# Car and truck collide

1. Apr 10, 2015

### Westin

1. The problem statement, all variables and given/known data
What is the x-component of the velocity of the stuck-together car and truck just after the collision?

speed= -7.32352941176471 m/s

You are correct.

c) In your analysis in part (b), why can you neglect the effect of the force of the road on the car and truck?

2. Relevant equations
Conservation of momentum and energy

3. The attempt at a solution
I believe that During the collision, external forces are negligible so the momentum of the system is conserved.

Am I on the right track here?

2. Apr 10, 2015

### rootone

In a vehicle collision most of the energy which was contained in the colliding vehicles will end up as kenitic and thermal energy contained in bits of flying debris.
If the collision is not very fast the energy will deform the vehicles, instead of making flying debris, but the deformed vehicles will still get hotter.
Some of the released energy might result in changing the state of other stuff at the location, such as the road.
No energy just gets lost.

Last edited: Apr 10, 2015
3. Apr 10, 2015

### SammyS

Staff Emeritus
You have only restated that external forces such as the force of the road on the car and truck can be neglected. You haven't given any reason as to why they can be neglected. However, you do mention that it's during the collision that they can be neglected. This is an important point.

So, why can those external forces be neglected during the collision?

4. Apr 11, 2015

### Westin

Im honestly kinda lost here, it it because the collision is between the car and truck, so the system is the car and truck, making the road be neglected?

5. Apr 11, 2015

### SammyS

Staff Emeritus
Have you studied "impulse"?

How do the collisional force compare with thise external forces in size and duration?

6. Apr 11, 2015

### Westin

The collision is so short that you can just neglect the road? And because the road will have such a tiny affect since the collision is so much larger?

7. Apr 11, 2015

### Westin

Larger, I mean the energy from the collision is so much larger than the road.

8. Apr 11, 2015

### SammyS

Staff Emeritus
It's the force that's at issue here, not energy.

Yes, the collisional forces are much larger than the external forces and occur over a very short time, during which time the external forces cause relatively little change in motion.