HardPhysicsHigh School (9–10)GeneralMechanics › Momentum and CollisionsMechanics › Newton's Laws of MotionConceptual
Some people say a rocket cannot work in space, because there is nothing to push against.
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
Some people say a rocket cannot work in space, because there is nothing to push against. Use the third law to explain why this is wrong. A space probe throws out of gas each second at : what force pushes it?
Need a reminder? The key ideaShow
Newton's third law: when A pushes B, B pushes A with a force of the same size in the opposite direction. The two forces act on different objects, so they never cancel each other.
With no outside force, the total momentum of a group of objects stays the same: . Give opposite directions opposite signs. Things that start at rest end with equal and opposite momenta.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. The idea. The rocket pushes its own exhaust gas backward; by the third law the gas pushes the rocket forward. No air or ground is needed.
Step 2. Thrust. .
Answer: It pushes on its own gas; .
Check. In space there is no air to slow it, so a rocket works even better there.
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