# Homework Help: Ok, how should I go about this one?

1. Jan 18, 2006

### matt85

Pendulum

A small 12.00g plastic ball is suspended by a string in a uniform, horizontal electric field with a magnitude of 10^3 N/C. If the ball is in equilibrium when the string makes a 30 ° angle with the vertical, what is the net charge on the ball?

Q = _________ C

I know Coloumb's Law, and I assume I want to implement it here somehow, however I don't know where to begin on setting this one up.

2. Jan 18, 2006

### matt85

Oh, and here is the visual.

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3. Jan 18, 2006

### Staff: Mentor

The ball is in equilibrium. What does that tell you?

4. Jan 18, 2006

### matt85

That there is no force acting on it?

5. Jan 18, 2006

### andrewchang

No, that isn't exactly the definition of equilibrium.

Equilibrium is when there is no _net_ force acting on it.

6. Jan 18, 2006

### Staff: Mentor

That there's no net force acting on it. Or, in other words: The net force is zero.

Start by identifying all the forces acting on the ball, then set up the equations for equilibrium. (Analyze the x and y components separately.)

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