Name at least three commonly used units of energy.

How is work done by a force measured? $A$ porter lifts a luggage of $20 \, kg$ from the ground and puts it on his head $1.7 \, m$ above the ground. Find the work done by the porter on the luggage. $(g = 10 \, m s^{-2})$

When the speed of a moving object is doubled,its:

State the law of conservation of energy.

The potential energy of a person is minimum when:

$A$ boy pushes the wall in the class for five minutes but the wall does not move. Another boy pushes the chair from the front to the back side of the class. In which case,more work is done?