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Home Minds On Momentum, Collisions and Explosions Mission MC3 Impulse and Momentum Change Variables Help Help 2

Mission MC3 Impulse and Momentum Change Variables

The Question

Cars are equipped with crumple zones. In a front-end collision, these crumple zones are designed to intentionally fold up and crumple during the short duration of the collision. One way that a crumple zone enhances safety is by preventing the car from rebounding. Preventing rebounding protects the passengers because it _____.

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

In a car collision, the momentum of the car is stopped (usually) by the action of a force for a given amount of time. To minimize the fatalities in such collisions, safety engineers use physics principles to design safety features that are incorporated into the cars. Crumple zones are examples of these safety features. Like air bags, crumple zones provide some additional give at collision time. A car with 50 000 units of momentum is going to be stopped due to the action of a force over some amount of time. The 50 000 units of momentum change will mean that somehow there must be 50 000 units of impulse (see the Know the Law section). The product of force and time (that is, the impulse) will have to equal 50 000 units. By equipping the car with a crumple zone, the time of impact is increased due to the give that it offers. Instead of a nearly immediate stopping of the car in maybe 10 milliseconds, the collision time might be extended to 100 milliseconds. This increase in time will not make the impulse (nor the momentum change) less; it will still be 50 000 units. However, and extending of the time allows the car to obtain 50 000 units of impulse with less force. With less force on the body of the car, there is less force transmitted to the passengers. The car becomes all crumpled up (in fact, it's probably totaled), but the passengers walk away alive (we hope). Now that's physics for better living.

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