Asked by Terri Davies
Rockets work a lot like letting go of a balloon. Imagine you have just inflated a balloon, and are pinching the end so that it is closed. The stretched rubber is exerting a force on the air molecules inside, forcing them to be at a higher pressure than the air outside. When you let go, these air molecules are forced out, allowing the balloon to get to the same pressure outside and propelling it.
In the same way, a rocket engine uses fuel to cause combustion, creating a product of hot gases that get forced out of the back of the rocket and propelling it in a similar fashion to a balloon. And, while one molecule in the reaction will only exert a small force, the effect of billions of combustion products leads to a large thrust force for the rocket.
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