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?