# Homework Help: Frictionless ball in a rotating reference frame.

1. Apr 4, 2013

### sbhit2001

1. The problem statement, all variables and given/known data
Imagine that a circular disc is rotating with a frictionless ball on it( ball is not at center of the disc) If we observe the motion of the ball from the rotating frame of reference, then how can we describe its motion?

2. Relevant equations

3. The attempt at a solution

As the motion of the ball is being observed from the rotating frame of reference, a centrifugal force must act on it but i know that it will appear as if the ball is moving in a circular path with its center at the center of the disc. If the net force on the ball is towards outside(centrifugal force), then how can it rotate because it has no inward acting fictitious centripetal force.. Please help...

2. Apr 4, 2013

### TSny

Besides the centrifugal force, there is another "fictitious" force acting on the ball.

3. Apr 4, 2013

### tiny-tim

welcome to pf!

hi sbhit2001! welcome to pf!
yes, it's frictionless, so it should stay where it is in the earth frame, which means that it rotates "the wrong way" in the rotating frame

this is the same as what happens to houses etc if you're standing on a rotating turntable …

as you say, there's a centrifugal force outwards, but there's also a Coriolis force inwards, which in this case is … ?