Momentum: Same for 200lb Barbell vs Rubber Bands?

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Discussion Overview

The discussion revolves around the concept of momentum, specifically comparing the momentum of a 200 lb barbell to that of rubber bands providing the same resistance during a bench press exercise. The conversation explores theoretical implications and intuitive understandings of momentum in different contexts, including the effects of mass and force.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant questions the assertion that both the barbell and rubber bands would have the same momentum, suggesting that the rubber bands would result in almost zero momentum due to their elastic nature.
  • Another participant agrees with the initial questioning and seeks clarification on how momentum is defined if not as mass times velocity.
  • A different perspective introduces a formula for momentum that incorporates force and gravitational acceleration, suggesting that under certain conditions, the effective momentum could be equivalent for both systems.
  • Concerns are raised about the variability of force provided by rubber bands compared to the constant force of a barbell's weight.
  • One participant asserts that the original questioning participant's understanding of momentum is more aligned with physics than that of their mathematically inclined friend.

Areas of Agreement / Disagreement

Participants express disagreement regarding the definition and implications of momentum in the context of the two scenarios. There is no consensus on whether the momentum of the barbell and rubber bands can be considered equivalent.

Contextual Notes

The discussion highlights potential limitations in understanding momentum, particularly regarding the definitions of mass and force in different contexts, as well as the implications of variable versus constant forces.

Ron77
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Hi Folks,

Question on momentum.

I have been talking with someone who is more educated than I, especially in math, but they are saying something that just doesn't make sense to me. I do realize that physics doesn't always make 'sense' so thought I'd ask on here. He said I would need calculus to understand why, but the answer, almost seems intuitive...

Here it is..

He says that both of these will have the same momentum even though one has about 20 times the mass of the other and V is the same.

1) A person laying on their back does a bench press exercise with a real barbell weighing 200 lbs at a certain velocity

2) Another person laying on their back does a bench press exercise at the same velocity, but the resistance is provided by rubber bands that provide 200 lbs of downward tension. (like a Soloflex exercise machine)

I am thinking that number 2 will have almost zero momentum. It seems that no matter how fast you moved your arms, if you stopped, the 10 pound bar with the bands attached would 'stick to your hands' so to speak, but an actual weight would have the tendency to 'keep in motion' moreso.

Also, I read that the definition of momentum is on the order of how hard it is to change the direction of an object. One could easily reverse the direction from traveling upward to downward if tension is provided by elastic materials, but if provided by a real weight with mass, it would be harder to change from upwards, to a downward movement... right?



Thanks,
Ron
 
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Ron77 said:
I am thinking that number 2 will have almost zero momentum. It seems that no matter how fast you moved your arms, if you stopped, the 10 pound bar with the bands attached would 'stick to your hands' so to speak, but an actual weight would have the tendency to 'keep in motion' moreso.
Your thinking is correct. So how does your friend define momentum if not mass times velocity?
 
krab said:
Your thinking is correct. So how does your friend define momentum if not mass times velocity?

I'm not sure, he said "Your using that formula?" Then said I needed to understand calculus for him to explain.
The only thing that throws me is he's one of the smartest and most knowledgeable people I've ever met, so if we differ, I always assume he's right and I'm wrong.
This one just bugged me as I am usually far above average with mechanical and spatial problems, and I could almost 'feel' how momentum would be by just thinking about it..
 
Well, there is an equivalence between the two.

Define momentum as p = m v, where m is mass of the object in motion and v is the velocity.

But what is m, or rather the effect of m?

Let's define m as F/g, or the force, F, divided by the local gravitational acceleration.

Then p = F/g v.

F can be provided by mg or kx, where k = spring constant and x is the displacement, so if one could devise a way so that F/g = kx over the range of x, then they would have the same effective momentum.

so one could also say p = (kx/g) v.

As far as the person doing the work there is no difference.

In reality though, kx provides a variable force F, whereas mg is constant. I am not familiar with Soloflex and other systems so I am not sure show they compensate for this.
 
youre mathematically inclined friend is not in touch with physics
you re thinking is more correct than his.
 

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