Posted by RSVP on .
K, so we are doing this lab in physics where a ball on a string is put through a glass tube and then different washers (masses) are added to the string. A radius is measured out, and then the string is spun in a horizontal direction...so with different radii and different masses, at different points of the lab. Anyways, I have all of my data, which is mass, rotation, and time..then this question asks us to calcualte the gravitational force, and it says that the centripetal fore will be the same as the gravitationla force...this leads to a problem...we are also asked to calculate frequeny and frequency squared, but this after the first calculation, but I did it first...then used the f^2 to find centripetal force..and I then found graviatationl force using f=mg....the values don't match....so why are the two calculations the same, and how do u calculate the gravitational force with the information I have (before the frequency) and why is it the same as the centripetal force?
Thanks, sorry this is long! I like to understand what I am doing!

Lab for physics 
bobpursley,
The lab involved two masses: one on the end of the rotating end of string (Me) and one (Mg)on the end hanging through the glass. What holds it in equilibrium?
Well centripetal force is the force on the rotating part of the string, and what is supplying it is gravity, so
me v^2/r = mg*g
NOTE: the masses are not usually the same.
now a note on procedure: You measured period T and radius r.
v= 2Pi*r/T or 2PI*r*frequency
v^2= (2PI)^2*(r/T)%2 or(2pi)^2 *(f*r)2
me v^2/r= me (2pi*f)^2 * r
you might check that, and then set it equal to mg*g and the results normally are pretty good for this lab (that is averaging several trials). 
Lab for physics 
RSVP,
I completely faill to understand where you are getting these equations from, and how & why you are using them. Firstly, the mass at the end of the string that hold the washers on it, it was never measured, none of the groups measured it, I didn't even think to measure it until last night, and it never said to measure it....also why are you finding v and then v^2? Please explain...and also why mgg? where does that come from? We never learned that before....Thanks, and sorry Im not understanding...Im trying!

Lab for physics 
bobpursley,
None of the groups measured it? Ask your teacher about it...I sense there is a great lack of prelab understanding of this classical lab.
The idea is to let centripetal force be balanced by a weight, thence to use that concept.
http://www.batesville.k12.in.us/physics/PhyNet/Mechanics/Circular%20Motion/labs/cf_and_speed.htm