# Misunderstanding in Fundamental Concept

1. Apr 24, 2012

### Bashyboy

Okay, say a car is moving at a constant velocity and crashes into a wall. Now, observation would clearly illustrate to us that a destructive force was imparted onto the wall and the car. But how can that be if there there is no force, because there was a constant velocity? I must have some concept very misconstrued.

2. Apr 24, 2012

### Staff: Mentor

The car is moving at constant velocity before it hits the wall. Once it hits the wall it is no longer moving at constant velocity--due to the force exerted on it by the wall.

3. Apr 24, 2012

### ingenvector

It may be worth considering exactly how force and momentum are related.

4. Apr 24, 2012

### Bashyboy

Oh, yes, okay. So, then as the wall applies a force against the car, the car mutually engages in exchanging an opposite force. I must really think about the whole system when considering problems as such; I was thinking in terms of the car, but clearly the wall can incite an interchange of forces. Say the car doesn't crush under the force the wall provides, is this because the frame of the car was able to exert a force that is exactly opposite?

5. Apr 24, 2012

### Staff: Mentor

No. Newton's 3rd law tells you that regardless of what happens to the car, both car and wall will always exert equal and opposite forces on each other. If the force is too much for the structure of the car to withstand, it will get crushed.

6. Apr 24, 2012

### ingenvector

Well, cars are usually engineered to have large impulse times, but considering an ideal frame made of some remarkably strong material, it could be that it wont be crushed in most normal car crash scenarios, but this is somewhat fanciful. However, the frame has a limited range of tolerance, and if the force is greater than it can bear, since both car and wall impart equal force upon one another, then the frame will be crushed regardless of the material.