1. Lift: This is the force that opposes gravity and pushes the object upwards. It's generated by the shape of the wings and the flow of air over them.

2. Weight: This is the force of gravity pulling the object downwards. It's determined by the object's mass.

3. Thrust: This is the force that propels the object forward. It's generated by the engines or propellers.

4. Drag: This is the force that resists the object's motion through the air. It's caused by friction between the object and the air.

These four forces interact with each other to determine the flight path of a flying object.