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lostcompass
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Homework Statement
A plastic sphere with a positive charge of 4.8 x 10^-19 C is held stationary in a gravitational field of strength 9.8 m/s^2 by an electric field of strength 1.2 x 10^5 N/C. What is the force of gravity on the sphere?
Homework Equations
F_e=qE=kQq/r^2
F_g=mg=GMm/r^2
g=GM/r^2
The Attempt at a Solution
I think the only way to find the force of gravity is to find the mass of the sphere first. Since r^2=kQ/E, g=GM/(kQ/E)=EGM/kQ. So M=gkQ/EG
M=(9.8 m/s^2)(9.0 x 10^9 N x m^2/C^2)(4.8 x 10^-19 C)/(1.2 x 10^5 N/C)(6.67 x 10^-11 N x m^2/kg^2)=5.3 x 10^-3 m
If this mass is the mass of the sphere and not some other random object, then the number can be substituted into F_g=mg
F_g=(5.3 x 10^-3 m)(9.8 m/s^2)=5.2 x 10^-2 N
If M isn't the mass of the sphere, then I'm completely wrong, but that's the only solution I've been able to come up with. Can anyone tell me if I'm right?