# What is the net charge of the ball?

#### elee123

1.A small 3.94 g plastic ball is suspended by a
27.9 cm long string in a uniform electric field
of 3400 N/C.
The acceleration of gravity is 9.8 m/s2 , and
the Coulomb constant is 8.99×109 N · m2/C2.
If the ball is in equilibrium when the string
makes a 43.2◦ angle with the vertical as in-
dicated, what is the net charge on the ball?
Answer in units of μC.

2. E=F/q
Fe=kq/r^2

3. Well I'm kind of lost. When they ask for the net charge, I just plugged the F, found using F=mg, and electric field into the equation E=F/q. And then I used that q and plugged it into the Fe=kq/r^2 equation, and found q. I thought that was the answer, but apparently not. I don't understand how to do the problem at ALL. please help. thanks in advance.

#### LowlyPion

Homework Helper
Welcome to PF.

Maybe make a drawing of the charged object and put all your forces acting on it in?

#### elee123

thanks.
I tried that. but after putting the forces in, I dunt know where to go from there.

#### LowlyPion

Homework Helper
Yeah ... but.

How might you explain the ball being at the angle described?

Isn't there an m*g force down and a E*q force horizontally?

Might that mean then that the vertical force is related to the horizontal force by some trig function of θ ?

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