# Centripetal Force

1. Oct 17, 2008

### Quincy

Suppose if you're spinning a thread by your finger that has a cup attached to it at the other end. You would need to keep wobbling your finger or the cup would stop spinning, right? Then why doesn't the Sun wobble to keep the planets spinning around it? Or does the cup stop spinning because of the downward gravitation force?

2. Oct 17, 2008

### Abdou

I still haven't finished high school so i might be wrong, so here goes :) ......

when you wobble your fingers, i think you might be applying the tension which acts as the centripetal. The tension would be equivalent to the gravitational pull of the sun towards the earth. In addition to that, the earth is moving at a constant speed (though i think its slowing down?), While the cup face frictional forces like air resistance, so tangentially its decreasing some of the force you apply by your fingers, most proabably also causes some sort of tangential acceleration too.

Please if I'm wrong correct me, I will be having a quiz on this topic, wich me luck :D

thanks,

3. Oct 17, 2008

### Quincy

lol same here.

I forgot about air resistance; so I guess if you spin the cup in outer space, it will keep on spinning without having to wobble your finger.

4. Oct 18, 2008

### Naty1

The sun keeps the planets in place via gravitational attraction...the curvature of spacetime.

Your finger has no gravitational attraction to a cup...at least it's negligible...you move the cup via the string via directly applied force...you apply centrifugal force to the cup; centripetal force holds in for accelerating from you...not gravity...