# Circular Motion

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1. Jan 20, 2015

### Bryan Tran

1. The problem statement, all variables and given/known data
A .8 kg lead ball is whirled on the end of a string 5 meters long. When the ball passes through point A at the top of the path, the tension in the string is 4.96 N.
What is the speed of the bob at point A?
*NOTE - POINT A is at the top of the circle...

2. Relevant equations
F=ma
F=m(v2/r)

3. The attempt at a solution
F=m(v2/r)
v = √(F*r)/m

V = √(4.96*5)/.8
V = 5.57 m/s

DOESN'T WORK...!

2. Jan 20, 2015

### rock.freak667

At the top of the circle, what are the forces acting and in what directions do they act (up or down) ?

The resultant of these forces is equal to the centripetal force mv2/r

3. Jan 20, 2015

### Bryan Tran

Umm force of weight and that's all... and it acts down.
But what does that do?

4. Jan 21, 2015

### siddharth23

Not just weight. Another force acts on it too. You've mentioned it in the problem itself.

5. Jan 21, 2015

### Staff: Mentor

Have you drawn a free body diagram showing the forces acting on the ball, or do you feel you have advanced beyond the point where you need to use free body diagrams?