Question

Anna says that if energy of 500J is done on a car of weight 6kg then the car will gain work of 480J in an ideal system. What would you say to her to correct her?

Answer 1

where did the other 20J go?

Answer 2

This means that energy input is not totally emerged in the system, meaning, the car has a given work efficiency, which is given by:
\[\eta=\frac{ Work output }{ Work input }*100\],
Then the efficiency of the car was \[\eta=\frac{ 480 }{ 500 }*100=96% \]
Efficiency=96%