Explain rotation in empty space

Question

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?

Answer 1

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).

Answer 2

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.

Answer 3

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.

Answer 4

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.

Answer 5

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