# How to answer question on momentum?

## Homework Statement

I am not sure wether I should take the value of velocity as a negative or positive in momentum questions.

A cart (cart 1) of mass 5kg is travelling to the right with a velocity of 5m/s and another cart (cart 2) of mass 5kg is travelling to the left with a velocity of 1m/s. After collision cart 1 is travelling to the right with a velocity of 1m/s. Cart 2 is also travelling to the right.

I need to find the velocity of cart 2 after collision. I basically need to understand when to take the velocity as a negative.

## Homework Equations

m1u1+m2u2= m1v1+m2v2

## The Attempt at a Solution

(5x5)+(5x-1)= (5x1)+(5v)
25-5=5+5v
20-5=5v
v= 3m/s

Last edited:

tiny-tim
Homework Helper
Hi Sixty3!
(5x5)+(5x-1)= (5x1)+(5v)
25-5=5+5v
20-5=5v
v= 3m/s

Yes, that's fine.
I am not sure wether I should take the value of velocity as a negative or positive in momentum questions.

From the example, you seem to be doing it right anyway …

you should always choose a direction to be the positive direction (in this case, you chose positive to be towards the right), and then all positions and velocities will be measured relative to that direction.