How does a rocket fly through empty space?

In space there is nothing for the exhausting gas to push against. Therefore rockets do not move because the exhausted gas pushes against the ground or air surrounding the craft. Instead, a rocket moves because of the unbalanced forces in action-reaction pairs.

Discover For Yourself!

It is easier to explain unblanced forces and action-reaction pairs by doing an experiment.

1. Inflate a balloon and tie a knot in the neck of the ballon so the air cannot escape. Hold the neck of the balloon, then release the balloon. What Happens?

Are you saying DUH!! It falls to the floor? If so, you are correct. It will fall because the only unbalanced force acting on the balloon is gravity. If something accelerates, it has an unbalanced force. Gravity causes things to accelerate toward Earth’s surface. This means that as something falls, its speed keeps increasing. For example, if one object is released at a height of 1 foot (0.3m) above the ground and a second object is released at a height of 3 feet (0.9m), the object at the greater height will have a greater speed when it hits the ground.

2. Inflate another balloon, but hold the neck closed with your fingers. Release the balloon.

What happens?

As shown in the diagram, your balloon will fly around until it deflates, then it falls. When an inflated balloon with an open end is released, the balloon flutters about as it flies due to the unbalanced forces making up action-reaction pairs of forces. Yes, gravity is pulling down on the balloon, but the force of the gas inside the balloon (A) is great enough to move the balloon in different directions until the balloon deflates. The balloon deflates because the balloon is pushing (B) on the gas inside the balloon.

Since there is an opening, the air is pushed out. The action-reaction pair of forces are force A and force B. Note that these are called unbalanced forces because they are not pushing on the same thing. Instead, they are pushing on different things as well as pushing in opposite directions. Force A is on the balloon due to the gas inside the balloon pushing on the wall of the balloon. Force B is the gas due to the balloon pushing on the gas insdie the balloon. Because of these unbalanced forces, force A pushes the balloon forward, and force B pushes the gas out of the opening.

Sir Issac Newton (1642-1727) is credited with explaining motion. He identified action-reaction forces. For more information and instructions for a balloon rocket, see BLAST OFF!

Photo by Crysti