That's a really good question. There are two
main forces acting on a plane in flight that
control how fast it can go. These are called
thrust and drag. Thrust is the force
that the airplane uses to move forward through the
air, and drag is the force that the air exerts on
the plane to slow it down.
Thrust in an airplane comes from its propellers
or jet engines. These act in the same way as a
propeller does on a boat. If you place a solid
sheet in a fluid (both water and air are fluids,
just water is much more dense) and move it back
and forth you will see it push the water. The
force that pushes the water backwards in return
pushes the sheet forward. Whenever a force pushes
on an object, it pushes back. The equal and
opposite force is what you feel if you push
against a wall. The reason that you do not fall
over is that the wall pushes back against you.
In the case of a propeller there are several
small sheets of metal that rotate very quickly.
Since the propellers are at an angle, as they spin
around they push the air in front of them towards
the back of the plane, and in return the air
pushes back and moves the plane forward. A jet
engine essentially has a series of small blades
rotating at a very high speed inside the engine.
Drag is a force that all planes feel. This is
a form of friction. Friction is the force that
pushes back on a textbook as you push it across
the table. Planes can be designed to minimize
their drag by building them in very streamlined
shapes. Imagine you are in a swimming pool and
you push yourself off of the wall. If you put
your arms straight out to your sides you are not
going to go very far before the friction of the
water against your body (this is drag) slows you
down. However, if you put your arms straight at
your side you will go farther because there is
less drag in this streamlined case. By having a
plane with engines that produce a large amount of
thrust, and designing it to be streamlined, an
airplane can travel very quickly.
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