Homework Help: Pith ball Charges

1. Jun 26, 2008

chukie

Two small pith balls, each of mass m = 12 g, are suspended by 1.2 m fine strings and are not moving. If the angle that each string makes with the vertical is q = 42.6 , and the charges on the two balls are equal, what is that charge (in microC)?

http://img386.imageshack.us/img386/5977/picmd8.th.jpg [Broken]

I equated the forces in x and y to zero:

Tcos42.6-117.6=0
T=159.76

Tsin42.6+Fe=0
plugging in 159.76
Fe=108

then i used the equation Fe=kQ^2/r^2

i solved for r by doing this:
r=2(1.2sin42.6)
r=1.6 m

I'm not sure if my steps are right. Can someone help me? Thanks in advance!

Last edited by a moderator: May 3, 2017
2. Jun 26, 2008

rootX

What's tan theta equal to?

Yep, it seems right what you have right now.

tan 42.6 = Electric force / gravitational force

Select to get more hint

3. Jun 26, 2008

chukie

let's a better way to find Fe, which is 108 N. Then plugging it into the equation then solving for Q, the answer I came up with is 178 microC, but the answer is wrong =(
I don't know where I've made a mistake.

4. Jun 26, 2008

alphysicist

Hi chukie,

The weight of 117.6 N you find here does not look correct to me. (You don't have units here, but your last post indicates that you meant it to be Newtons.) Do you see what went wrong?

Last edited by a moderator: May 3, 2017
5. Jun 26, 2008

chukie

ahhh!! i see now! i forgot to convert grams into kg! thanks so much!