Circular Motion: Find Tension at Top of 1.11kg Disc

[electricheart]

Homework Statement

A 1.11 kg disc is whirled in a vertical circle with radius 0.680m about a fixed point. Find the tension at the top if the speed at the top is 8.76 m/s.

Homework Equations

F= ma
F= m* Acp
Acp= m(w^2*r)

3. The Attempt at a Solution
I figured centripetal force was acting on the mass so I used the equation w=v/r to find omega(w), then I found the Acp. But that answer is incorrect. If at the top of a circle will the tension be the same Acp.

 

[rock.freak667]

At the top of the circle, the forces acting are the normal reaction of the 1.11kg mass and the tension. Both acting downwards. The resultant of those two is the centripetal force mv²/r

 

[electricheart]

So F + M g = Acp.
Alright, thanks.

 

[Dr.D]

Centripetal force is not really a force at all, it is a mass*acceleration term.

You will get a much better understanding of this problem (as opposed to simply getting an answer quickly) if you draw a free body diagram and write Sum F = m*a where you recognize that for a body in uniform circular motion, a = - r * omega^2 * er, a vector pointing towards the center of the circle.