Q1.
In the previous chapter, a weightlifter is shown holding a barbell steady in her hands (Fig. 6.8). Is she doing any work on the barbell while holding it steady?
Answer
No. She does zero work on the barbell, because the barbell has no displacement.
W = F × s
The upward force she applies = weight of the barbell = mg
Displacement of the barbell, s = 0
W = mg × 0 = 0 J
The upward force she applies = weight of the barbell = mg
Displacement of the barbell, s = 0
W = mg × 0 = 0 J
Why it happens: in science, work needs a displacement in the direction of the force. The barbell stays where it is, so no energy is transferred to it — its kinetic energy stays zero and its potential energy stays the same.
But she does get tired. To keep applying the force, the fibres in her muscles repeatedly contract and relax. That uses up the chemical energy stored in her body and turns much of it into heat. So the energy is spent inside her body, not on the barbell. Feeling tired is not the scientific test for work being done.