# Homework Help: Mass question

1. Jun 9, 2006

### c4iscool

I can't figure out this problem. A 1000kg car traveling east at 3m/sec collides with and sticks to a vehicle traveling due west at 2m/sec. Immediately after the collision, the vehicular combination is moving due west at 1m/sec. the mass of the second vhicle is ? kg. How do I set this up?

2. Jun 9, 2006

### Staff: Mentor

What's conserved during the collision?

3. Jun 9, 2006

### c4iscool

I think the mass would be conserved.

4. Jun 9, 2006

### neutrino

Anything else? Think important conservation laws.

5. Jun 9, 2006

### c4iscool

Are u guys getting at conservation of energy laws? But can I only get the velocity with that?

6. Jun 9, 2006

### Hootenanny

Staff Emeritus
I'll give you a clue, it starts with an 'M' and its not mass

Last edited: Jun 9, 2006
7. Jun 9, 2006

### c4iscool

conservation of matter

8. Jun 9, 2006

### Hootenanny

Staff Emeritus
Thats the same as mass. Are you sure you know no other conservation laws? I'll give you another clue, its symbol is 'P'

9. Jun 9, 2006

### c4iscool

I don't know of any others. sorry I'm a noob!=(

10. Jun 9, 2006

### c4iscool

oh momentum

11. Jun 9, 2006

### Hootenanny

Staff Emeritus
No problem . The law you need to use is 'conservation of momentum'. Have you heard of momentum before? Do you know how it is defined / how to calculate it?

12. Jun 9, 2006

### c4iscool

is it m1v1=m2v2 or something?

13. Jun 9, 2006

### Hootenanny

Staff Emeritus
Thats the one. Initial momentum (the product of mass and velocity) must equal the final momentum. Take note that momentum is a vector quanitity, so direction matters.

14. Jun 9, 2006

### c4iscool

but how do I figure out the final momentum without the mass?

15. Jun 9, 2006

### Hootenanny

Staff Emeritus
You can calculate the intial momentum. You know the final velocity. You know the two vehicles are stuck together. Can you formulate an equation using the above facts?

16. Jun 9, 2006

### c4iscool

would the equation be (1/2)Massf(1)=(1/2)1000(3). or can I drop the 1/2 in front of the equation.

17. Jun 9, 2006

### Hootenanny

Staff Emeritus
Hmm. How about this taking east to be positive and letting M = 1000kg, m be the unknown mass, v1 = 3, v2 = -2 and vf = -1;

$$Mv_{1} + mv_{2} = (M+m)v_{f}$$

18. Jun 9, 2006

### c4iscool

is the mass about 2000 or 4000?

Last edited: Jun 9, 2006
19. Jun 9, 2006

### Hootenanny

Staff Emeritus
I got the mass to be about 1333kg. Perhaps you have made an arithmetic error? If you show your working I'll see if I can see where you've gone wrong.

20. Jun 9, 2006

### c4iscool

I just reworked it and now I have more!
(1000*3)+m(-2)=(1000+m)-1
3000-2m=-1000-m
4000=m