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Physics Question - ma = mg
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TimFox:
I agree about the confusion in English-language contexts between pound as unit of force or weight, and as unit of mass.  The usual statement on a US food package label is “net wt 1 lb 2 oz” (on a Cheerios box), which in strict physics language would be “mass that weighs 1 lb 2 oz at mean sea level”.  In metric lands, however, I have seen pressure gauges calibrated in “kgf/cm2” instead of N/m2 or Pa or dyn/cm2.
The infamous NASA mission failure a while back involved changing vendors on small rockets and misreading the force units.
By the way, my mnemonic for conversion between N and lb is to remember that 1 kg weighs 9.8 N and 2.2 lb (approximately).
RJSV:
Ok, Tim I think you've supplied some KEY info, to clarify portions of this thread.
   So, if I read right, a definite precise definition and use of 'MOTION' was given by Newton. Fantastic, even tho proves me wrong, somewhat.
   So then, as MOTION was given the precise definition, as what we today might call 'momentum'.
Then, will the units be kg-meters/ seconds (squared) ?
  Plus, maybe the English system direct equiv would be
   Slug-feet / second (squared) ?
Although I realize, maybe the English system avoids that form, and using 'pounds' as an equation substitute is ok as long as a standard is consistent, and even though the language structure isn't strictly correct?
   ?? Did I get that right ?

   As to relativistic effects, after 2 years college courses on CLASSIC Physics, I enrolled in a nuclear physics intro where the instructor described the so- called 'NEUTRINO PARADOX'.
   In this effect, Neutrinos streaming out from the sun would last too long: They were measuring too many reaching sea level, before the very short decay time kicked in. (They decay to mu-mesons).
   The answer came in time dialation where time ticks slower, in the neutrino moving at >90 % light speed.
But, again, I'm saying some problems in this thread are due to injecting relativistic effects too early, into a sort-of 'word salad'  including getting mixed up because MASS does increase (or equiv perceived mass) at relativistic speeds (90% light speed). So, increase in momentum, and now I realize no worries about calling it ' motion of a body', that increase gets all smushed into word salad that reverts to including relativistic mass increase (due to high speed).
WHEW, Thanks Tim, for clarifying your contribution!

But still,.  Nominal Animal for President
TimFox:
A couple of side comments about gravitational acceleration g and weight.
1.  Scales.  It was mentioned above that "scales" measure force or weight, not mass.  This is certainly true for spring scales (or modern units with strain gauges) that measure force as an extension of a compliant spring.  However, "balances" that compare the device under test to a standard mass/weight, using leverage as appropriate in a steelyard balance, compare the masses of the DUT and the standard, since the two will balance regardless of the local value of g, but the weight of the standard will change when you take the experiment to the Moon.
2.  Measurement of g.  It is not necessary to know the mass of the object in an experiment to measure the uniform gravitational acceleration in a limited region.  Galileo worked out the period of a simple pendulum around 1602, where the period is independent of the bob mass and the amplitude of the swing (so long as the amplitude is small enough to use the approximation sin(x) = x, the error is small), but depends on the length of the string.  The period T = 2pi x sqrt (L/g) in that limit of small amplitude.  A more precise experimental method is the Kater Pendulum, where the length in the experiment can be better defined.  See  https://en.wikipedia.org/wiki/Kater%27s_pendulum
TimFox:

--- Quote from: RJHayward on June 25, 2021, 11:17:44 pm ---Ok, Tim I think you've supplied some KEY info, to clarify portions of this thread.
   So, if I read right, a definite precise definition and use of 'MOTION' was given by Newton. Fantastic, even tho proves me wrong, somewhat.
   So then, as MOTION was given the precise definition, as what we today might call 'momentum'.
Then, will the units be kg-meters/ seconds (squared) ?
  Plus, maybe the English system direct equiv would be
   Slug-feet / second (squared) ?
Although I realize, maybe the English system avoids that form, and using 'pounds' as an equation substitute is ok as long as a standard is consistent, and even though the language structure isn't strictly correct?
   ?? Did I get that right ?

