Question

A 9kg box is sliding across a horizontal floor on an elevator. determine the kinetic frictional force that acts on the box when the elevator is

A) Stationary
B) moving upward at a=1.4 m/s^2
C) moving downward at a=1.4 m/s^2
Let: uk=0.35

Answer 1

option C is right. ..

Answer 2

frictional force is directly proportional to the Normal force N.
in first case the elevator is stationary so acceleration on box is g in downward direction.

so the frictional force is (UK)mg=0.4(9)(10)=36N.
in second case elevator is moving upward with acceleration 1.4m/s^2.
so resultant acceleration on box is g+1.4=11.4m/s^2.because when elevator is going up,the pseudo force acts downward on box.
so friction=(0.4)(9)(g+1.4)=3.6(11.4)=41.04N.
in third case resultant acceleration is g-1.4=8.6m/s^2.
friction=0.4(9)(8.6)=30.96N