physics
posted by nyasha .
A 3.50kg box is sliding across the horizontal floor of an elevator. The coefficient of kinetic friction between the box and the floor is 0.330. Determine the kinetic frictional force that acts on the box when the elevator is
(a) stationary,
(b) accelerating upward with an acceleration whose magnitude is 2.10 m/s2, and
(c) accelerating downward with an acceleration whose magnitude is 2.10 m/s2.

a) fnormal= mg
b) fnormal= mg+ ma
c) fnormal= mgma 
(a) mg is the normal force between the floor and the box, and μmg is the frictional force between them.
(b) m(g+2.1) is the normal force between the box and the floor. So the frictional force is ...
(c) m(g2.1) is the normal force. The frictional force is ....