# Calculate the magnitude of the force exerted by the spring

1. Oct 17, 2004

### MusicMonkey

Calculate the magnitude of the force exerted by the spring on mass Mb=300g, moving in a circle of radius r=22cm. The mass makes 10 revolutions in 4 seconds. Determine the mass m, suspended over the pulley which stretch the spring by the same amount as during the rotation.

For the magnitude of the force I used the equation V=2(pi)f*r. Then I plugged in f=4/10 and r=22 cm. Then I used F=mv^2/R and calculated for Force. Is this correct?

For the second part of the question I do not understand how to do it. :grumpy:

2. Oct 17, 2004

### BLaH!

Yep, it sounds like you did the first part correctly. For the second part you will need to remember the relation between the spring force and the extension of the spring. That is,

$$F_{spring} = k\Delta x$$

From the first part of the problem

$$k\Delta x = M_b\frac{v^2}{R}$$

(Of course you don't even need to know what $$k$$ or $$\Delta x$$ are to do the first part.)

In the second part you have a mass $$m$$ that is being suspended by the spring. Thus,

$$F_{spring} = k\Delta x' = mg$$

The problem states that $$\Delta x' = \Delta x$$. Thus,

$$k\Delta x' = k\Delta x = M_b\frac{v^2}{R} = mg$$

Now just solve for $$m$$.