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**1. Homework Statement**

A proton orbits a 1.0-cm-diameter metal ball 1.90 mm above the surface. The orbital period is 1.50 [tex]\mu s[/tex].

What is the charge on the ball?

**2. Homework Equations**

[tex]F_{cp} = \frac{m v^2}{r}[/tex]

[tex]F_c = \frac{k q_1 q_2}{r^2}[/tex]

**3. The Attempt at a Solution**

Since the proton is orbiting the metal ball, there is obviously a centripetal force involved, but first I need to velocity.

[tex]v_{cp} = {\omega r} = \frac{2 \pi r}{T} = \frac{2 \pi (0.005)}{1.5*10^{-6}} = 2.1*10^4 m/s[/tex]

I was wondering whether this statement is true [tex] F_{cp} = F_c[/tex]