(a) The block has a weight of (27.0 kg) <em>g</em>, and the normal force of the surface pushing upward on the block has the same magnitude, so that static friction exerts a maximum force of
<em>µ</em> (27.0 kg) <em>g</em> = 70.0 N
where <em>µ</em> is the coefficient of <u>static</u> friction. Solving for <em>µ</em> gives
<em>µ</em> = (70.0 N) / ((27.0 kg) <em>g</em>) ≈ 0.265
(b) As it's moving, the block still has the same weight and thus feels the same normal force, (27.0 kg) <em>g</em>. In order to move at a constant speed, kinetic friction must exert the same force as the push, so
<em>µ</em> (27.0 kg) <em>g</em> = 64.0 N
where <em>µ</em> is now the coefficient of <u>kinetic</u> friction. Solve for <em>µ</em> :
<em>µ</em> = (64.0 N) / ((27.0 kg) <em>g</em>) ≈ 0.242