Forces acting on a paper plane: thrust, lift, drag, and gravity.

This diagram shows the four forces that act on a paper plane (or any flying object). Here's what each force does:

1. Thrust (yellow arrow pointing left)
- This is the forward force that moves the plane ahead
- For a paper plane, thrust comes from your arm when you throw it
- Once thrown, there's no more thrust - the plane just coasts forward

2. Lift (green arrow pointing up)
- This force pushes the plane upward, fighting against gravity
- The wings create lift as air flows over and under them
- The shape of the wings helps generate this upward force

3. Gravity/Weight (red arrow pointing down)
- This pulls the plane downward toward the ground
- It's caused by Earth's gravitational pull on the plane's mass
- Gravity is always acting on the plane, trying to bring it down

4. Drag/Air Resistance (blue arrow pointing right)
- This force opposes the motion of the plane through the air
- It slows the plane down as it moves forward
- Air resistance increases with speed

How These Forces Work Together:

When you throw a paper plane:
- Thrust gets it moving forward initially
- As it moves forward, the wings create lift to keep it up
- Gravity constantly tries to pull it down
- Drag slows it down gradually

For the plane to stay in the air:
- Lift must be strong enough to overcome gravity
- The initial thrust must be strong enough to overcome drag and keep the plane moving so the wings can generate lift

Eventually, drag slows the plane down, reducing lift, and gravity wins - causing the plane to descend and land.