Newton's Laws and Car Crash Safety
Discover how the laws of physics affect what happens when cars crash at different speeds
How the Crash Tests Were Done
Scientists from the AAA Foundation for Traffic Safety, Insurance Institute for Highway Safety, and Humanetics Innovative Solutions worked together to test what happens when cars crash at different speeds.
Red Numbers
Throughout this website, red numbers show values that are much higher and more dangerous than the normal 40 mph test. These represent forces and speeds that could cause serious injuries.
The Test Cars
- Model: 2010 Honda CR-V EX
- Age: About 11 years old (average car age)
- Safety Rating: Excellent safety ratings
- Number of Tests: 3 identical cars
Why this car? Scientists chose cars that represent what most people drive and that are known to be safe.
The Test Speeds
- Test 1: 40 mph (normal test speed)
- Test 2: 50 mph (10 mph faster)
- Test 3: 55.9 mph (15.9 mph faster)
Why these speeds? To see what happens when cars crash just a little bit faster than normal.
The Crash Test
- Type: Moderate overlap frontal crash
- Contact: 40% of the car hits the barrier
- Barrier: Fixed wall with aluminum honeycomb
- Location: Driver's side of the car
What this simulates: A head-on crash between two cars of the same size going the same speed.
The Test Dummy
- Type: Hybrid III 50th percentile male
- Size: Average-sized adult male
- Sensors: Head, neck, chest, legs
- Position: Driver's seat with seatbelt
Why use dummies? They have sensors that measure forces and accelerations just like a real person would experience.
Newton's First Law: The Law of Inertia
The Law: An object at rest stays at rest, and an object in motion stays in motion at constant speed in a straight line, unless acted upon by an unbalanced force.
Inertia: The tendency of objects to resist changes in motion
Everyday Example: When you're in a car that suddenly stops, your body keeps moving forward because of inertia. That's why we wear seatbelts!
When the Dummy Hits the Airbag
| Test Speed | Time Face Hits Airbag | Airbag Deployment Time |
|---|---|---|
| 40 mph | 88 milliseconds | 54 milliseconds |
| 50 mph | 80 milliseconds | 46 milliseconds |
| 55.9 mph | 78 milliseconds | 44 milliseconds |
Millisecond: One thousandth of a second (0.001 seconds)
Red numbers: Show when the crash was so fast that safety systems couldn't deploy in time
How the Steering Wheel Moved
| Test Speed | Forward/Backward | Up/Down |
|---|---|---|
| 40 mph | 3 cm forward | 3 cm up |
| 50 mph | 2 cm backward | 11 cm up |
| 55.9 mph | 8 cm backward | 18 cm up |
Red numbers: Show dangerous steering wheel movement that puts the airbag in the wrong position
Timeline of Dummy Motion
- Seatbelt tightens: 22-26 milliseconds
- Dummy begins moving back: ~150 milliseconds
- Dummy stops moving: ~200 milliseconds
- Total crash time: ~0.2 seconds
Newton's Second Law: Force = Mass × Acceleration
The Law: The acceleration of an object is directly proportional to the force applied to it and inversely proportional to its mass.
Formula: Force = mass × acceleration (F = ma)
Acceleration: How quickly something speeds up, slows down, or changes direction
Everyday Example: A bowling ball and a tennis ball falling from the same height. They accelerate at the same rate due to gravity, but the bowling ball hits with much more force because it has more mass!
Forces on the Car
| Test Speed | Peak Car Deceleration | Speed Change |
|---|---|---|
| 40 mph | 36.9 g's | 45.3 mph |
| 50 mph | 46.8 g's | 54.5 mph |
| 55.9 mph | 55.5 g's | 61.5 mph |
g-force: A measure of acceleration. 1 g = normal Earth gravity. 36.9 g's means 36.9 times stronger than gravity!
Red numbers: Show forces so high they could seriously damage the car and hurt people inside
Forces on the Dummy's Head
| Test Speed | Peak Head Acceleration | Head Injury Score |
|---|---|---|
| 40 mph | 75 g's | 548 |
| 50 mph | 139 g's | 1,780 |
| 55.9 mph | 157 g's | 2,399 |
Head Injury Score: Higher numbers mean more dangerous. Over 1,000 is considered very dangerous.
Red numbers: Show head injury levels that could cause serious brain damage or facial fractures
Forces on Chest and Legs
| Test Speed | Chest Force (g's) | Leg Force (kN) |
|---|---|---|
| 40 mph | 39 g's | 0.25 kN |
| 50 mph | 69 g's | 3.35 kN |
| 55.9 mph | 92 g's | 7.39 kN |
kN (kilonewton): A unit of force. 1 kN = about 225 pounds of force
Red numbers: Show forces strong enough to break bones in the chest and legs
Newton's Third Law: Action and Reaction
The Law: For every action, there is an equal and opposite reaction. When one object exerts a force on another object, the second object exerts an equal force back on the first object.
Action-Reaction Pair: Two forces that are equal in strength but opposite in direction
Everyday Example: When you walk, you push backward on the ground with your feet (action), and the ground pushes forward on you (reaction), making you move forward!
Action-Reaction: Dummy and Seatbelt
| Test Speed | Dummy pulls on seatbelt | Seatbelt pulls back on dummy |
|---|---|---|
| 40 mph | 0.9 kN tension | 0.9 kN tension |
| 50 mph | 2.5 kN tension | 2.5 kN tension |
| 55.9 mph | 4.0 kN tension | 4.0 kN tension |
Action-Reaction: The forces are always equal but in opposite directions
Red numbers: Show how much stronger the forces become at higher crash speeds
Action-Reaction: Dummy and Car Floor
| Test Speed | Dummy pushes on floor | Floor pushes back on dummy |
|---|---|---|
| 40 mph | 5.1 kN downward | 5.1 kN upward |
| 50 mph | 8.9 kN downward | 8.9 kN upward |
| 55.9 mph | 11.1 kN downward | 11.1 kN upward |
Safety System Response Times
- Seatbelt tightens: 22-26 ms
- Front airbag inflates: 44-54 ms
- Side airbag inflates: 52-70 ms
- Head hits steering wheel: 90-99 ms (high speeds only)
ms (milliseconds): One thousandth of a second. Your eye blink takes about 300-400 ms!