Momentum question: Object dropped onto a moving railcar

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Discussion Overview

The discussion revolves around the concept of momentum in the context of an object being dropped onto a moving railcar. Participants explore the implications of this scenario on the momentum of the railcar and the object, as well as the nature of the interaction between them during the drop.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants inquire about the momentum of the railcar and the dropped object before and after the collision, suggesting a need for clarity on the masses involved.
  • There is a question regarding the distinction between this scenario and a perfectly inelastic collision, indicating a potential overlap in concepts.
  • One participant asks whether the drop is considered from the perspective of the ground or the railcar, highlighting the importance of reference frames.
  • Concerns are raised about the orientation of the railcar and the object, with some participants questioning whether the railcar is indeed perpendicular to the ground.
  • A participant discusses the interaction between the dropped object and the railcar, suggesting that the object will exert a force on the railcar upon impact, which could affect its momentum.
  • Another participant notes that if the object moves at the same speed as the railcar after the drop, there must have been an interaction that caused this horizontal acceleration.
  • Some participants express confusion about focusing on the interaction with the side of the cart rather than the overall dynamics of the drop.
  • There is a mention of the conservation laws and their predictive power in analyzing the situation without needing to delve into detailed interactions.

Areas of Agreement / Disagreement

Participants express various viewpoints on the mechanics of the scenario, with no clear consensus reached on the specifics of the momentum changes or the nature of the interactions involved.

Contextual Notes

Participants have not fully resolved assumptions regarding the reference frames and the nature of the collision, which may affect their interpretations of the momentum changes.

kasheee7zark7
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Could someone please explain, in terms of the momentum, a rail car slowing down if an

item is dropped vertically downwards on to it. Thanks.
 
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Can you state the momentum of the cart (say its mass is ##M##) and weight (say its mass is ##m##) before and after the collision?
 
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What's the difference (if any) between this problem and a perfectly inelastic collision?
 
kasheee7zark7 said:
Could someone please explain, in terms of the momentum, a rail car slowing down if an item is dropped vertically downwards on to it.
Vertically in the rest frame of the ground or of the rail car?
 
The object and train are perpendicular to the ground.​
 

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kasheee7zark7 said:
The object and train are perpendicular to the ground.
Please show some work.

I doubt the train is perpendictular to the ground. Is it vertical and riding on its caboose?
 
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See the video below (question 3, 6 minutes 54 seconds from the start).

The explanation given describes the interaction between the vertical side of the cart and the rock dropped, but what if there is no interaction between the two and it is dropped, say, in the middle of the cart? While I am writing these words, I am thinking that at the moment it hits the cart, the rock will move to the right, creating a leftward force on the cart (using Newton’s 3rd law), slowing the cart down. Presumably the interaction here would the friction of the floor of the cart and the rock.

 
kasheee7zark7 said:
Presumably the interaction here would the friction of the floor of the cart and the rock.
If the rock ends up moving at the same horizontal speed as the cart, then there must have been some interaction that accelerated the rock horizontally. For the final speed of the cart the details of that interaction do not matter.
 
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Thanks for that. I was geting caught up on focussing on the interaction with the side of the cart.
 
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  • #10
kasheee7zark7 said:
Thanks for that. I was geting caught up on focussing on the interaction with the side of the cart.
The power of conservation laws is the ability to make certain predictions without having to know much about the details.
 
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