Drag: When a plane moves through the air, it is slowed down by the friction of the air. If it slows to much, the plane stalls and quickly loses altitude. Pilots must keep enough speed to make sure the plane stays in flight.

Gravity: is the force that pulls the plane down. When a plane is on the ground, standing still in calm air, only the force of gravity is working on it.

Thrust: causes the forward motion of the plane. The plane can be pulled through the air by propellers or pushed by a jet engine.

Lift: Air is a gas, and air has pressure. Picture the air surrounding a plane as putting pressure on various parts of the plane. Lift is a force of this air pressure acting on the wings of the plane. It is generated when the air pressure on the upper surface of a wing is less than the air pressure on its lower surface. The faster the gas moves, the lower the pressure on the surface of the wing.The wing of a plane is curved so that the air moving past its upper surface travels further and moves faster than the air moving past its lower surface. The difference in pressures is lift.

Lift on a wing depends on 3 things:

• the shape of its cross section


• its velocity, or airspeed


• its angle relative to other parts of the aircraft

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