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I have a doubt, general one.

If we place a rod in empty space and apply a force perpendicular to the rod at one end would the rod rotate like it does in real world?

I think it wouldn't rotate because there is no point or axis of rotation for it rotate.

To be precise:(Note:Experiments conducted in empty space only)

If I hold a rod about one end and apply a force about its other end ,it would rotate about the holding point... Now if I left the rod and applied a force about any end what would happen?

Banchin
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  • Why do you suppose there is no point or axis of rotation? It's still a physical object in physical space. The rod will still spin around its center of mass. – Nuclear Hoagie Jul 18 '18 at 14:38
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    So how was the space shuttle turned to allow the heat shield tiles to face the atmosphere on reentry? –  Jul 18 '18 at 14:38
  • Possible duplicate of Will there be translation + rotational motion? – V.F. Jul 18 '18 at 17:56
  • Sry about the unclear question,I edited it please give a check – Banchin Jul 19 '18 at 13:20
  • I read an answer in some other website(quora)which is as follows:If we push near the end, away from the center of mass the force propagating towards the bulk of the object has a much larger mass to move, and so its acceleration is lower. The force propagating towards the other side has much less mass to move, and so it accelerates faster. This results in the object twisting as you push it....Is this explanation correct? (sry for the trouble...it's just that I don't trust that website as much as I trust this one) – Banchin Jul 20 '18 at 14:19

5 Answers5

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What you are saying is that no object can rotate in empty space, because there is no axis of rotation.

Now in your case, the Earth would not be rotating. But the Earth is rotating, and there is an axis of rotation (3D), and the axis is actually the 3d combination of the x,y,z rotations.

Now what you are asking I think is, if there would be nothing else in space, how would we know that the object is rotating? Because there would be nothing to compare it to.

Even with Earth in the early times they figured the rotation out because of the movement of stars in the sky.

Now if there would be nothing else in space, just Earth, how would we know it is rotating?

Well, if nothing else works, we could send up a rocket to space, outside Earth's gravity and looking back you would see the Earth rotate, because the rocket would not follow Earth's rotation (so the rocket would have to be sent out to space not like a satellite, so not orbiting, and further out to space).

Árpád Szendrei
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The object would move, and that movement can be written as the sum of two:

translational movement + rotation about the center of mass (CM)

So the "default" axis would be the CM.

Why is it that? The answer is "because it is easy". In fact, you can say it moves about any axis you want, but then the movement will be described much more difficultly.

For example, you can say it will rotate about an axis in the other extreme, but that will probably imply a non uniform angular acceleration, plus a translational movement, plus some other kind of movements.

In sum: the movement is translation + rotation about the CM, but only because this is the simplest way to model what happens.

FGSUZ
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  • If we consider an axis at an extreme end ,do we need to consider pseudo forces ,I mean does the pseudo force cause torque? – Banchin Jul 19 '18 at 13:38
  • Pseudo forces depend on the RF you choose, not the axis. You're free to choose your reference frame for coordinates, and also the point where you refer torques to. – FGSUZ Jul 19 '18 at 13:41
  • If my frame is sitting about one end and a force if applied about other end then as I'm rotating about COM (as told by you)(default) my end is in rotation and definitely has some angular acceleration .Now that I'm a non inertial frame would the pseudo force cause torque ? – Banchin Jul 19 '18 at 13:55
  • I didn't mean "default" in that sense. Default was meant to be "the usual one we always take" because of its simplicity. If your frame is inertial or not depends on wether you are accelerating or not. Zoom outside the rod: is the extreme fixed or not? That will tell you if you're inertial or not. – FGSUZ Jul 19 '18 at 14:41
  • Forget all those stuff ,I'm asking one basic question: Can a pseudoforce cause torque and if yes ,is there any pseudoforce, if reference frame is one end of the rod (not where force is applied),and if pseudoforce exists ,is it causing torque? – Banchin Jul 19 '18 at 14:45
  • Sure, pseudoforces behave like usual forces, so they cause torque in NI Frames. – FGSUZ Jul 19 '18 at 14:51
  • I have one more doubt: is the motion of com along a straight line? – Banchin Jul 19 '18 at 14:53
  • This is getting too long. I'm moving this to chat. The motion of the COM depends on the forces there are. – FGSUZ Jul 19 '18 at 14:54
  • Let us continue this discussion in chat. – Banchin Jul 19 '18 at 14:55
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The axis of rotation you are looking for is through the center of mass, mutually perpendicular to the rod and to the applied force.

The only case in which the object does not rotate is when the force is applied at the center of mass.

AbstractArgumentType
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Very good question. The words that give away the answer are “if you apply a force”. This force can’t come from nothing. And since it sets something rotating, the thing that creates this force has to be anti-symmetric of shape in a way. Thus giving the universe you’re talking about a certain “sense of direction” or “an axis”. Also, rotation can also be seen by how an object deforms, not only its trajectory.

7th808s
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As pointed out here, a conducting sphere in a perfect vacuum will slow down by emitting electromagnetic radiation.

Count Iblis
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