Question

Assuming energy wasn’t destroyed or created in this activity, why do you think the efficiency
isn’t 100%?

Answer 1

The efficiency of the activity is determined by the following expression: efficiency = useful work/input energy.

Even if the energy wasn’t destroyed or created it cannot be spent on the useful work completely, according to the second law of thermodynamics. There always some losses on the entropy, thus, the efficiency isn’t 100%