Let's take a closer look at how planes achieve speeds sufficient to create enough lift to leave the ground.
The force that pushes a plane forward is known as "thrust." A great deal of thrust is necessary, not only to get the plane up to speed, but also to keep it there. As the plane pushes forward through the air, however, the air, which as we've seen has its own mass, is also pushing back on the plane. This force is known as "drag." Planes achieve sufficient thrust to overcome drag in one of two basic ways:
Propellers. From the time of the Wright Brothers until World War II, virtually every plane relied on propellers to generate speed. An internal combustion engine (similar to that in a car) keeps the propellers spinning. The propellers work like a fan, pushing air toward the back of the plane.
As we've seen, air has mass. And according to Sir Isaac Newton's third law, every action has an equal and opposite reaction. So, as the plane is pushing air backward, the air is pushing just as hard against the plane. Get the propellers spinning at high speeds and they'll push enough air backward to move the plane forward at high speeds.
Jet engines. Invented by German scientists during World War II, jet engines work on a very different system. They take in air and combine it with jet fuel in what's called a "compression chamber." The compression process heats the mixture to a very high temperature, causing the fuel to ignite and burn.
In the resulting explosion, the mixture of fuel and air is forced out of the airplane at very high speeds. This expulsion is directed backward, and its force pushes the plane forward (Newton's third law once again). But before leaving the engine entirely, the exhaust passes by a fan, which spins as a result. The energy from the spinning fan is used to compress the fuel and air mixture in the compression chamber, where the whole process starts over again.