Momentum Conservation & External Force: Explained with an Example

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Discussion Overview

The discussion revolves around the concept of momentum conservation in the presence of external forces. Participants explore the conditions under which momentum is conserved and provide examples to illustrate their points. The conversation touches on theoretical aspects, derivations, and interpretations of momentum conservation.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant questions why momentum is not conserved when an external force acts on a system and requests an example.
  • Another participant explains that momentum is defined as mass times velocity and that external forces change the velocity, thus changing momentum and preventing conservation.
  • A participant challenges the assumption that momentum should be conserved and asks if others understand the derivation of the conservation of momentum.
  • One participant states that the rate of change of momentum is zero only when there is no external force acting on the system.
  • Another participant asserts that momentum is conserved if the system includes all sources of external force, suggesting that conservation of momentum is valid under specific conditions.

Areas of Agreement / Disagreement

Participants express differing views on the conditions for momentum conservation, with some arguing that external forces prevent conservation while others contend that including all forces in the system allows for conservation. The discussion remains unresolved with multiple competing perspectives.

Contextual Notes

There are assumptions regarding the definitions of systems and external forces that are not fully explored. The derivation of conservation laws is referenced but not detailed, leaving some mathematical steps and implications unresolved.

Gurasees
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Why ain't momentum conserved when external force acts on the system?
Please explain with an example.
 
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Momentum is the mass of the object times the velocity. Force is defined as the mass of an object times the change in velocity. If the system is exposed to an external force, the velocity of the system changes. Since the velocity of the system changes, so too does the momentum hence it is no longer conserved.

If your system is a single billiards table with one ball and no momentum, applying a force to the table will cause the ball to move.
 
Why do you think it should be conserved? Do you know how conservation of momentum is derived?
 
weirdoguy said:
Why do you think it should be conserved? Do you know how conservation of momentum is derived?
dP/dt = 0 = F (external). hence P is constant.
 
Gurasees said:
dP/dt = 0 = F (external). hence P is constant.
Only time this is true is the case Fexternal = 0, hence no external force.
 
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Gurasees said:
Why ain't momentum conserved when external force acts on the system?
Momentum is conserved in a system that includes all the sources of your 'external force'.
That is the basis of COM. There is no contradiction or exception.
 

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