Confusion over 2nd law and conservation of momentum

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Discussion Overview

The discussion revolves around the application of the second law of motion and the conservation of momentum, particularly in scenarios involving a moving cart and the addition or removal of weight. Participants explore how these principles apply to both linear and rotational motion.

Discussion Character

  • Exploratory, Technical explanation, Conceptual clarification, Debate/contested

Main Points Raised

  • One participant questions how a moving cart can accelerate when a weight is added, suggesting that conservation of momentum should cause the cart to slow down.
  • Another participant proposes that the effect on the cart depends on how the weight is added, specifically noting that if the weight is added at zero speed relative to the ground, it may not slow the cart down as expected.
  • There is a discussion about the forces involved, with one participant explaining that a force exists between the cart and the new weight, causing the weight to accelerate forward while the cart accelerates backward.
  • A follow-up question is raised regarding the scenario of removing the weight by lifting it straight up, indicating a desire to explore further implications of these principles.
  • Another participant asks for clarification on what is meant by "carefully" in the context of adding or removing weight, suggesting that the method of addition or removal could influence the outcome.

Areas of Agreement / Disagreement

Participants express differing views on how the addition or removal of weight affects the motion of the cart, indicating that multiple competing perspectives remain unresolved.

Contextual Notes

There are assumptions about the initial conditions of the weight being added or removed, as well as the definitions of "carefully" that are not fully explored, which may affect the conclusions drawn from the discussion.

Who May Find This Useful

This discussion may be of interest to those studying mechanics, particularly in understanding the nuances of momentum conservation and forces in dynamic systems.

steven george
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Hi everybody, I apologize if something like this has been asked before but I have been unable to find an answer through searching.

If a weight is carefully added to a moving cart then the cart should slow down due to conservation of momentum. How is it that the cart has accelerated without a horizontal force?

This issue originally came up while discussing rotational motion. If a figure skater is spinning and brings his hands in he will speed up. How can he accelerate without any torque?

There must be something very basic that I am missing here, so thank you to anybody who can help with an answer.
 
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steven george said:
If a weight is carefully added to a moving cart then the cart should slow down due to conservation of momentum.
It depends on how the weight is added. I'll guess: You add things at zero speed relative to the ground.
steven george said:
How is it that the cart has accelerated without a horizontal force?
There is a force between cart and new weight, speeding up the weight to match the speed of the car. The new weight accelerates forwards, the remaining cart accelerates backwards..
 
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mfb said:
It depends on how the weight is added. I'll guess: You add things at zero speed relative to the ground.There is a force between cart and new weight, speeding up the weight to match the speed of the car. The new weight accelerates forwards, the remaining cart accelerates backwards..

I knew there must be a simple answer. Thanks for that. Now, what if that weight is taken off by carefully lifting it straight up?
 
What do you mean by "carefully"? The answer may depend on this.
 
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