# Homework Help: Relationship between centripetal force and radius

1. Oct 23, 2006

### kiss89

hi i'm a new member in this forum and i'm taking physics(University level) grade 12.......in one of our labs about the relationship between centripetal force and radius, we must use a constant centripetal force( we used 2N) and find the frequency in RPM for different radii, and this is what we got:
Fc:2N ( Constant)
0.8 88
0.6 113
0.4 138
0.2 192
then he asked to plot a graph of Fc versus frequency squared.
(Note:also it says after that to draw a line from each point to the origin because Fc for frequency 0Hz is 0)
my question is do we just square the frequency we got in experiment, or is there some kind of a trick he wants us to understand?????
If u could explain this to me please, i'd be very grateful.
THANK YOU.

2. Oct 23, 2006

### OlderDan

Are you certain you are supposed to be plotting Fc versus frequency squared? Since you held Fc constant and varied r and frequency, it would seem more reasonable to be plotting r vs frequency (or maybe 1/r or 1/r^2 since r is decreasing as the frequency increases). Maybe you were asked to try some different graphs, drawing the best straight line you can from the orign through the data points?

3. Oct 23, 2006

### kiss89

well in this lab we had to spin a string with mass ( which is constant 20 g) attached to its end above us and this is what it says in the lab:

In this part of the experiment, we will calculate the centripetal force and the radius of the circle with frequency as the constant.

1) Adjust the string to provide a radius of rotation of 0.8m
2) Swing the rubber stopper in a horizontal circle keeping the scale reading constant at 2N or 200g
3) Record the number of rotations and time in a data table. Calculate the frequency and frequency squared and record these numbers in your data table.
4) Repeat steps 1 to 3 for radii of 0.8m, 0.6m, 0.4 m, 0.2 m

4. Oct 23, 2006

### kiss89

can anybody help me with this plzz

5. Oct 23, 2006

### kiss89

does anyone have any idea please, my due date is getting closer

6. Oct 23, 2006

### OlderDan

All of that makes sense. The next logical step is to try to find a relationship between the angular frequency or angular frequency squared (there is probably a reason they asked you to calculate that) and the radii.

If you do the graph you said you were asked to draw, all the points will have the same value for Fc. The only reason for drawing a line from each point to the origin is that you will see a set of lines with different slopes. The slopes of these lines are probably what your teacher wants you to compute. (You don't need a graph for this, but let's not worry about that.) If you compute Fc/frequency^2 for each point, do you see any connection between those values and the radii? If you don't see a connection, I suggest you make a graph of those computed values vs the radii. What kind of curve can you draw that goes through or near all the points on that graph?

7. Oct 23, 2006

### kiss89

i dont know if i'm right , but the computed values i got from Fc/Freq.^2 are the same as the radii , and i got a line where it almost passes through 3 points out of 4 , and the last point is far away from the line , as in the picture attached

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8. Oct 23, 2006

### kiss89

i dont know if i'm right , but the computed values i got from Fc/Freq.^2 are the same as the radii , and i got a line where it almost passes through 3 points out of 4 , and the last point is far away from the line , as in the picture attached

9. Oct 24, 2006

### OlderDan

I won't be able to see your graph for a while, but theoretically it should be a straight line through the origin, showing that the centripetal force divided by angular frequency squared is directly proportional to the radius of the circle. If you use the frequency and the circumference of the circle to figure out how fast the object was moving, you could rearrange terms to show that the centripetal force is the mass of the object times its speed squared divided by the radius.

10. Oct 24, 2006

### kiss89

ok thanks for your help i think i understood what he wants