II. Conservation of Momentum





Collision/3 Laws





Total Momentum
Before Interaction



Total Momentum
After Interaction


m1v1 + m2v2 = m1v1′ + m2v2





Momentum is only conserved
in a totally elastic collision

Totally Elastic Collision?





What is the momentum

of the big fish after it

 swallows the little fish?







-4 kgm/s

Total momentum before =
 Total momentum after




Ex 4) What is the magnitude of the total momentum of these carts?

Total momentum = m1v1 + m2v2

Total mom. =

(4.0 kg)(3 m/s)+6.0kg(-3.0m/s)


= 12. kgm/s + -18. kgm/s 


= -6.0 kgm/s






"Left train momentum

= 30 kgm/s.

What is the momentum

 of the right train?"

What's the total momentum before the collision?

After the collision?








Answer: -30 kgm/s





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