Kinetic friction on a moving object

[KingNothing]

Hey...this problem arose:

A force of 40N is required to start a 5 kg box moving across a horizontal concrete floor.
A) What is the coefficient of static friction between the box and the floor? (its .82, I got this no problem)

B) If the 40N force continues, the box accelerates at 0.7 m/s^2. What is the coefficient of kinetic friction?

I don't see how to do part B. I know it's .74, but I don't know how. Please explain in detail by showing equation(s).

[jamesrc]

Use Newton's second law. You know the net force is equal to the mass times the acceleration. Since the net force is equal to the applied force less the friction, you can calculate the coefficient of friction.

F_{net} = ma = 40 - f
f = \mu_k mg