# Homework Help: Who can confirm this for me?

1. Apr 11, 2012

### domyy

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

A railroad car of mass 12,000 kg collides and couples with a second car of mass 18,000 kg that is initially at rest. After the collision, both move at a speed of 4.8 m/s.

What is the initial momentum and initial speed of the first car?

2. Relevant equations

The formulas that I´ve used are:

Pi = pf = mv
vi = pf/m

3. The attempt at a solution

So, I´d like someone to confirm my answers:

For Pi (initial momentum) I did:

Pi = pf = mv = (12,000kg + 18,000kg)(4.8m/s) = 144,000kg.m/s

For Vi (initial speed) = Pf/m = 144,000/12,000 = 12m/s.

2. Apr 11, 2012

### tal444

They appear to be correct. Nice job.

3. Apr 11, 2012

### domyy

Thankss!!! =D

4. Apr 11, 2012

### domyy

I have another question. Could you confirm the other as well? Please??

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

A rifle with a mass 2.2kg fires a bullet with a mass of 5g. The bullet moves with a velocity of 600m/s.

1) What is the momentum of the bullet after it´s fired?
2) What is the recoil velocity of the rifle assuming that no external forces act on the rifle?

2. Relevant equations

P = mv

3. The attempt at a solution

For question 1) P = 0.005kg x 600m/s = 3kg. m/s.

For question 2) I wrote 0.

5. Apr 11, 2012

### tal444

1) Should be 30 kgm/s, not sure if you just had a typo.
2) 0 is incorrect. You have the 30 kgm/s of momentum from the bullet, right? Therefore, there must be an equal and opposite momentum acting on the rifle.

6. Apr 11, 2012

### domyy

I wish it were at typo. Actually, the calculation is correct. Am I using the wrong information?

Now for question number 2, you mean p = -p, so the answer would be -30?

7. Apr 11, 2012

### domyy

I just would like to say that I am truly appreciating your help!! :)

8. Apr 11, 2012

### tal444

Whoops, my bad on number 1. MY typo on the calculator xD. As for 2), yes, the momentum of the rifle would be -3, but you're looking for the velocity.

9. Apr 11, 2012

### TomTheBoss

help me with this 1 please
A teddy bear of mass 400 grams is hung from the end of a spring. The spring measures 51.0cm long in the rest position; when the teddy bear is attached to the end of the spring, the spring extends to 72.0cm. Calculate the elastic potential energy stored in the spring when extended to 72.0cm.

10. Apr 11, 2012

### domyy

hahaha no problem :) You are still being EXTREMELY helpful.

The question: A rifle with a mass 2.2kg fires a bullet with a mass of 5g. The bullet moves with a velocity of 600m/s.

I am looking for recoil velocity,

would that be 2.2 divided by 600m/s ?

11. Apr 11, 2012

### domyy

I am sorry but I am not sure about I should find the recoil velocity.

12. Apr 11, 2012

### tal444

No, your momentum is -3, right? P=mv. Mass of the rifle is 2.2kg. What is your velocity?

As for Tom, you should probably post your problem in your own thread. You also need to post an attempt at a solution first.

13. Apr 11, 2012

### domyy

v = p/m
v = -3kg.m/s divided by 2.2kg = -1.36 m/s.

So to find recoil velocity I should just use v=p/m. That´s how I find it, right?

Last edited: Apr 11, 2012
14. Apr 11, 2012

### tal444

Divide, not multiply ;)

Edit: Yes, that's correct :)

Last edited: Apr 11, 2012
15. Apr 11, 2012

### domyy

Thank you so much (again!) xD

Last edited: Apr 11, 2012
16. Apr 11, 2012

### tal444

You did most of the work yourself ;), nice work. Now I'm off to bed :zzz:

17. Apr 11, 2012

### domyy

have a good night! :)