David learns
                                                             how car safety
                                                             belts and air 
                                                             bags can save
                                                             your life. 


How do airbags prevent automobile injuries?


Moving objects have momentum. Newton's First Law of Motion says that unless an outside force acts on an object, the object will continue to move at its present speed and direction. Automobiles consist of several objects, including the vehicle itself, the passengers inside and any other loose objects in the vehicle. Unless the objects inside the car are restrained they will continue moving at whatever speed the car is travelling even if the car is stopped by a crash.

Changing or stopping an object's momentum requires a force acting over a period of time. If momentum changes instantly, as in a car crash, the force is very, very great! If the momentum can be changed over a period of time, even a fraction of a second, much less force needs to be applied with less damage or injury.

In a head-on collision, if a passenger flies into the dashboard of a car, their momentum is instantly stopped, and serious injury is often the result. If the passenger is restrained by a seatbelt, their momentum is reduced more gradually by the constant and smaller force of the belt acting over a longer period of time. Seatbelts can reduce the impact of a passenger to one-fifth of the impact suffered by the body of the car.

Passive restraint laws, combined with an interest in air bags have made vehicle safety a selling feature on automobiles. An air bag is made of a coated fabric and is stored in a module mounted on the steering wheel. Crash sensors, which activate upon impact at speeds of 10-15 miles per hour, are mounted in several locations on the car chassis.

In a crash, the sensors ignite a chemical, sodium azide, which releases harmless nitrogen gas to instantly inflate the bag. As the driver or passenger is thrown into the bag, it applies a restraining force. Even though this entire process happens in only 1/25th of a second, the added time is enough to prevent serious injury.

Air bags are not intended to replace seat belts. They are part of a supplemental restraint system. Seat belts are still necessary because air bags only work in front-end collisions of more than 10 miles per hour. Only a seat belt can help in side impacts, rear-end collisions, side swipes and secondary impacts.

Things to Talk About

  1. How does an air bag resemble a parachute? How is it different?
  2. Is it possible to invent a springy car that would absorb the impact of accidents by itself?
  3. Why can't air bags help when a car is rear ended?

Momentum--The product of the mass and the velocity of an object.

Velocity--Speed in a certain direction (e.g., a car travelling along a straight road with a speed of 70 km/hour also has a velocity of 70 km/hour. A car traveling at 70 km/hour round a bend in a road has a velocity that is continuously changing as the road is not in a straight line.

Force--A push or pull which causes acceleration, or a change in the shape of an object, or a reaction. A force is measured by the change in momentum produced in one second. A force cannot be seen, only its effects can be seen.


Additional Sources of Information

Activity Page

Egg Pitching!

You can find ways to throw eggs at high speed and not have them break!

Main Activity

Design ways to cushion an egg that is thrown through the air. Using the theories behind air bags in automobiles, find the best way to protect it from impact so you can throw it faster and further.


1. Turn under the bottom edge of the sheet about 10 cm. Sew the flap up and insert the broom stick handles into the top cuff and the one you have just sewn.

2. Have four classmates hold the corners of the sheet out horizontally to the ground.

3. Have a fifth student take aim, wind up and pitch the egg up and over onto the sheet.

4. Experiment with different speeds and distances to see how far and how fast you can throw the egg without breaking it.


1. What happens to the egg's momentum? What would happen if you were dropping the egg on a concrete floor?

2. How could you cushion the egg itself? Would that transfer the momentum of the egg?

3. Would a parachute attached to the egg provide enough cushion to keep it from breaking? How high could you drop an egg attached to a parachute?

4. Does the size of the sheet make a difference in your experiments? What if you pulled the sheet taut?

See if you can find other examples of impact protection devices. Where are they most common? Why do you use pads for gymnastics? What about helmets and knee pads? Do they transfer momentum?

Toss a balloon filled with water into the air. Stop and analyze what happens when you successfully catch the balloon. What ways have you helped transfer the momentum?

Design a few posters that promote seat belt use. Hang them up around the school to promote automobile safety. Why would parents want to put small children in safety seats in the back seat of a car?

People have different opinions about mandatory seat-belt and air- bag use. In fact, this discussion often includes bicycle and motorcycle helmets for safety, too. Divide your class into two debating teams and discuss the following question: Can a society that believes in individual decisions regarding safety issues require individuals to use devices that may save their lives in an accident?

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Educational materials developed with the National Science Teachers Association.

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