Understanding Centripetal Force Lab Calculations

[Stevedye56]

Hey i just had a question on a centripital force lab I did in school today. We had what was essentiall a hollow tube with a string through it, at one end was a hanging mass and on the other was a rubber stopper. The radius was kept constant by a clip marking the radius so it could not move higher or lower. The stopper was spun around with increacing masses hanging. We then had to calculate the V_T which i had no problem with. The Actual centripital force was the hanging mass converted into weight. When calculating the theoretical centripital force we used the formula
mv^2
r

Then to get percent error actual-theoretical/actual x 100.

My question is i have percent error for a few in the hundreds, is there a reason for this such as the radius kept changing, or counting the revolutions could have been wrong considering how fast it was.

P.S I am sorry if i didnt make this clear if i didnt ill clear it up to the best of my ability.


Thanks for taking the time to look,

-Steve

 

[Chi Meson]

Hmm, sounds familiar.

A few common errors in this lab:

Which mass did you multiply by g to get the "actual" Fc ?

Which mass did you use in the mv^2/r formula?

Did you square the speed?

 

[Stevedye56]

I multiplied for the first trial by .055kg and i used the same mass for the remainder of the calculations. And yes, i did remeber to square the speed, i did it separately to make sure i did not forget that step. Heres the calculations I had.

hanging mass: .055kg
time for n rev.= 5.12s
n of rev.=10

n2pir/T= 102pi(.37m)/5.12s = 4.5 m/s

actual F_c 9.8(.055)= .54N

theoretical F_c= mv^2/r=.055(4.5)^2/.37=3.0N

%difference
.54-3.0/.54= 4.6 x 100= 460% error ?

I must have done something wrong

 

[Chi Meson]

Yes, you did something wrong. The "m" in the "theoretical" Fc is not the hanging mass, but the mass of the stopper. What was it, 0.012 kg?

 

[Stevedye56]

Yes, you did something wrong. The "m" in the "theoretical" Fc is not the hanging mass, but the mass of the stopper. What was it, 0.012 kg?

Wow. Yes, it was close to that at .0102kg.

Thank you.

-Steve