Momentum Conservation & External Force: Explained with an Example

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Momentum is not conserved in a system when an external force acts upon it, as the force causes a change in velocity, which in turn alters the momentum. For example, applying a force to a billiards table moves the ball, demonstrating that momentum changes when external forces are introduced. Conservation of momentum applies only in closed systems where no external forces are present. To maintain momentum conservation, all sources of external forces must be included in the system's analysis. Thus, momentum conservation is upheld only when external forces are absent or accounted for.
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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