You are holding a briefcase by the handle, with your arm straight down by your side. Does the force your hands exert do work on the briefcase when (a) you walk at a constant speed down a horizontal halfway and (b) you ride an escalator from the first to second floor of a building? In both cases justify your answer.

The total work done is given by:

${\mathbf{W}}_{\mathbf{tot}}\mathbf{=}{\mathbf{K}}_{\mathbf{2}}\mathbf{‐}{\mathbf{K}}_{\mathbf{1}}\mathbf{=}\mathbf{∆}\mathbf{K}$

When forces act on a particle while it undergoes a displacement, the particle’s kinetic energy changes by an amount equal to the total work done on the particle by all the forces.

(a)

The force your hands exert does not do any work on the briefcase in the absence of air resistance when you walk at a constant speed down a horizontal walkway as there is no horizontal force your hand exerts on the briefcase.

The force your hand exerts does a small amount of positive work which is equal to the negative work done by the air resistance force.

The vertical component of the force your hands exert does no work as there is no vertical displacement of the briefcase.

(b)

The upward force your hand exerts does positive work on the briefcase as there is an upward displacement of the briefcase.

The positive work done by your hand is equal to the magnitude of the negative work done by gravity provided the initial and final speed of the briefcase remains the same.