# Homework Help: Uniform Circular Motion - Find Force

1. Jul 15, 2006

### webren

Hello,
The first part of the problem confuses me, but I have solved the second part.

"In the Bohr model of the hydrogen atom, the speed of the electron is approximately 2.20 x 10^6 m/s. Find (a) the force acting on the electron as it revolves in a circular orbit of radius 0.530 x 10^-10 m and (b) find the centripetal acceleration of the electron."

I don't understand how to find part (a).

In finding part (b), I used the formula for centripetal acceleration which is m(v^2/r). This is kind of confusing because there isn't a mass given, so it seems like the book just disregards the mass and simply divides the velocity(squared) by the radius. In other problems when a mass is given, they will throw the mass in the forumla like I have stated above. Is this the correct approach of going about these kind of problems? If a mass isn't given, leave it out, but if it is, use it in the above forumla???

So for part (b), my answer came out to be 9.13 x 10^22 N, and the book agrees.

Any help in solving part (a) and possibly explaining my question regarding part (b) would be appreciated. Thank you.

2. Jul 15, 2006

### neutrino

The centripetal accelaration is $$\frac{v^2}{r}$$. When you want to find the (centripetal) force you multiply it with the electron's mass. This is F = ma in the radial direction.

3. Jul 15, 2006

### webren

Ah, okay. That makes sense. Thanks for clearing that up, neutrino.

So in part (a), they are asking for the centripetal force, but to do that we need the electron's mass. If we don't know its mass, how is it possible to find the centripetal force?

4. Jul 15, 2006

### neutrino

Generally, the electron's mass is "a given" ~ 9.11 10e-31 Kg. This is known as the electron's rest mass.

5. Jul 15, 2006

### webren

Understood. Thanks.