Humans can walk on land in the absence of automobiles, they can swim in the water without boats, but without planes and rockets, we cannot fly. Leaving the ground is in defiance with the gravitational pull of the earth and therefore requires a certain work needs to be done by a body to propel it. This propulsive force is known as ‘thrust’. If we consider a rocket for example, when it lifts off, chemical reactions take place in its combustion chamber producing large amounts of trapped energy which explodes out at high speeds through a nozzle and exerts force on the atmosphere. The atmosphere reacts in accordance with newton’s third law to push the rocket upwards.