Proving that the earth isn't an inertial frame?

Question

I keep trying to think of ways to answer this question but it just isn't working. Can anyone help me out or give me some hints or ideas?

We usually treat a frame of reference attached to the earth as an inertial frame. This is in fact not true. Describe an experiment you could do, using material one is likely to find in the store rooms under the Physics Lecture Hall (i.e., what might be used for classroom demonstrations), which would show that the Physics Lecture Hall itself is NOT an inertial frame.

thanks for your help!

Answer 1

The earth's rotation causes two pseudoforces:
Coriolis force
Centrifugal force

The centrifugal force is very hard to detect, because it just cancels out gravity ever so slightly (and makes the earth somewhat fatter in the middle).

The coriolis force can be detected if you are not too close to the equator. The smallest experiment I can think of is a Foucalt (spelling?) pendulum--the one whose path rotates around every day or two (the period depends on your lattitude) because of the coriolis force.

And NO, you cannot observe the coriolis force in a toilet. And NO, they do not flush counter-clockwise in Australia unless the toilet manufacturers there choose to build them that way.

Answer 2

I agree with Mistress Bekki. The Foucault pendulum is a classic experiment to prove the earth's rotation, but it's a very delicate apparatus and hard to set up, so I'd be surprised if you could throw together one that actually works, from material in the store room.

I remember hearing once about another experiment, involving a torus, filled with a liquid that had small particles suspended in it. You mounted the torus vertically, and attached it to an east/west horizontal axis attached on two diametrically opposite points on the torus. You let the liquid inside "settle down," and then you give the torus a quick 180° "flip," swapping the top half with the bottom half.

The idea was that the top half of the (stationary) torus is moving slightly faster than the bottom half, since it's slightly farther away from the earth's axis. When you flip the torus, the "fast" half of the fluid moves to the "slow" half of the apparatus, and vice versa; so the original momentum in the fluid causes it to swirl a bit, in a direction opposite of the earth's rotation. As I recall, there was a microscope hooked up to the thing, through which you could observe the motion of the suspended particles.

This is all from memory--unfortunately, I can't remember what the experiment was called, and I can't seem to find it mentioned on the 'net.

Answer 3

Bekki is correct, but I would like to point out that there is in fact an experiment you can do with water in a bowl to detect the Coriolis force. You need to fill a large (several meters in radius) perfectly hemispherical bowl with water. The bowl should have a plug exactly in the bottom of the bowl, which can be pulled out from underneath. Place the bowl in a closed box, so that there is no wind to interact with the water surface. Wait about a month for transient motions of the water to die away. Pull plug rapidly, so as not to impart any forces to the water.

I have done this experiment, and it works. Water flows through the hole directly downwards for about 15-20 minutes, with no spin whatsoever. After about 20 minutes, rotation builds up in the direction predicted by coriolis.

To ensure that this is not an effect of the direction in which the water was poured into the bowl, I always pour the water the wrong way. This was performed at about 41 degrees latitude.