Chapter 7, Problem 40A

Some people used to think that a rocket could not travel to the moon because it would have no air to push against once it left Earth’s atmosphere. We now know that idea was mistaken. What force propels a rocket when it is in a vacuum?

To determine

The force that propels a rocket in a vacuum.

Explanation of Solution

Introduction:

As per Newton’s first law of motion, an object maintains its state of motion until an unbalanced external force acts on it.

As per Newton’s third law of motion, there exists an equal but opposite reaction force for an action force.

When fuel is burnt, the gases exhaust out from the nozzle of the rocket with some force. This causes the generation of opposite yet an equal amount of force which propels the rocket forward. No air or ground is required to be pushed against for a rocket to move. A rocket can continue to move in a certain direction in a vacuum even without burning the fuel. This can be understood by the law of inertia. When it is required to change the direction or change the speed, fuel can be burnt which would propel the rocket.

Conclusion:

Thus, as per Newton’s third law of motion, there exists a pair of forces that are opposite yet equal in nature. When the fuel is burnt, the gas which exhausts out in the backward direction causes the rocket to propel in a forward direction.