Physics Fact
Thrust is the force that moves an airplane forward through the air. A propeller spinning or a jet engine expelling hot air produce thrust. Your hand provides thrust when it throws a paper airplane. According to the scientist Sir Isaac Newton, "If an object is not moving, it will not start moving by itself." In other words, something needs to push the plane forward in order for it to fly. Depending on how heavy or big the plane is, the pilot will need more thrust to overcome its "drag" or weight.

Wright Fact
The Wright brothers wanted to find an engine for their new glider so they wrote to all the top manufacturers for help. Unfortunately, all of the available engines that were powerful enough were also extremely heavy (about 100 pounds per horsepower!). Orville and Wilbur feared that the weight of the engine might crash their plane. So they asked Charlie Tate, their mechanic from the bicycle shop, to design a small lightweight engine. It took the whole winter for Charlie to get it working but when he did the Wright brothers were very happy with his results.