# Newton's Space Tug

High\-school Newton's laws in low Earth orbit: inertia, F = ma and action\-reaction, by firing a space tug's thrusters, throwing a wrench and docking a cargo pod

By Numa

Topic: Physics & Chemistry

[Open the interactive Jig](https://ask.jig.com/j/07EQgSr6rRCo)

## Transcript

### 1. Newton's Space Tug

Four hundred kilometres above Earth, a small tug is closing on a cargo pod\. Almost nothing rubs or drags on moving objects up here, so Newton's three laws are easy to see\. Watch it dock, then you fly it\.

Gravity still acts here, but everything falls around Earth together\.

### 2. First Law: Inertia

Newton's first law: with no net force, an object keeps its speed and direction\. With the engines off, the marks stay evenly spaced\. Drag the tug to fire it, let go, and watch the new speed hold\.

On Earth, friction and air drag are what make things slow down\.

### 3. Second Law: F = ma

Both tugs push with the same force\. Which pod speeds up faster? Drag either tug to fire both and find out\. The empty pod wins: the same force gives a smaller mass more acceleration\.

Mass here means everything being pushed: the pod and its tug\.

### 4. Third Law: Push and Push Back

Forces come in equal, opposite pairs\. A thruster pushes gas one way, and the gas pushes the tug the other way just as hard\. Drag the tug and watch it shoot away from its own plume\.

That's why rockets work in empty space: they push on their own exhaust, not on air\.

### 5. Third Law: The Wrench Throw

What happens when a floating astronaut throws a wrench? Hand and wrench push on each other equally\. The light wrench flies off fast, and the much heavier astronaut drifts back slowly\. Drag from her hand to aim and throw\.

Astronauts get around by pushing off handrails\.

### 6. Dock with the Station

Fly the pod's ring into the station's port\. Go slowly: the latches only catch a pod moving at 0\.20 m/s or less, within 0\.15 m of the centre line\. Every push stays with the pod until you push the other way, so brake before you arrive\.

### 7. Nudging a 420\-Tonne Station

Docked, the same push now has to move 426,000 kg: station, pod and tug\. With so much mass, the acceleration is tiny\. In 12 seconds, the whole stack creeps just 20 cm\. Drag the tug to try it\.

Thin air slowly drags the real ISS down, so docked spacecraft push it back up\.