How to answer question on momentum?

  • Thread starter Sixty3
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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:

Answers and Replies

  • #2
tiny-tim
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Hi Sixty3! :wink:
(5x5)+(5x-1)= (5x1)+(5v)
25-5=5+5v
20-5=5v
v= 3m/s

Yes, that's fine. :smile:
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. :wink:
 

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