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Physics Question - ma = mg

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SiliconWizard:

--- Quote from: TimFox on June 25, 2021, 05:57:09 pm ---An accelerating force is that in Newton's First Law, usually stated now as "an object at rest will stay at rest, and an object in motion will stay in motion unless acted on by a net external force".

--- End quote ---

I just do not like this term "accelerating force" much, hence my remark.

But as to what I said about weight and freefall, and that was also mentioned later on in the thread, I think it's an important, yet not always fully understood point. And not so trivial after all.

It's actually the thought of weight having virtually no effet in freefall that led Einstein to formulate the equivalence principle (that, true, was something already more or less "known" before, but in a more restricted way), and eventually led to the theory of general relativity. So, even though it's in itself nothing extremely special when you know Newton's laws, the idea is still key in "recent" physics history.

What Einstein noticed thinking about weight in freefall is that, in short, the effect of gravity, for an object in freefall, would sort of cancel gravity. That was seminal enough to have inspired the theory that followed...

TimFox:
Should a force effect an acceleration, pursuant to Newton's Second Law, what should one call it?
If a strong wind damages your property, it is called a "damaging wind", etc.

bostonman:

--- Quote ---I just do not like this term "accelerating force" much, hence my remark
--- End quote ---


Certain terminology is confusing, especially when you've spent your entire life having a specific naming convention. As an example, Weight has always been known to me as weight, but it's technically Mass. Then you say you're picking up X kg of weight, but you're really using Newtons. Personally it's hard for me to comprehend that a car hitting a wall is X Newtons, but it's easier to think of some logical comparison like it's the equivalent of bench pressing 1000lbs.

Anyway, just some input about how spending a lifetime using wrong terminology can get confusing when you need to learn the correct way.

Acceleration can be confusing. This entire time we've talked about picking up stuff, so "acceleration" is 9.8 due to gravity. Now what if I'm pushing an object (leaving out anomalies like friction)? I need to accelerate the object in order for it to move, but in this case, am I applying Kinetic energy to perform Work instead of using an acceleration formula?

Another Work related question, if I walk on a treadmill at a normal pace, I believe it will read approximately 200 calories are burned (not that important) - obviously this is the computer doing a theoretical calculation based off distance. If I decide to burn calories by picking up a 25kg object several times at a distance of 0.5m (let's forget about Work exerted by putting down the object), that is 245N, which is 122J of Work.

Due to laziness, I used an online conversion calculator and got 29 calories burned just by picking up that object once (although it seems high to me).

Does this mean I can save myself an hour on the treadmill by picking up that item seven times?



TimFox:
I must refer you to a good freshman physics textbook to keep the technical terms straight.
For example, you don't "apply kinetic energy"  to accelerate an object, you apply a force to accelerate the object, which thereafter has kinetic energy.
Similarly, weight is not mass.  There is confusion in the way that weight is used in commerce (at the deli counter, for example) for what is really mass.
If I take 1 kg of sausage from the deli and carry it to the moon, it no longer weighs 9.8 N.
Questions about calories (technically, kilocalories) burned during exercise are not Newtonian physics, but physiology:  complicated biological processes.
In your example of lifting a weight, the (Newtonian physical) energy for one lift is the force (245 N) times the distance of the lift (0.5 m), and your body does 122 J of work.
If you lift the weight very slowly, so the acceleration is negligible, then after one lift, the object has 122 J of potential energy, and zero kinetic energy.  If you then drop it, it will accelerate downwards and will have 122 J of kinetic energy after dropping 0.5 m, illustrating the conservation of energy.

CatalinaWOW:
I agree with TomFox that you need time with Resnick and Halliday or other good introductory physics course.

While improper nomenclature is hurting you, improper concepts are your bigger problem.  It will be worthwhile for you to do the experiments described in the text and work the end of chapter problems.  In general if you find the problems hard it is because you didn't understand the presented concepts. Go back and reread and redo the experiments.

Think of your body as a very inefficient car.  It burns something like 1000-1500 calories a day sitting in a chair doing nothing.  The work perform lifting a weight or walking a treadmill is almost negligible.

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