Airplanes are complex machines. Despite their size and thousands of parts, they only need 4 forces to get lift off and stay in flight.
Thrust moves a plane forward. Drag slows it down. Gravity pulls a plane down toward the Earth, and lift raises it up into the sky.
To stay in the air, the plane must have enough thrust to fight against the drag. It must also have enough lift to overcome the Earth's gravity. To slow down and land, the plane will need more drag than thrust, and the lift must be weaker than gravity.
Thrust, drag, gravity, and lift all work together to get heavy airplanes, and their passengers, safely around the world.
In this game, the students are the aircrafts, subject to the four forces of flight.