Why Can External Road Forces Be Neglected in a Collision Analysis?

[Westin]

Homework Statement

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?

Homework Equations

Conservation of momentum and energy[/B]

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?

 

[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.

 

[SammyS]

Homework Statement

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?

Homework Equations

Conservation of momentum and energy[/B]

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?

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?

 

[Westin]

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?

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?

 

[SammyS]

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?

Have you studied "impulse"?

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

 

[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?

 

[Westin]

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

 

[SammyS]

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

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.