The physics of flight

Welcome! I'm excited to explore the physics of flight with you. To help me tailor this explanation, what is your current interest or background in aviation? (e.g., are you a student, a flight simmer, or just curious about the science?)

I'm just curious about how planes fly.

It is great to have you here! Since you are curious about how these massive metal machines defy gravity, let us start by looking at the four fundamental forces that make flight possible.

The Four Forces of Flight

Every second a plane is in the air, it is in the middle of a continuous, four-way aerodynamic tug-of-war. These four forces dictate exactly how an aircraft moves, climbs, descends, and stays aloft:

Lift: The upward force generated by the wings that pulls the airplane into the sky, working directly against weight.
Weight: The downward pull of gravity that constantly tries to bring the aircraft back to the Earth.
Thrust: The forward force created by the engines (whether jet engines or propellers) that pushes the aircraft through the air, fighting against drag.
Drag: The rearward, resistive force caused by air disruption and friction as the airplane slices forward through the atmosphere.

To understand how these forces interact on a real plane, we can look at a visual representation of how they balance each other out in the sky.

The four fundamental forces of flight acting on a horizontal airplane in equilibrium.

This diagram shows how these four forces are aligned. When a plane is cruising steadily at a constant speed and altitude, these opposing forces are in a state of perfect balance, or equilibrium. This means that your upward lift exactly matches the downward pull of gravity, and your forward engine thrust exactly matches the backward tug of air resistance. If any of these forces change—for instance, if the pilot increases thrust—the forces become unbalanced, and the plane will accelerate, climb, or change its flight path.

newton's second law of motion

You have hit on the exact mathematical engine behind how planes change their state of motion! Newton's Second Law—commonly written as the formula $F = ma$ —tells us that force equals mass times acceleration.

When we apply this to flight, any imbalance in our four forces creates a net force ( $F$ ) acting on the plane's mass ( $m$ ). This net force causes the aircraft to accelerate ( $a$ ), meaning it will speed up, slow down, climb, or descend.

F_{\text{net}} = m \cdot a

Think of it like driving a car. When you step on the gas, the engine generates extra force. Because that force is greater than the resistance from the road and air, you have a net positive force, and you accelerate forward.

In a plane, if a pilot pushes the throttle forward to increase engine thrust, the thrust exceeds the aerodynamic drag. According to Newton's Second Law, this net forward force causes the plane to accelerate. As the plane speeds up, more air rushes over the wings, which in turn increases lift and allows the plane to climb. Conversely, if drag is greater than thrust (like when pulling the throttle back), the net force is backward, causing the plane to decelerate.