I'm a bit confused with how the foucault pendulum works.

EDIT for clarity:

The Foucault pendulum shows an effect which is commonly described as follows: the plane of oscillation of the pendulum seems to rotate from the perspective of the person observing, who is on the same frame of reference as the Earth, due to the Earth rotating beneath it. My question is, if the Earth, the atmosphere, the pendulum and me are all rotating with the Earth at the same rate, shouldn't we observe an oscillation on a plane which never rotates?

  • $\begingroup$ Hello. Welcome to PSE. This question is vague. Yes, the atmosphere is rotating with the earth but I cannot understand what you are asking. Can you provide a diagram? What do you mean by "push the pendulum along?" Perhaps you could check the wiki article en.wikipedia.org/wiki/Foucault_pendulum to get a look at the mechanics of a Foucault pendulum. $\endgroup$ – joseph h Sep 3 '20 at 1:59
  • $\begingroup$ Sorry for not expressing it clearly. What I meant was, why doesn't the plane of oscillation of the pendulum always rotate along with the Earth at the same rate? I don't get why we observe the plane of oscillation rotating from our frame of reference since me, the pendulum, the Earth and the atmosphere are rotating at the same rate, shouldn't I see the pendulum oscillating on a plane that never rotates? $\endgroup$ – Cromen Sep 3 '20 at 2:01

A Foucault pendulum swings back and forth in a plane and the Earth rotates beneath it, so that a relative motion exists between this plane and the earth's surface, so that the plane of the pendulum as viewed from above appears to rotate. This is the basic way the Foucault pendulum works.

Now for simplicity consider yourself at the north pole, and you are looking down at the pendulum setup. If you picture a line that represents the direction of oscillation of the pendulum, you will notice that this line will rotate at a rate of 15 degrees per hour, and therefore the line will return to it's initial position after one day has passed, or in other words after 24 hours, since 24 x 15 degrees = 360 degrees.

The exact same thing would happen if you did the same experiment at the south pole, only the direction the line would rotate would be opposite to what it was at the North pole.

There is no reason for the atmosphere to affect the motion of the pendulum in such a manner. The atmosphere does rotate with the earth due to drag that has occurred over a very long period of time. Your question assumes that the atmosphere is a "solid body" moving everything along as it rotates with the earth, and clearly this is not the case.

  • $\begingroup$ I see. So, if I understand it correctly, let's say you throw a ball high up in the air, when it comes down, the Earth would have slightly rotated so it wouldn't fall in the same position anymore. The atmosphere doesn't keep the ball rotating with me? Is this right? $\endgroup$ – Cromen Sep 3 '20 at 2:30
  • $\begingroup$ no! that is actually incorrect. When you throw a ball in the air, although you give it an upward velocity, it still has a horizontal velocity imparted to it by the earths motion. This is called conservation of momentum. $\endgroup$ – joseph h Sep 3 '20 at 3:17
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    $\begingroup$ That makes sense! I understand your point too about the pendulum although there’s one last piece. When the pendulum starts moving, doesn’t it also have the same velocity components as in the ball example? $\endgroup$ – Cromen Sep 3 '20 at 3:33
  • $\begingroup$ Yes, but obviously the pendulum is not being thrown in the air! So the "x-value" of velocity would be similar. $\endgroup$ – joseph h Sep 3 '20 at 3:45

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