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Starting to Move

Jet engines operate by producing a hot gas and expelling it from the exhaust of the engine at high speed. The exhaust gases have a large momentum and need a large force to be exerted on them to obtain this speed. Newton's Third Law of motion then tells us that they exert an equal and opposite force to send the jet engine in the opposite direction. This is most simply illustrated with a balloon full of air.

When you let it go, the air at high pressure inside the balloon escapes at high speed and the balloon is pushed in the opposite direction to the escaping air. The action of the ballon on the air produces a reaction of the air on the balloon.

Cars operate on the same 'action – reaction' principle. A car is not though, made to move because the exhaust gases are ejected at high speed. Instead, as long as the tyre grips the road and does not spin, the tyre exerts a backwards force on the road, resulting by Newton's Third Law, in an equal forwards force being exerted by the road on the tyre. This happens at every wheel connected to the driving force, so the car moves forward.