ignored, and the centrifugal effect of the rotation is taken as part of gravity.
Try this in class
- Q1Why does a rider on the disc say a force pushes the puck sideways, while a person watching from above says no sideways force acts?
- Q2Why do hurricanes spin anticlockwise in the Northern Hemisphere, when the Coriolis effect turns moving air to the right?
- Q3Why do hurricanes almost never form within about 5° of the equator?
- Q4A sink is about 0.5 m across and empties in 20 s. Use f = 2Ω sin φ to explain why the Coriolis effect does not decide which way it drains.
- Q5Long-range artillery and missiles have to allow for the Coriolis effect, but footballs do not. Why?
Frequently asked questions
Is the Coriolis force a real force?
It is called a fictitious or inertial force. Nothing pushes the puck sideways. It appears only when you describe the motion from a rotating frame such as the disc or the Earth, and it is needed there to make the laws of motion work.
Does the Coriolis effect make toilets drain in opposite directions in each hemisphere?
No. For water in a sink or toilet the Coriolis acceleration is millions of times smaller than other effects, such as the shape of the bowl and how the water was poured. It matters only for motion over large distances or long times, such as weather systems and ocean currents.
Why does the deflection depend on latitude?
Only the part of the rotation of the Earth about the local vertical turns horizontal motion. At the poles the ground spins fully about the vertical, while at the equator it does not turn about the vertical at all, so f = 2Ω sin φ goes from its largest value to zero.