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I Conservation of momentum is valid here?

  1. Nov 22, 2016 #1
    Question says" When a car comes to a normal stop its momentum is not conserved. Does this violate the principle of conservation of momentum? Explain. What happens to the car's kinetic energy?"

    My question is why momentum is not conserved when a car comes to a normal stop?
     
  2. jcsd
  3. Nov 22, 2016 #2

    Orodruin

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    What is the momentum before and what is the momentum after the stop?
     
  4. Nov 22, 2016 #3
    Before stop the momentum is decreasing and after the stop it is zero.
     
  5. Nov 22, 2016 #4
    Then what about the principle of conservation of momentum?
     
  6. Nov 22, 2016 #5

    Orodruin

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    Yes, what about it? What are your thoughts?
     
  7. Nov 22, 2016 #6
    Momentum is conserved when no external force acts on the system. When the car comes to normal stop, the external force from the road or from the brake acts on the car to stop. Hence, the conservation of principle is not valid in this process. Am I right.
     
  8. Nov 22, 2016 #7
    And the kinetic energy of the car is lost as heat to the car and the surrounding.
     
  9. Nov 22, 2016 #8

    Orodruin

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    And if you consider the entire car-earth system?
     
  10. Nov 22, 2016 #9
    Then momentum is conserved. Is it not?
     
  11. Nov 22, 2016 #10
    It sounds like you got it.
     
  12. Nov 23, 2016 #11

    Orodruin

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    Yes, it should be. Yet the car momentum after is zero and the one before non zero.

    This is a conundrum for many people who are just learning conservation of momentum.
     
  13. Nov 24, 2016 #12
    Thanks for your kind help.
     
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