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obog t1_j0fdjxw wrote

Reply to comment by starfyredragon in Does rotation break relativity? by starfyredragon

Linear movement continues without any force, however rotation needs a constant force to continue, that being the centripetal force. Seems odd because a force doesn't seem to be applied to a spinning object in space, but technically there has to be some force on anywhere outside of the center of mass, likely some sort of tension force (or in the case of larger objects like planets, gravity).

Edit: I want to clarify that that necessary force isn't necessarily external. A rotating object will continue rotating without an external force, however within the single object, particles near the outside of rotation will have a force of tension pulling them back towards the center, which is also the centripetal force. If that force didn't exist, the object would break apart.

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JCSterlace t1_j0ggxgj wrote

The rotation of an isolated solid object does not need a constant force to continue - angular momentum is conserved. Some internal forces holding the object together (to make it solid) would be acting centripetally, but that's internal to the object, not acting on the object.

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obog t1_j0gjdr9 wrote

Those internal forces are what I meant, as such the object is still technically accelerating

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VoilaVoilaWashington t1_j0gmqc8 wrote

The internal forces are causing the outer parts to accelerate around the center. If you swing a hammer while you spin in a circle, it's your hand that's accelerating the hammer and keeping it moving around you. Let go, and it goes flying.

That was their point - you need something keeping it all together outside the center of mass, or it will just fall apart.

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[deleted] t1_j0fkpbk wrote

[removed]

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Quartersharp t1_j0fnrfp wrote

It does… a little bit. A rotating object will have its outer parts under tension because of the centripetal acceleration, which wants to pull them outward. Unless the object flies apart, it is doing a tiny amount of work by staying intact while rotating, because its outer parts have to keep changing direction. Because of this, the object will also radiate gravitational waves, and will very gradually slow down, probably over thousands and millions of years.

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rpetre t1_j0forax wrote

Nitpick: "centripetal" means "towards the centre", so in your example the tension IS the centripetal acceleration, it's what keeps the parts of the body on their respective circular trajectories.

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wasmic t1_j0g22v9 wrote

No. There's no work being done from the rotation itself, if the object is perfectly rigid. Of course, in the ideal case there is work being done due to tiny stretches all over the place, but that should all cancel out because it both stretches and contracts to keep the same shape. With no overall radial motion coaxial with the force, there is no work being done.

Also, a spinning sphere, or a cylinder spinning around its own axis, will not emit gravitational waves. But something like a rotating cog would emit gravitational waves.

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Putnam3145 t1_j0g7zxw wrote

A perfectly rigid object has a faster-than-light speed of sound, among other problems, and is thus unphysical

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PefferPack t1_j0fu27b wrote

>it is doing a tiny amount of work by staying intact while rotating, because its outer parts have to keep changing direction

Really? This is kindof mind-blowing if true. Work is force dot displacement, so if there's no displacement in the radial direction, then there's no work done. If the body can be considered rigid then there is no radial displacement. Even in a flexible body, there would only be a short transient period of radial displacement as it stretches out from the rotational forces.

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wasmic t1_j0g1tt6 wrote

Yeah, I don't think there's a connection between gravitational waves here, and there's no work being done to keep a rotating object together.

Gravitational waves are not emitted from spherically or cylindrically symmetric objects, either (provided the cylindrical axis is the rotational axis).

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TheSmartestBanana t1_j0gjwf6 wrote

I believe the net centripetal force (toward the center) is balanced out by the net centrifugal force (toward the outside/perimeter). Therefore there is zero net force and zero net acceleration on an object rotating at constant angular velocity.

A force does not need to be applied to keep an object spinning in space (think planets and moons). It is valid to say that an object that is rotating will remain in rotation unless acted upon by an outside force (conservation of angular momentum).

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obog t1_j0gnqyo wrote

The centrifugal force isn't real, it's only an observed force when you're inside the rotating frame of reference. A constant force does still have to be applied, it's just often an internal force, not an external one. In the case of an object rotating in space, there is a force of tension acting in the outer parts of the object pulling them back towards the center. Due to that the object will stay in rotation without any outside force, but the key term there is outside. there still has to be some force to keep an object rotating, it can just be an internal force. That internal force (as I said, the force of tension for an object rotating) is the centripetal force.

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TheSmartestBanana t1_j0gqq4u wrote

Centripetal force can only be felt by a point inside the rotating body as well. That point obviously has an acceleration because the directional component of its velocity is changing constantly. The rotating object as a whole is not accelerating and therefore requires no net force. There are a lot of forces that hold an object together, but those forces do no cause an acceleration on that object and therefore cause no force to act on the object itself as a whole.

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atmsk90 t1_j0hgbik wrote

I think there is some confusion between static and dynamic equilibrium here. Centrifugal "force" is simply inertia. It's much easier to visualize by considering a point mass on a massless arm. The arm has to exert a force on the mass not as a result of any applied force from somewhere else, but just as a result of needing to accelerate the mass toward the center of rotation.

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hydroxypcp t1_j0kuo20 wrote

for this question you do have to look at individual particles of the rotating body though. If we take a human body as the rotating body, then the eyes are accelerating and thus not an inertial frame of reference

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