# Plane Lift\-Off on a Moving Runway

By lexmirnov

Topic: Physics & Chemistry

[Open the interactive Jig](https://ask.jig.com/j/XLAk1qVQcfzv)

## Guide

### 1. A runway that runs backward

This runway is a giant treadmill belt\. As the plane rolls forward, the belt runs backward at exactly the plane's speed\. Will the plane ever take off? Drag the scene to look around\.

Spoiler: yes, it takes off\. Let's find out why\.

### 2. Wheels just spin freely

First, run the belt on its own\. A plane's wheels are not driven by its engine\. They roll freely on bearings, like the wheels of a shopping trolley, so the belt mostly just spins them\.

Wheel bearings add only a tiny drag, so the belt can hardly tug on the plane\.

### 3. The propeller pushes on air

Now the engine roars\. The propeller grabs air and throws it backward, and the air pushes the plane forward\. This push is called thrust, and the belt cannot touch it\.

It is like standing on roller skates on a treadmill while a friend pulls you with a rope\.

### 4. The belt matches the plane

The plane speeds up, and the belt runs backward just as fast\. Watch the runway lights beside the belt: the plane still moves forward past them\. Its wheels simply spin twice as fast\.

At 50 km/h forward with the belt at 50 km/h backward, the wheels turn as if rolling at 100 km/h\.

### 5. Lift comes from moving air

Wings make lift only when air flows over them\. What counts is the plane's speed through the air, called airspeed\. The belt moves only the ground under the wheels, so the airspeed keeps climbing\.

A small plane like this lifts off at roughly 100 km/h of airspeed\.

### 6. Full power: lift\-off\!

Your turn\. Press Full power and follow the whole run, with the belt matching the plane's speed all the way\. When the airspeed is high enough, the wings lift the plane off the belt\.

### 7. Tested for real

In 2008 the TV show MythBusters tried it\. They pulled a long sheet backward under a light plane, and it took off as usual\. A car would stay put, because its engine turns its wheels to push on the ground\.

A plane pushes on the air, so the moving ground under its wheels hardly matters\.