# What is the physics behind car crashes?

We did this test in physics where we crashed cars into walls and other cars. I was wondering how you would explain what happens during a car crash, and what variables can you change to change the car crash. I know that mass and acceleration are two variables. Any help would be appreciated.

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• Anonymous

I am on the faculty of a science museum, and we do this sort of thing all the time.

Let's keep it a bit simple and apply a car crash to Newton's Three Laws of motion.

1) A body at rest will remain at rest and a body in motion will remain in motion in a straight line until acted upon by an outside force.

---When a car crashes into something, even if the front part of the car has been stopped, the back part of the car is continuing in a straight line. This is why cars crumple in.

2) Force = Mass times Acceleration

The greater the mass of the crashing car, the greater the resulting force. Same thing for the Acceleration. The more acceleration inherent in the car, the greater the force. Of course, if two cars are heading straight for each other, the Accelerations are added. Which is why head-on collisions are so frightful.

3) For every action there is an equal and opposite reaction. If the car exerts a force on a wall, then the wall exerts an equal and opposite force back onto the car. (Otherwise the car wouldn't get damaged at all.

That's probably the main thing your teacher's wanting to get you to understand.

But here are a few other things that could be taught when discussing collisions:

--Heat created by the friction of the two cars smashing into each other.

--Coefficients of friction if the car tried to break before the impact.

--Projectile and trajectory if there was someone in the car who gets thrown out of the car.

Anyway, I hope that's all helpful.

Science smiles,

Quill.

• Physics Of Car Collisions

• Car crashes are frequently used as teaching tools to reveal the relationship between momentum, energy, velocity and mass. Generally all the kinetic energy (½mv²) is turned into heat unless there is some residual velocity after the crash. Momentum (m*v) is always conserved and equal to the average force*time of a collision.

• Energy absorbing design.

Side crashes - sills need to be designed so's to keep the impacting car down and out of the side windows.

Mass and acceleration are not really variables - as impact tests are at specified speeds. Deceleration rates are the most important as they relate to the energy absorbing design, hence the time rate change of velocity.

The slower the time rate change of velocity is - the safer it is for the passengers.

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• All of Newton's Laws apply. Those being, "An object in motion will stay in motion," "An object at rest will stay at rest," and "Every action has an equal and opposite reaction.

Variables include:

Mass

Velocity

Acceleration

Friction

Air Resistance

Gravity (although not exactly a variable)

Angle of impact

Intertia

Momentum

Hope I helped!

• Anonymous