# Circular motion

#### electricheart

1. The problem statement, all variables and given/known data

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.

2. Relevant 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

Homework Helper
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.

### The Physics Forums Way

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving