Conservation of momentum is valid here?

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Discussion Overview

The discussion revolves around the principle of conservation of momentum in the context of a car coming to a stop. Participants explore whether momentum is conserved during this process and what happens to the car's kinetic energy, addressing both theoretical and conceptual aspects.

Discussion Character

  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • One participant questions why momentum is not conserved when a car comes to a normal stop.
  • Another participant notes that the momentum before the stop is decreasing and becomes zero after the stop.
  • There is a challenge regarding the principle of conservation of momentum in this scenario.
  • Some participants argue that momentum is conserved only when no external forces act on the system, citing the external forces from the road or brakes as reasons for the lack of conservation during stopping.
  • One participant suggests that if the entire car-earth system is considered, momentum should be conserved.
  • There is acknowledgment that the car's momentum is zero after stopping and non-zero before stopping, leading to confusion for learners of the concept.
  • Another participant mentions that the kinetic energy of the car is lost as heat to the car and the surrounding environment.

Areas of Agreement / Disagreement

Participants express differing views on whether momentum is conserved during the stopping process, with some asserting that it is not due to external forces, while others suggest that considering the broader system may lead to conservation. The discussion remains unresolved regarding the application of the conservation principle in this context.

Contextual Notes

Participants highlight the dependence on definitions of the system and the role of external forces, but do not resolve the implications of these factors on the conservation of momentum.

Mohammed Shoaib
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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?
 
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What is the momentum before and what is the momentum after the stop?
 
Before stop the momentum is decreasing and after the stop it is zero.
 
Then what about the principle of conservation of momentum?
 
Yes, what about it? What are your thoughts?
 
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.
 
And the kinetic energy of the car is lost as heat to the car and the surrounding.
 
Mohammed Shoaib said:
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.
And if you consider the entire car-earth system?
 
Then momentum is conserved. Is it not?
 
  • #10
It sounds like you got it.
 
  • #11
Mohammed Shoaib said:
Then momentum is conserved. Is it not?
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.
 
  • #12
Orodruin said:
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.
Thanks for your kind help.
 

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