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Safety, while important, is not the main factor in determining the number of engines on planes. Instead, it depends on how much thrust is needed to get them off the ground and keep them in the air.

There are four forces of flight: lift, weight, drag and thrust. Engines produce thrust, the force of flight that pushes a plane forward to overcome drag. In an engine, a mixture of air and fuel is ignited, causing them to combust. This combusted air and fuel create hot gasses that push against the front of the engine, but not against the hole in the back. This unbalanced push thrusts the plane forward.