A Simple Problem

What's the issue? (That's how zero my physics is) Every action having an opposite reaction?

Turn one wheel clockwise, the other simultaneously counter-clockwise? Wouldn't the two opposite forces simultaneously balance and neutralize the ... torque?

Or ... telekinesis. Cause TK works in zero gravity ... which is why it's never been proven here on Earth.

You'll need something to counteract the motion of setting the bicycle wheel spinning in any particular direction. Otherwise, you're going to spin in the direction of any force applied.

The easiest way I can think of is to position yourself at the wheel with one arm extended and one grasping it. You must simultaneously push one arm in one direction (the one presumably grasping the wheel) and the other in exactly the opposite direction with exactly the same amount of force. In addition, you must let go of the wheel with the one hand at precisely the exact moment your other hand equalizes the force being used to start the wheel spinning.

If you can do this precisely enough, the opposing forces should balance one another enough to prevent you from spinning.

I say "should" because the degrees of force and counterforce involved need to be very precise otherwise you're going to start spinning in one direction or another.

One of my physics profs proposed a similar thought experiment involving 3 astronauts once. It was a very interesting experiment.

Astronomer said:

the thought experiment

Click to expand...

See ... I knew it would eventually have to come back to telekinesis!

The problem with any solution that requires arm-flapping or something similar to counteract the rotation you're putting on the wheel is that it has to be precisely done, and that's tricky. I mean, how many people can accurately measure the torque they're applying without any instruments? There is an easier way, though.

If you pushed the wheel such that the line of the force went through your centre of mass, but not the wheel's, then the wheel would rotate due to the eccentric force, but you would not, because there would be no moment. So then all you need to know is where your centre of mass is - which is something that anyone used to working in null-g would know, because they'd be used to feeling their body rotate around it, and if they'd ever used thruster packs of any kind then that thrust would have to be balanced around their CoG to stop them from tumbling. For a naked human, it's somewhere near the small of your back, roughly in line with an imaginary line connecting the top of your hips. A spacesuit and backpack would likely shift it a little, but hey, if you get it wrong then you can just arm-flap a little

Astronomer, as I recall the problem dealt with 3 astronauts, each weighing 200 lbs. Two of them are close enough together to push off each other. The experiment asks simply what happens to all 3 astronauts if one of the two closest together pushes the other on an intercept course with the third.

It's a relatively simple thought experiment really.

Ie,

A B. C

A pushes B towards C

All three have exactly the same mass.

Astronomer said:

Anaximander, that's the answer I was looking for, but as others have surmised, it's not the only possible answer. The downside of this method is that the wheel will float away from you (and you from it) for all eternity (or about one orbit, depending on conditions), but at least it's spinning. So that didn't take long.

Click to expand...

If you held the wheel at the hub, and then pushed... although then the resultant force on the arm doing the holding would also have to go through your CoG, which could be tricky.

Astronomer, all things being equal, and obeying the Laws of Physics, when A accelerates B he begins moving in the opposite direction. B is accelerated toward C in accordance with the kinetic energy imparted by A (actually, it's 1/2 of 1/2mv^2 as I recall since the energy imparted by A on B acts on both A and B equally, but in opposite directions). B floats toward C. On impact, in theory, all of B's kinetic energy could be imparted to C such that B comes to a stop and C goes flailing off into space OR B could impact C and impart only a smaller amount of the kinetic energy of B to C such that both are accelerated in the same frame.

There's supposed to be a way to save all 3 astronauts, but I've never seen an answer.

For what it's worth, the question was on the final for my second year in Undergrad physics and I received full value for my answer (A and C are doomed, but B gets to watch and die alone too).

Ice-skaters might be a better example (or Hockey players), but the principle is essentially the same - billiard balls in space/micro-gravity.