# Homework Help: Momentum problem: 2 carts, finding initial speed

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1. Sep 21, 2016

### emily081715

1. The problem statement, all variables and given/known data
A 2.0-kg cart collides with a 1.0-kg cart that is initially at rest on a low-friction track. After the collision, the 1.0-kg cart moves to the right at0.50 m/s and the 2.0-kg cart moves to the right at0.30 m/s .If the positive direction is to the right, what was the initial velocity of the 2.0-kg cart?

2. Relevant equations
i know that it is a momentum problem so i am assuming we will use p=mv

3. The attempt at a solution
i originally tried solving for the initial velocity by setting the equation up like this; m1(v2-v1)= m2(v2-v1). when plugging in all the variables and isolating for V1 of the 2.0kg cart i got 0.05 m/s. that is not the correct answer and i am unsure is i missed steps or am doing the problem completely wrong

2. Sep 21, 2016

### Orodruin

Staff Emeritus
Is momentum conserved if this equation is fulfilled? The equation states that both carts get the same change in momentum.

Also, it is not clear what you mean by v1 and v2.

3. Sep 21, 2016

### emily081715

i mean intial velocity for V1 and finial velocity for V2. truthfully i am very confused how to go about this problem

4. Sep 21, 2016

### Orodruin

Staff Emeritus
Well, you have two objects with velocity before and after so this notation is still ambiguous. I suggest you try to think of what I said in the previous post.

5. Sep 21, 2016

### Staff: Mentor

What is the final momentum of the two carts?

6. Sep 21, 2016

### emily081715

would the final momentum
would the final momentum be 1.1? i added the momentum of each cart after the collision

7. Sep 21, 2016

### haruspex

In the units being used, yes. But you should always specify the units.

8. Sep 21, 2016

### Staff: Mentor

Okay. Now if V is the initial velocity of the mass that is moving, in terms of V, what is the initial momentum of the combination of two masses?

9. Sep 21, 2016

### emily081715

i'm a little confused by the question. wouldn't the initial momentum have to be 1.1 for the initial since the one cart is at rest? so is the initial then 1.1?

10. Sep 21, 2016

### Staff: Mentor

Okay. Now, what did its velocity have to be for this to be its initial momentum?

11. Sep 21, 2016

### emily081715

would it need to be 0.55?

12. Sep 21, 2016

### emily081715

actually that's the correct answer, thanks for the help and helping me through

13. Sep 22, 2016

### Orodruin

Staff Emeritus
Please use units. The number 1.1 has no meaning as a momentum without a specification of the units used. They could be kg m/s (as they should here), but without specifying we cannot know this and you may as well be referring to solar masses * light years / hour.

Always specify the units!

14. Sep 27, 2016

### Staff: Mentor

Khan academy has many good videos on physics. Here's a list of the ones related to momentum: