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.