About expt re Newton's 2nd law

AI Thread Summary
The discussion centers on an experiment designed to demonstrate Newton's second law, specifically the relationship between acceleration and resultant force on a constant mass. One participant questions the validity of using a pulley system with a hanging mass to provide the resultant force, arguing that it requires applying Newton's second law to determine tension, which contradicts the principle of proving the law itself. Others acknowledge that while calculating tension may complicate the experiment, it is still possible to apply Newton's second law to predict and verify acceleration. Ultimately, the conversation highlights different approaches to conducting the experiment while maintaining the integrity of the physics involved. The exchange leads to a clearer understanding of the experiment's execution.
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.
 
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Now I have a better insight into this experiment.
Thank you.
 

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