# Homework Help: Centripetal acceleration

1. Oct 5, 2008

### HelloMotto

You are whirling a rubber stopper of mass something attached to a string in a vertical circle at high constant speed. At the bottom of the circle, the net force that causes acceleration is

my answer was Vertically upward and greater in magnitude than MG.
but the answer in the book says
vertically upward and lower in magnitude than mg.

WHY? I dont get this.
At the bottom of the circle, The Centripetal force is the difference of Force tension pointing upwards and force of gravity pointing downwards. So if the netforce is smaller than MG, then there can't be a centripetal force....

2. Oct 5, 2008

### tornzaer

If I'm understanding you correctly, you are talking about the tension of bottom vs. top?

The top has less magnitude because the tension [up] is reduced by the Fg [down]. At the bottom, the magnitude is greater because it's tension [down] PLUS Fg [down].

3. Oct 5, 2008

### Staff: Mentor

You are thinking about the tension while they are asking about net force. As long as the tension is greater than MG, there will be some net centripetal force.