   As to relativistic effects, after 2 years college courses on CLASSIC Physics, I enrolled in a nuclear physics intro where the instructor described the so- called 'NEUTRINO PARADOX'.
   In this effect, Neutrinos streaming out from the sun would last too long: They were measuring too many reaching sea level, before the very short decay time kicked in. (They decay to mu-mesons).
   The answer came in time dialation where time ticks slower, in the neutrino moving at >90 % light speed.
But, again, I'm saying some problems in this thread are due to injecting relativistic effects too early, into a sort-of 'word salad'  including getting mixed up because MASS does increase (or equiv perceived mass) at relativistic speeds (90% light speed). So, increase in momentum, and now I realize no worries about calling it ' motion of a body', that increase gets all smushed into word salad that reverts to including relativistic mass increase (due to high speed).
WHEW, Thanks Tim, for clarifying your contribution!

But still,.  Nominal Animal for President

--- End quote ---

In Principia, Newton defined his terms before going into his laws.  In modern nomenclature, momentum (vector) is the product of mass (scalar) and velocity (vector),
P = m v, and has units of kg m/s, not s2.
Force is the product of mass and acceleration, and therefore has the units kg m/s2.  Kinetic energy E = (1/2) m v2, where all terms are scalars, has the units of kg m2/s2.
When you get relativistic, look up the meaning of "rest mass", which is invariant.
bostonman:

--- Quote ---It does make sort-of intuitive sense – at least, if you've ever compared the effort needed to do say pull-ups slowly versus fast.  I can do a dozen fast pullups (not the swingy ones – those are cheating via angular momentum –, nor using your legs to kick in the air to get some extra help; just fast but proper ones), but ask me to stay stationary midway, and I'll drop in a few seconds, much faster than it would take to do those pullups.  I think.  Need to verify at the gym (on separate days).
--- End quote ---

As someone pointed out, this thread has some confusion over people messing terms and units. Hopefully I'm not one of those people as I'm just trying to grasp (what I feel are) basic concepts that confuse me.

I try avoiding the human factor in all this because of the complexity, however, gym topics do help because it's where (at least) I do most "lifting".


--- Quote ---This longish thread has shown how much meat there is in even simple problems.  And how important it is to frame the question properly.  Things which seem obvious to those with long exposure can be either quite confusing or devilishly simple depending on presentation and/or point of view.

--- End quote ---

I think the "longish thread" is due to me asking the same question, but doing different activities that involve a different part of physics each time. As an example, I started off simply asking why ma=mg based off TBBT. I researched it before asking the question and kept seeing discussions that they are equal because of Earth. This didn't make sense to me because of what I've already stated and now I believe I understand why they are "equal". Then I deviated asking about lifting a box off the floor, and that got moved towards "Work".

As for speed of lifting something, this actually also came from TBBT. Sheldon and the group were moving a time machine up the stairs and (I believe) Howard said why don't we push faster. Sheldon replied stating Work done doesn't matter how fast you push the object.

That got me thinking because if I pick up a box 1m really fast, it feels like I've done far more work than if I lifted it slowly. Now if I lifted the same box 1m really fast ten times, I'd be out of breath whereas ten times slowly and I'd have far more energy at the end.

From what I understand due to this thread, one thing to consider is that I've gave the box kinetic energy picking it up faster because now it needs to transfer that kinetic energy when it stops at 1m than it would be if I picked it up slower. If I understand correctly, it would attribute to me being "out of breath" because I've transferred my kinetic energy to the box and then used energy to stop it at 1m.

It does seem acceleration should factor into some equation because if Ke = 1/2mv^2, then without acceleration, how did the object achieve velocity?

As for doing pullups, I think when discussing the "human factor", too many factors come into play as for getting out of breath and fatigue muscles. If I'm correct, say I use a wench and a motor to lift a box (i.e. no human interaction at all). Excluding friction on the pulleys, wire resistance, etc.. I imagine the Work done by the motor to lift the box would be equal to the Work done by a human to lift the same box the same height.

In other words, the calculation is identical, and I can safely say I burned X calories lifting the box ten times based off converting Joules (i.e. Work) into calories.

Getting back to lifting a box very fast ten times, at a distance of 1m, it's the same Work, however, now I've wasted energy because the box needs to stop at 1m thus the energy being thrown out the window; therefore I'd get tired sooner. On the other hand, if I lifted the box very slow, my arms would begin getting tired.
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