How does a top ‘know’ who is stationary and who is spinning?
Related thought experiment:
An observer sits on a spinning top and perceives a centrifugal force.
Suppose this happens in a completely empty universe, in which there is therefore no other reference to determine that the top is spinning. Is the centrifugal force still there, and if so, how can our observer explain the force he then feels?
In that case, does the top also try to maintain its torque, in other words, will someone who pushes against the shaft also have to push ‘extra hard’?
Answer
The speed of a mass depends on the coordinate system in which you are located: your colleague next to you in the train is at rest relative to you, but has a speed relative to an observer on the platform. However, the acceleration of a mass (and thus the forces on that mass) are observed the same in all coordinate systems that move with a constant speed relative to each other. So you can determine that the top rotates because the rotation implies an acceleration.
The axis of the toll will keep its direction if no force is applied to it. The difference compared to the movement of the top on Earth is that you will not have the ‘precession’ movement (the spinning of the top around the vertical axis), because that is a result of gravity.
Answered by
Prof Walter Lauriks
Physics Acoustics
Old Market 13 3000 Leuven
https://www.kuleuven.be/
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