Mary applies a force of 70 N to push a box with an acceleration of 0.49 m/s2. When she increases the pushing force to 80 N, the box's acceleration changes to 0.72 m/s2. There is a constant friction force present between the floor and the box.
(a) What is the mass of the box?
kg
(b) What is the coefficient of kinetic friction between the floor and the box?