Physics
posted by George .
In moving a 35.0 kg desk from one side of a classroom to the other, a professor finds that a horizontal force of 300 N is necessary to set the desk in motion, and a force of 185 N is necessary to keep it in motion at a constant speed.
(a) What is the coefficient of static friction between the desk and the floor?
(b) What is the coefficient of kinetic friction between the desk and the floor?
I know that F=ma, but how do you do this?

You don't use F = ma. The desk is not accelerating. All that equation tells you is that the pusing force is equal to the opposiing friction force. The net force on the desk is zero.
The static coefficient of friction is 300N/(M g)
The kinetic coefficient of friction is
185/(M g)
Note that the denominator is the weight, not the mass.
Plug in for mass M and g (the acceleration of gravity) and crank away. 
Thanks!