Circular Motion: Find Speed at Point A

[Bryan Tran]

Homework Statement

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...

Homework Equations

F=ma
F=m(v²/r)

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...!

 

[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

 

[Bryan Tran]

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

 

[siddharth23]

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

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

 

[Chestermiller]

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

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?