B Car spinning on a frictionless sheet of ice = absolute motion?

[ESponge2000]

TL;DR Summary

Relative velocities are relative but what about the case of a vehicle with a steering wheel stuck all the way down to the left that’s skidding doing 360 after 360 round and round and round? If the car is at rest then wouldn’t that not be possible ? I thought objects are always at rest with themselves but this
Challenges that, it can’t be assumed the whole universe is spinning around the car ?

Is this absolute motion ?

 

[phinds]

Yes, from the FOR of the car, the whole universe is spinning around it. There IS NO "absolute motion".

Since you are unsure about this, you might want to read up on the difference between inertial motion and accelerated motion (doesn't change the answer to your question, just some additional information).

 

[Orodruin]

Velocity is relative. Angular velocity isn’t. (Edit: As long as we are talking inertial frames in classical mechanics)

 

[DaveC426913]

Yes, from the FOR of the car, the whole universe is spinning around it. There IS NO "absolute motion".

Except you then have to explain the additional forces the passengers experience.

 

[Ibix]

Is this absolute motion ?

Depends on what you mean by "absolute motion".

You can detect rotation (and acceleration in general) with a "closed box" experiment. A free mass on a spring will extend the spring in some direction if you are rotating or accelerating, and you may find subtle variations in the extension as a function of position.

This stands in contrast to inertial motion, where whatever your state of motion any experiment will give you the same answer unless it looks outside the box at something else. So you can say you are moving inertially with respect to something, but not that you are moving inertially.

So if you use "absolute" to mean "can be detected in a closed box", then rotation and acceleration are absolute, while inertial motion is not.

However, you are still free to regard yourself as "at rest" even when you are accelerating. You just end up with so-called fictitious or inertial forces in the maths. This sometimes makes life easier and sometimes doesn't.

 

[PeterDonis]

you then have to explain the additional forces the passengers experience

If you mean "centrifugal forces", those aren't felt by the passengers, they're "fictitious forces", which are already known to be frame-dependent and don't require a physical explanation; they're conventions.

If you mean the forces the passengers actually feel, those are due to contact with something that pushes on them--in the OP scenario, the car is pushing on them. Those forces are invariants, they're the same in any frame, and their explanation is the same in any frame: the car is pushing on them.

 

[hutchphd]

the car is pushing on them.

Does this argument require a turtle to push on the car? (and then all the way down....) I must not understand your point

 

[Dale]

Is this absolute motion ?

So “absolute” motion usually refers to motion with respect to some absolute reference frame. Such a frame does not exist, so such motion does not exist. Some authors may use the term differently.

Related terms are covariant and invariant, which mean that a quantity is a geometric object that is the same geometric object regardless of what coordinates are being used. For example, if I am directly facing you, 3 m away, and you are facing directly to my left, then we both agree that my “1 m forward and 0 m to the right” and your “0 m forward and -2 m to the right” are referring to the same place and if there were some object located there we would agree it is the same object.

So, proper acceleration is invariant. It is the Minkowski norm of the covariant four-acceleration. The car spinning around is unambiguously accelerating. This is invariant/covariant, not “absolute”

 

[PeterDonis]

Does this argument require a turtle to push on the car? (and then all the way down....)

The car will be pushing on the Earth, and the Earth will be pushing back on the car. But no turtle needs to push on the Earth; it's in free-fall motion.

More generally, there will always be a finite chain of pushes that will end with some overall system whose center of mass is in free-fall motion. It's never an infinite chain of turtles.

I must not understand your point

My point is that only the forces that are actually felt require physical explanations, and those explanations are invariant; they don't depend on what frame you choose. So the physical explanation of why the people in the car feel a force is the same whether you choose a frame in which the car is rotating and the rest of the universe is at rest, or a frame in which the car is at rest and the rest of the universe is rotating.

The post I was responding to was implicitly claiming that that is not the case--that you need some kind of different explanation in the car's rest frame than in the "car is rotating" frame.

 

[Herman Trivilino]

TL;DR Summary: Relative velocities are relative but what about the case of a vehicle with a steering wheel stuck all the way down to the left that’s skidding doing 360 after 360 round and round and round? If the car is at rest then wouldn’t that not be possible ?

An accelerometer will tell you if you are spinning or not. What it won't do is tell you whether you're at rest or moving in a straight line at a steady speed.

 

[Ibix]

To make a possibly related point, imagine a mouse living inside your tyres. He can determine whether the wheel is rotating or not by how his gyroscopes and pendulums behave. But he cannot tell whether the car is moving down a road or sitting on rollers on a test rig.

 

[Lnewqban]

If the car is at rest then wouldn’t that not be possible ?
...
Is this absolute motion ?