Questions about conservation of momentum

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Discussion Overview

The discussion revolves around the concept of conservation of momentum in the context of collisions within a closed system. Participants explore the implications of external forces on momentum conservation, particularly when one object is accelerating while another moves at a constant speed.

Discussion Character

  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions the meaning of having no external force on a closed system, particularly in scenarios involving collisions where one object accelerates.
  • Another participant asserts that an object cannot accelerate without an applied force and states that momentum is not conserved if there is an external force acting on the system.
  • A follow-up inquiry seeks clarification on whether the change in momentum is equal to the external force multiplied by the duration of its application.
  • One participant reiterates the initial question about the implications of external forces during collisions and whether it affects the ability to solve for final velocities.
  • Another participant explains that "no external force" implies that all objects travel at constant velocity between collisions and notes that while forces are present during collisions, the total change in momentum is zero due to Newton's third law.

Areas of Agreement / Disagreement

Participants express differing views on the role of external forces in momentum conservation, indicating that the discussion remains unresolved regarding the implications of these forces during collisions.

Contextual Notes

Participants have not fully defined the conditions under which momentum conservation applies, particularly in relation to external forces and the nature of interactions during collisions.

sgstudent
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What does having no external force on a closed system mean? For example if I have 2 objects colliding. One travels at a constant speed while the other travels with a constant acceleration. In this case is an external force being applied on the system?

If so, only the m1u1+m2u2=m1v1+m2v2 can be used if the two objects have a constant velocity?

Also, during the collision won't there be a force being applied on the object? So how would momentum be conserved? During the collision the formula F=change in mv/time is used. So is the conservation only after this collision?

Thanks for the help :smile:
 
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An object can't accelerate without some applied force. Momentum isn't conserved if you have an external force on the system. Actually, the change in total momentum is equal to the external force.
 
Khashishi said:
An object can't accelerate without some applied force. Momentum isn't conserved if you have an external force on the system. Actually, the change in total momentum is equal to the external force.

Oh, you mean the change in momentum is equal to the external force multiplied by the duration of it?

So if there is an external force, how will the collision be like? Or are we unable to solve for the final velocities.

Thanks :)
 
sgstudent said:
What does having no external force on a closed system mean? For example if I have 2 objects colliding. One travels at a constant speed while the other travels with a constant acceleration. In this case is an external force being applied on the system?

If so, only the m1u1+m2u2=m1v1+m2v2 can be used if the two objects have a constant velocity?

Also, during the collision won't there be a force being applied on the object? So how would momentum be conserved? During the collision the formula F=change in mv/time is used. So is the conservation only after this collision?

Thanks for the help :smile:

If the only interactions that you are considering are collisions, then "no external force" means that all objects travel at constant velocity between collisions.

During the collision, you're certainly right that there are forces involved. But Newton's third law implies that the total change in momentum due to a collision is zero. So the vectorial sum of the momenta of all the objects is the same before and after the collision. (Assuming once again that the only interaction is collisions).
 

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