# Homework Help: Momentum/Impusle problem

1. Dec 13, 2007

### harelo

1. The problem statement, all variables and given/known data

So my problem says: "Car A (in a frictionless surface) travels at a velocity and weighs 4000 kg, Car B is parked and weighs 3000 KG, they collide in an inelastic collision (meaning they stick together) and they move a distance of 2.8 M together."

2. Relevant equations

Find the velocity of the Car A before impact

3. The attempt at a solution

I have to approach this through momentum/impulse/work/energy, but I have no clue how to do it since I am missing so much information. Thank you for any help :)

Last edited: Dec 13, 2007
2. Dec 13, 2007

### Dick

If you have a variable you need and you aren't given a numeric value, then just give it a symbolic name, like 'v'=velocity. Similarly for coefficient of friction, and give the answer in terms of those unknowns. That's all you can do.

3. Dec 13, 2007

### harelo

|m1| =======> |m2| --------------> |m1+m2|

V1= ?
V2= 0
$$\Delta$$d = 2.8 m
V = ?

Find V1

so I know that since this is an inelastic collision, the conservation of momentum would look like:

m1V1 = (m1+m2)V

However, I'm missing two variables, I do know that:

m1V1 = F$$\Delta$$d

Then again, I am still missing F, so I've tried to use numerous kinematic equations with no success..

I'm sorry for not having any more work, but this is all I can do :( I would really appreciate some help.

4. Dec 13, 2007

### harelo

Ok no matter what I try I can't find anything, I don't have any force, any time, velocity nor work I can work with, so I just can't find a possible numerical answer for this. I'm pretty much begging for help now, as lame as it sounds.

5. Dec 13, 2007

### Dick

I can't find a numerical answer for that either. You don't have enough information to do that, as I think you know. To know F you need to know a friction coefficient. You can't get a numerical answer unless you've overlooked that it is part of a multipart question where you were given these numbers. Relax.

6. Dec 13, 2007

### harelo

Maaan... this is typical of my physics teacher, sorry for cause trouble

7. Dec 13, 2007

### Dick

Asking a question does not constitute causing trouble.

8. Dec 13, 2007

### harelo

Would any Potential Energy be produced in this case?

9. Dec 13, 2007

### Dick

For there to be 'potential' energy there has to be a potential to work against, like gravitation or a spring, or some such thing. I don't see anything like that here.

10. Dec 13, 2007

### harelo

thought so... then I'm out of options, I only have masses which I can find the kinetic ration with (KEf/KEi), but that won't do much.

11. Dec 13, 2007

### Dick

The only way there is a numerical answer forthcoming is if you have omitted information pertaining to the problem. As I said, you can only express the answer in terms of an unknown 'v' and 'mu'.

12. Dec 13, 2007

### harelo

alright, thanks a lot and I'm pretty sure this is not the last you'll see of me.

13. Dec 13, 2007

### haha

i think i can do this here is how u do this
4000(v)+3000(0)= (3000+4000)2.8
4000v=19600
v=19600/4000
v=4.9m/s
i think this is the right answer
base on the law of conservation of momentum, momentum before have to be equal to momentum after

14. Dec 13, 2007

### Dick

That's fine. If the statement is "the final velocity is 2.8m/sec". Which is not the question posed. Is that what the question is, harelo?

15. Dec 13, 2007

### harelo

How did you just replace the final combined velocity with the distance covered by both cars?