# Finding mass using charges and accerleration

1. Sep 19, 2015

### Ryan060

1. The problem statement, all variables and given/known data
A rod carrying a charge of 8.0uc held above a balloon carrying a charge of -.70 uc. at a height of 30cm. find the mass of the balloon

2. Relevant equations
fe= (q1*q2)/r^2

F=ma

3. The attempt at a solution
I do not want this problem solved for me I am very trapped at this moment. finding the electric force is easy but I cannot see how to find the mass because the fg is depended on the mass to calculate so if I do a force chart and set the sum of the forces in the y to Fe-Fg=ma with fg=-(mg) makes me stuck i need some assistance in how to actually work this problem like what correlation am I not seeing because the answer alone does not help me.
Thank you

2. Sep 19, 2015

### Staff: Mentor

It is not clearly stated, but I think the balloon is in equilibrium here, so you know its acceleration.

3. Sep 19, 2015

### Ryan060

oh I forgot to add that the question lists the acceleration which was 1.5m/s/s toward the rod. what it wants me to find is the mass of the balloon using the charges listed and the distance between the two objects and somewhere in there using the acceleration I am just completely stumped on how to find the Fg in order to calculate the mass.

4. Sep 19, 2015

### Staff: Mentor

Oh, okay.

Fine, you can still use Fe-Fg=ma. Plug in Fg and Fe you found (you don't have a numerical value for Fg yet, but that does not matter), you also know a, so the mass is the last unknown variable in the equation.