# Magnitude of Tension

1. Oct 10, 2004

### RumDragon

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Hey everyone, I just need help with a problem that is giving me trouble. I have scoured my physics book and can't find the solution or even a starting point.
First we have a drawing of a circle with mg and ft1 with two arrows pointing down at the top of the circle and at the bottom of the circle it has mg and ft2 with the ft2 arrow pointing up and the mg arrow pointing down. The question is: If its speed is 4.02 m/s and its mass is 0.291 kg, calculate the magnitude of the tension in the string when the ball is at the top of its path. If anyone could help me by just telling me how to figure out the magnitude of tension I'd be forever greatful, thanks.

2. Oct 10, 2004

### arildno

Welcome to PF!
The sum of forces must provide the centripetal acceleration of the ball..
Don't double post, BTW.

3. Oct 10, 2004

### RumDragon

Sorry about the repeat. Hmm, I'm still having trouble wrapping my brain around your answer (I'm a rather science-stupid English major) so is there a general formula or something for determining magnitude of tension or what? Thanks.

4. Oct 10, 2004

### arildno

1) What is the expression for centripetal acceleration?
2) Formulate Newton's 2.law of motion at the moment when the ball is on top.
Which forces act on it, and what are their directions?

5. Oct 10, 2004

### RumDragon

Ok I know that centripetal acceleration can be written as a=v^2/r (i believe) and newton's 2nd law states that torque= moment of intertia * angular acceleration. That being said it also states the radius is 95.0 cm. So the centripetal acceleration would be around .17 So the forces acting on the ball attached to the string at the top would be gravity and Ft1, both downward. So would the force be the centripetal acceleration times the mass?

6. Oct 10, 2004

### RumDragon

This is due tonight and I can't finish it.

7. Oct 10, 2004

### arildno

Whatever has torques to do with Newton's 2.law?
(Torques appear in the moment-of momentum equations, which can be deduced from Newton's 2.law)
You are right about the expression for the centripetal accelerations.
Now write down which forces act upon the ball when it is at the top; the centripetal acceleration are then downwards, WHAT ARE THE DOWNWARDS FORCES ACTING ON THE BALL THEN??????????????????????????????

8. Oct 10, 2004

### RumDragon

The downward forces would be gravity and the centripetal acceleration would they not?