Calculating Initial Momentum and Speed of Colliding Railroad Cars

[domyy]

Homework Statement

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?

Homework Equations

The formulas that I´ve used are:

Pi = pf = mv
vi = pf/m

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.

Are my answers correct?

 

[tal444]

They appear to be correct. Nice job.

 

[domyy]

Thankss! =D

 

[domyy]

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

1. Homework Statement

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. Homework 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.

Are my answers correct?

 

[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.

 

[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?

 

[domyy]

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

 

[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.

 

[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.

 

[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 ?

 

[domyy]

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

 

[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.

 

[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?

 

[tal444]

Divide, not multiply ;)

Edit: Yes, that's correct :)

 

[domyy]

Thank you so much (again!) xD

 

[tal444]

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

 

[domyy]

have a good night! :)