A15.0 kg block is pulled by a horizontal force of 30.0 n along a rough horizontal surface at a constant velocity. what is the coefficient of friction?

If the block is moving at a constant velocity, it means there is no net force, which means the frictional force is same in magnitude as the pulling force!

F_pull = Friction

Now the frictional force (F = cR) is related to the coefficient of friction & normal force, where the mass of block is important (R = mg). Putting all these small equations together:

F_pull = cR
F_pull = cmg

Let's plug in the values we know

(30) = c (15)(9.8)

... and use the calculator to find that coefficient, c!

The answer would be a