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Home Minds On Momentum, Collisions and Explosions Mission MC3 Impulse and Momentum Change Variables Help Help 3

Mission MC3 Impulse and Momentum Change Variables

The Question

Air bags are used in cars as a safety feature to protect front-seat passengers. This safety feature helps to protect the passengers of the car by increasing the ____ and decreasing the ____.

Physics Rules

Momentum Change - Impulse Theorem:
When a force is exerted upon an object in a collision, the object is said to have encountered an impulse. The impulse is simply the mathematical product of the force exerted on the object and the amount of time over which it was exerted. The impulse changes the object's momentum and is equal to the amount of momentum change.
 
Impulse = Momentum Change
or

F•t = m•∆v

Minds On Time

The theory behind dashboards and the more extreme air bag is to provide a collision surface to the passenger that has a greater give. This give serves to extend the time over which the force is exerted upon the passenger in order to slow them to a stop. If a passenger with 1000 units of momentum encounters a collision with a windshield, the time of impact will be short (small) and the force will be large and damaging. If the same passenger at the same speed contacts an air bag, the impulse and momentum change will still be 1000 units. Only now, the time of collision will be large due to the give of the collision surface. With the time increased, the same impulse can be achieved with less force on the passenger. Of course, less force means a greater chance of surviving the accident. Now that's physics for better living.

Don't be Fooled

Many students have the conception that the use of an air bag alters the amount of impulse and momentum change that the passenger experiences. In the collision, a passenger with 1000 units of initial momentum will have a 1000 unit momentum change. This value is dependent upon the passenger's mass and pre-collision speed. A 50-kg passenger moving at 20 m/s is going to have a 1000 kg•m/s momentum change whether using an air bag or not. And if the momentum change is 1000 units, the impulse required to obtain it will be 1000 units as well (see the Know the Law section). Don't be fooled! An air bag doesn't change the amount of impulse required to stop a passenger, it simply changes the manner in which the impulse is achieved - by doing so with more time and less force.

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