# Rotating disk/ sphere and moving mass along it

1. May 11, 2013

### Edi

Coriolis effect - In a non-friction system, f I roll something along the surface of the planet from on of the poles to the equator, it will appear to move to the west, it will essentially stay behind the planets rotation and actually rotate it in the opposite direction. Now, if we add friction to the system, the planet will accelerate the ball. The question is - will the planet system as a whole slow down? As I understand it should work the same with a spinning ballerina, who rotates with arms pulled to chest, expands them straight out and slows herself rotating.

And the opposite - if we roll the ball from equator to pole in non friction system, it will "go ahead" the planet, as it has more energy/ speed than lower points of the planet. Then, if we ad friction, the planet will slow down the ball and and the whole planet system will accelerate again? (again the ballerina effect)

(more questions will come after this, if the answer is, in shorts, - yes)

2. May 11, 2013

### Bandersnatch

The angular momentum has to be conserved, so having the planet accelerate the ball will affect the planet's rotation accordingly.

Note that you won't be able to get a rolling ball back to the poles, in a frictionless system. At best you can make it cross the poles while travelling in a circle along the meridian lines.
The only way to do it is to stop the ball first.

3. May 11, 2013

### Edi

Then the answer is "yes", yes?

Well, yes, the ball wont get back to the pole, but it will go .. up/ down/ away from the equator, right? And the "And the opposite - " part of my first post will be true, right?

So, yes, the angular momentum has to be conserved - just as I thought.
BUT, in that case - if the friction that accelerates the ball is a, say, piezoelectric crystal attached to the surface of the planet in with the ball smacks in to and when the ball strikes it, it not only accelerates (deceleration is just acceleration in the opposite direction anyway) the planet, but also generates electricity from the impact? What then? The electric energy eventually turns in heat and some of that will even radiate away from the planet, in space.
So.. the planet slows down .. because energy is somehow pumped out of its rotation or.. what?

4. May 11, 2013

### Bandersnatch

Yessir. It's a "yes".

Both regular friction, and having some more esoteric setup like the one you've described, convert the kinetic energy of the planet's rotation into other kinds of energy.

In the idealised situation, where both bodies are completely rigid and the collisions are perfectly elastic, then all of the momentum of the planet would be transferred into the ball.

But since there are no such ideal setups in reality, there are always additional losses, mostly in the form of heat. So the total kinetic energy of the system(and angular momentum) after the ball is accelerated(whichever way) will be less than before due to the heating of the ball/surface due to friction, or/and other energy siphons you might put in there.

In other words, yes, the planet can be slowed down this way.

5. May 12, 2013

### Edi

So.. does that mean that sufficiently industrial species (us, humans?) can use a mechanism such as this to pump out energy from the planets rotation and use it to generate more usable forms of energy for our use (heat to electricity..) ?

6. May 12, 2013

### Staff: Mentor

Rotation would be very hard to do, but you can google for "tidal power generation" to see how we routinely capture some of the moon's kinetic energy as it orbits the earth.

7. May 12, 2013