# Crash Lab Momentum

How momentum is conserved in elastic and locked\-together carts, and how a crumple zone and airbag stretch a crash's stopping time to cut the force\.

By Numa

Topic: Physics & Chemistry

[Open the interactive Jig](https://ask.jig.com/j/EeYSit3wTLmX)

## Transcript

### 1. Crash Lab, After Dark

A test sled hits a steel barrier at about 34 mph\. In less than a blink, its nose folds, the airbag fires and the dummy stops\. Why can a person survive that? The answer starts with momentum\.

Slow motion here runs 30 times slower than real time\.

### 2. Momentum: p = m·v

Momentum is mass times velocity\. Double either one and the momentum doubles\. Drag orange cart A back against the launcher, drag steel plates from the trolley onto its deck, then let go\.

Momentum has a direction\. Here, rolling toward the barrier counts as positive\.

### 3. Elastic Collision: Momentum Moves Across

With spring bumpers, the carts bounce and keep all their kinetic energy\. This is an elastic collision\. With equal masses, A stops and B rolls away at A's speed\. Change the plates or the pull: on the monitor, the total after always matches the total before\.

### 4. Inelastic Collision: Locked Together

Now the bumpers latch, and the carts move off as one\. This is an inelastic collision\. Total momentum stays the same, but some kinetic energy turns into heat, sound and bent metal\.

Momentum is conserved in every collision\. Kinetic energy is conserved only in elastic ones\.

### 5. Impulse: The Hard Stop

This car has a rigid nose and no airbag, so the dummy stops in about 10 milliseconds\. Impulse is force times time, and it equals the change in momentum\. Squeezed into so little time, the force is huge: like 19 tonnes on the chest\.

On the graph, impulse is the shaded area under the curve\.

### 6. Crumple Zone and Airbag

Switch between Rigid and Crumple\. The crumple zone and airbag make the stop 12 times longer, so the peak force is 12 times smaller\. The momentum change is the same\.

### 7. Challenge: Stop on the Line

Cart B is towed toward cart A\. Set cart A's mass and pull so the two latch together on the green line and stop right there\. Hint: a locked pair stays put only if their total momentum is zero\.

There is more than one answer\. Can you find two?

### 8. Real Life: Buying Milliseconds

Crumple zones, airbags and seat belts all do one job: they make the stop take longer\. Same momentum change, more time, less force\. A gymnast bends her knees on landing for the same reason, and you pull your hands back when you catch a fast ball\.