# Homework Help: Centripetal Force and free body diagram

1. Oct 17, 2009

### KL90

1. The problem statement, all variables and given/known data
a. Draw a free body diagram for mass Mb while in motion (fig. 2a). Identify the centripetal force. assume that the mass hangs vertically

b. Calculate the magnitude of the force exerted by the spring on mass Mb = 402g, moving in a circle of radius r=18cm (fig. 2a). The mass makes 20 revolutions in 12 seconds. Determine the mass m, suspended over the pulley (fig 2b), which stretch the spring by the same amount as during the rotation.

2. Relevant equations

3. The attempt at a solution
a. Mb would have 3 forces: F spring (Fsp), F centripetal force (Fc) and F gravity (Fg).
Fsp is point outwards to the right, Fc pointing inwards to the left, and Fg is pointing downwards.
I was wondering if it would have tension from the string supporting Mb?

b. Fsp=Fc = mv2/r = m(2$$\pi$$f)2r
here, v = 2$$\pi$$r/T = circumference/Period and frequency = 1/T

1) Fsp = 0.402kg x (2$$\pi$$ 20rev/12sec)2 x 0.18m'
= 7.967 Nt
I'm not entirely sure if what i did is correct.

2) I'm not entirely sure how to calculate m after this point.

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2. Oct 17, 2009

### turin

I agree with the number of applied forces, but you are double-counting one of them, and you are neglecting another one.

Actually, if you want to count pseudo-forces (other than weight), then you can list four, and you have listed all but one of them.

I think that you should reread the attachment.

Good thinking. Ask yourself: Is the mass in (instantaneous) translational equilibrium (in the rotating frame)? Should there be a force that counteracts the weight, and, if so, what supplies this force?