# Forces in flight

As you run around the yard, you can feel air pushing against your face and arms.  If you throw a piece of paper (flat, not crumpled up) into the air, it will fly and fall differently than if you throw the same piece of paper after you crumple it into a ball.  Understanding how air moves and how it reacts to objects moving through it is important in designing airplanes.

##### Aerodynamics is the study of how air moves and how it reacts to objects moving through it.

In addition to aerodynamics, there are forces that push and pull on an airplane as it is in the air.  Engineers have to look at and understand these forces to be able to design planes that will fly well.  This picture shows the forces pushing and pulling on the plane.

Thrust is the force used to make you move forward.  When you run, your legs and feet move to make you move forward, to push you forward.  When you throw a paper airplane, your arm creates the thrust/force to push the plane.  In an airplane, the engine produces the force to move the plane forward.

Lift is another force engineers have to understand.  (This one requires a good understanding of aerodynamics!)  Wings have to be designed to allow air to flow over and under in just the right way.  As the air rushes around the wing, the different pressures of the air over and under the wing create lift.  Lift is the force that actually helps get the plane up off the ground.

Gravity is the force that pulls you and everything else toward the earth.  If you throw a ball in the air, it comes back down because it is pulled by gravity.

Drag is the force slowing you down as you push against the air in front of you.  (Newton’s Laws of Motion state that every action has an equal and opposite reaction.  As you push against the air, the air is pushing back against you.)

When engineers understand these forces, they can design planes that create enough lift to overcome the power of the gravity force and enough thrust to overcome the drag.  When that happens, flight is achieved!