Help with equations for Momentum Question

[weasal1975]

Homework Statement

I have absolutely no idea how to setup the equation for the following homework problem.

2.3-kg cart is rolling across a frictionless, horizontal track toward a 1.5-kg cart held initially at rest. The carts are loaded with strong magnets that cause them to attract one another. Thus, the speed of each is increasing. At a certain instant before they collide, the 1st carts velocity is +4.5m/s and 2nd cart is -1.9m/s.

A) total momentum of the system of 2 carts at this instant
B) velocity of 1st cart when 2nd cart is at rest.

I don't want the answer, I can find those on google. I want to under which equations to use and why.

Homework Equations

The Attempt at a Solution

 

[CompuChip]

Let's start with A), hat will help you solve B) as well.

The equation for momentum is
p = m v,
where m is the mass and v the velocity.

The boldface letters mean that the quantities are vectors. If you don't know what that means, then all you need to remember is that they have a direction (3 m/s and -3 m/s are different things).

 

[weasal1975]

Thank you. Would I need to do this for both cars and then subtract?