Rockets: Action, Reaction and Mass (With a Chance of Thrust)

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Newton's Rocket

Whether you're building it in your backyard or in a multimillion-dollar aerospace lab, a rocket relies on a basic physics principle to move. Just like the recoil of a gun and the thrashing of an untended fire hose, a rocket moves according to Newton's third law of motion. That's the idea that every action has an equal and opposite reaction.

Rocketing Around the Rink

It can be tricky to think of motion in terms of actions and reactions, but you can get a basic idea of how it works by imagining yourself on roller skates (or ice skates if you prefer). If you wanted to move backward, you could just push off from the railing. But a rocket meant to move through a vacuum doesn't have the luxury of matter to push off from.

Another way to move backward on the rink would be to throw your best fastball. As long as you kept your balance while throwing, you'd move backward a little, in the opposite direction of the ball. That's what a rocket does, but it uses exhaust in place of a baseball. This is also what happens when a bullet leaving a rifle causes it to kick against a marksman's arm, and why the end of a fire hose will flail if not held down.

A Fiery Mass of ... Mass

Burning fuel is one of the most common ways to power a rocket. As the fuel burns, the gas it produces has nowhere to go but out. It leaves the rocket at high speed -- that's the action. The rocket moves in the opposite direction -- that's the reaction. You can also steer the rocket using this principle, either by directing the departing gas through a nozzle or by firing separate thrusters with their own fuel sources.

Variably Massive With a Chance of Thrust

Newton's third law describes why the rocket moves. Newton's second law, on the other hand, describes how fast the rocket moves. This law boils down to a simple equation -- force equals mass times acceleration. If the gas has more mass, it exerts more force. The same is true if the gas is moving faster. Either way, the rocket moves faster, too.

The tricky part of this scenario is that the rocket's mass is always changing. After all, the gas leaves the rocket because it's the product of burning fuel. As that fuel burns up, the rocket gets less and less massive. To guarantee that a rocket reaches its destination, this perpetually dropping mass has to be part of the flight plan.