### Crumple Zones of a Car

Modern cars are easier to crumple in an accident than old cars. Crash test research has shown that this can reduce injury to the passenger. Modern cars are equipped with large bumpers made of relatively soft materials, so that the front and the rear sections will crumple into a controllable size during a crash. The "crumple zones" reduce much of the impact force by lengthening the impact time. Old cars have poor crumple zones, so they decelerate extremely rapidly in a crash and exert an enormous force on the passengers. That is why passengers in an old car suffer from more serious injuries in a traffic accident.

 Fig. 4-4 The crumple zones and the passenger section of a car.

The passenger section is between the front and the rear sections of a car. The bumpers in the front and the rear section are designed to protect the passenger section. Modern cars have a very strong and rigid passenger section which can withstand high energy load in a crash. Some cars have steel tubes embedded inside the doors to protect passengers against a side-on collision.

Car manufacturers are developing a new technology that can make a car crumple in a progressive manner. Physics principles can help scientists to understand how accidents happen, so they improve car safety.

 Activity: Designing a crumple zone

Example

 Fig. 4-5 A car collides with a stationary truck.

A truck of mass 3000 kg initially at rest is hit by a car of mass 1000 kg from the front The car has an initial velocity of 20 . The car and the truck move together after collision.

1. What is their common velocity just after collision?
2. What is the impact force on the car if
1. the car has a good crumble zone (impact time: 0.2 s), and
2. the car has a poor crumble zone (impact time : 0.1 s)?

Solution

1. By conservation of momentum,

1. Applying , we have

2. Applying , we have