Lift

Lift is the hardest force to understand - it is the upward force that allows planes to fly. It lifts them off the ground and holds them in the air.

Lift is caused when there is more pressure below the wing than above it. This sounds complicated, but it's really quite easy. When two people arm wrestle, whoever is strongest produces the most pressure. This causes their arm to move towards the weaker person, who is producing less pressure. Things that have more, or higher pressure move towards things that have less, or lower pressure.

The lift force of a plane is mainly caused by the shape of the wings. They are flat on the bottom and curved on the top. As the plane moves through the air on the runway, the air that rushes past has further to travel over the curved top of the wing than it does underneath it. This means that the air on top of the wing is more spread out, causing lower pressure above the wing than below it. The difference between the low pressure above the wing and the high pressure below it is called the lift force. The wing is forced up by the higher pressure from below the wing until it eventually lifts into the air.

On take-off, the plane moves faster and faster down the runway. As it gets faster, more and more air passes above and below the wings and the lift force increases. When the lift force is more than the weight of the plane, the plane takes off.

And that's how planes fly!