By Newton's second law,
• the net vertical force on the box is
∑ F = n - mg = 0
where n = magnitude of the normal force, m = mass of the box, and g = acceleration due to gravity; then
n = mg = (20 kg) (9.8 m/s²) = 196 N
• the net horizontal force on the box is
∑ F = -f = -ma
where f = mag. of kinetic friction and a = acceleration of the box. Since
f = µn
where m is the coefficient of kinetic friction, so that
f = 0.5 (196 N) = 98 N
Then the box's acceleration is
-98 N = - (20 kg) a → a = (98 N)/(20 kg) = 4.9 m/s²