Forces, Accelerations, & Car Accidents

Below is a list of available QuickTime video clips and explanations of the physics of the crash.

Notice in this test crash of an older car, the car has a very rigid frame, so it comes to rest almost immediately while the body of the car keeps going at its current velocity. The body has no crumple zone so it comes to rest as it hits the wall and the people are then brought to rest immediately as they strike very solid items within the car. Remember, it takes the same impulse (J=Ft) to stop the people in the car. This car uses a small time, so the force must be huge...and quite deadly.

Cars are designed with crumple zones so they may slow down over a longer period of time, which keeps the force smaller (see impulse). However, this safety feature alone will usually not prevent serious injury or death to the occupants of a car during an accident. The crumple zone only slows the car more gradually. The only way it slows the occupants more gradually is if they are attached to the car. Otherwise, the car may come to rest more slowly but the people come to rest immediately upon striking the already stopped interior of the car. Stopping in a small amount of time means the force must be very large. This video clip shows some very dramatic scenes of car crash tests with test dummies who are not wearing seat belts. Specifically look for cars crumpling and people stopping in very small amounts of time.

This clip illustrates what happens to children in a car accident if they or their car seat are not properly belted in. (This particular car seat was flawed.) Remember, if the car was going 55mph, then any person or object in the car is also going that fast and will continue to go that fast until a net force acts on them to slow them down.

It should be pointed out that the rotating dial makes 1 rotation in 1/10th of a second. It was used to measure small intervals of time on the film (pre-digital clock)

This clip clearly gives the driver a good reason to make sure occupants in the rear of the car are wearing their seat belts. Not wearing a seatbelt not only puts your life in danger but also anyone else who happens to be riding with you. The force from the seatbelt safely decelerates the driver, but the child in the back seat follows Newton's Law of Inertia and continues moving in the absence of a net force. The 60mph "kid" not only breaks its own neck but also the neck of the driver.

In this clip we see a "parent" holding a "child" and neither of them wearing a seatbelt. As you can see, the "parent" not only allows harm to come to the "child" but actually hurts the child more as the child becomes an ineffective "air bag" for the parent.

In this clip, we see that seat belts and child seats not only protect you in a frontal impact, they could also prevent a tragedy in rear end collision. In this clip, the station wagon literally gets accelerated out from under the "kids" sitting in the back. They were at rest originally, and in the absence of a net force (from the seat belt) they remained at rest while the car they were in was accelerated by the net force from the car that hit them. Newton's first law can be a killer!

Guard rails are intended to protect people by preventing cars from leaving the sides of the road near cliffs, rivers, curves, and bridges. Most of the time they do this well. Unfortunately, some older guard rails actually pose a threat to lives. If the guard rail does not end by being curved to the ground, or in a breakaway design, the rail may be capable of impailing a moving car and its occupants. The ends of these older rails are usually sharp and difficult to see from the end. A rapidly moving car upon running into one is met with a very large force and high pressure as that force is exerted on a very small edge of the rail. The effect is known as "guard rail spearing".

Seatbelts use two main ideas to protect passengers during a car accident. First, they slow the passenger down more slowly than the passenger running into steering wheel or dashboard. This keeps the force required to stop them smaller. (see impulse) It also prevents the person from contacting any of the glass windows in the car or continuing on to be stopped abruptly by the road, tree, or another automobile. The video clip above shows the role of the seatbelt during an accident.