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Have you ever done this? You blow up a balloon, but instead of tying a knot to hold in the air, you let go—and air rushes out. The balloon swerves, circles, and zigzags but keeps flying as long as the air keeps rushing out. When the air stops flowing, the balloon drops to the ground.
The balloon demonstrates something first described by British scientist Sir Isaac Newton (1643–1727). The idea is that for every action there is an equal and opposite reaction. Here’s how that works with the balloon. The action is the air rushing out, moving with a certain amount of force in a certain direction. What is the equal and opposite reaction? That’s the balloon flying around, with the same amount of force but in the opposite direction.
Scientists use this same idea to launch a rocket, and here’s how it works.
