About expt re Newton's 2nd law

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Discussion Overview

The discussion revolves around an experiment intended to demonstrate Newton's second law, specifically the relationship between acceleration and resultant force on a constant mass. Participants explore different methods of applying force, including the use of elastic bands and pulleys, and debate the validity of these approaches in the context of the law being tested.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant proposes an experiment using elastic bands to demonstrate that acceleration is directly proportional to resultant force, given a constant mass.
  • Another participant questions the validity of using a pulley system to provide the resultant force, suggesting that it may not accurately reflect the principles of Newton's second law.
  • A participant argues that calculating the tension in the pulley system involves using Newton's second law, which they believe undermines the experiment's purpose of proving that law.
  • Another participant suggests an alternative approach by using the weight of the hanging mass directly to apply force, indicating that this method is more straightforward but still acknowledges the complexity of the situation.
  • There is a discussion about the equivalence of methods with and without calculating tension, with some participants expressing differing views on the implications of this equivalence.

Areas of Agreement / Disagreement

Participants express differing opinions on the appropriateness of using a pulley system to demonstrate Newton's second law. Some agree with the concerns raised about the use of tension, while others suggest that both methods can be valid under certain conditions. The discussion remains unresolved regarding the best approach to the experiment.

Contextual Notes

Participants highlight the assumption of a smooth pulley and the implications of using Newton's second law in the analysis of the pulley system, indicating potential limitations in the experimental design.

grzz
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hello all

Consider an experiment to prove that the acceleration in a certain direction on a given mass is directly proportional to the resultant force in that direction given that the given mass is constant. The acceleration can be measured using light gates. This resultant force can be varied by pulling on the given mass by elastic bands taking care to keep the extension of the band constant for a given resultant force.
I heard a discussion re this experiment where the resultant force was provided by hanging a mass on a pulley. i do not think this is correct Physics.
I would appreciate any reply whether I was correct in this.
Thanks.
 
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grzz said:
I heard a discussion re this experiment where the resultant force was provided by hanging a mass on a pulley. i do not think this is correct Physics.
What's your objection to it? It's not quite as direct as the example you gave. Certainly the force on the primary mass does not simply equal the weight of the hanging mass, if that's your point.
 
Doc Al said:
What's your objection to it? It's not quite as direct as the example you gave. Certainly the force on the primary mass does not simply equal the weight of the hanging mass, if that's your point.

Thanks for the reply.
My objection is, IMHO, that in finding the tension on the hanging mass which is the resultant force on the given mass ( we are assuming that contact between pulley and string is smooth so that pulley does not rotate) one has to use Newton's 2nd law itself. And we were always taught that in proving some law one cannot make use of that same law.
 
grzz said:
My objection is, IMHO, that in finding the tension on the hanging mass which is the resultant force on the given mass ( we are assuming that contact between pulley and string is smooth so that pulley does not rotate) one has to use Newton's 2nd law itself. And we were always taught that in proving some law one cannot make use of that same law.
I agree. Were I to do that 'experiment' I would not calculate the tension. Instead I'd use a force of m2g (the weight of the hanging mass) acting on the total mass of m1 + m2. Still not quite as direct as your first example, but workable.

But even if you calculate the tension, you can always think of it as applying Newton's 2nd law to predict the acceleration, then checking your prediction.

The two methods--with and without the tension calculated--are entirely equivalent, of course.
 
Last edited:
Now I have a better insight into this experiment.
Thank you.
 

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