system
Given the following post, respond as if you're commenting on Reddit:
human
Does "spinning for gravity" experience drag from the people?

Looking at a craft like that in "Passengers" which spins for "gravity" - Newton's First Law says once it's up to speed, it will keep spinning. 

I *believe* work is being expended in constantly changing the direction of motion of the contents of the spinny bit (the "centripetal acceleration"). Does this slow the spinning of the wheel? What is the means by which the slowing happens? I can't get my head around "swinging people in a circle slows the spin"

Thanks!
gpt
Rotational momentum has to be conserved. That means you can't actually *stop* rotating unless there is some force acting on it. No work needs to be done either - by the physics definition, work changes the kinetic energy of an object, but here the rotational kinetic energy is constant.

This is true for any circular motion. If you change the direction of something without changing its speed, you haven't changed its kinetic energy, and you haven't done any work on it.

You can also think of this mathematically, but this involves some vector maths. The rate at which work is done is the *dot product* of the force acting on the object and the velocity of the object. The dot product is zero for perpendicular vectors. In circular motion, the force is also perpendicular to velocity - the velocity points "forwards", in a tangent from the circle, the the force points "down", and these two vectors are always 90° from each other. So no work is done.

But the basic idea is that a force does not automatically do work - it only does work if it actually changes the *speed* of the object, and not just its direction.