Why is it not possible to hold a weight for longer time?

[roser777]

It is probably a simple question. I am curious to know 'Why do we feel tired while holding a weight in our hand or heads for a longer time?' It is probably very simple question and most of the people can alswer it. But I am just curious to know the actual causes and their impact in terms of law of physics. Is it (tiredness/loss of energy) due to body metabolism or the net impact of laws of force, energy and mass etc. How does it behave when the same weight is placed on a stand? How long can one hold a weight in his hands?

 

[N_Molson]

Your question aims towards 2 fields : biology & physics.

There is a link, that is expressed in Newtons.

The newton is the SI unit for force; it is equal to the amount of net force required to accelerate a mass of one kilogram at a rate of one meter per second squared. In dimensional analysis, F = ma, multiplying m (kg) by a (m/s2), the dimension for 1 newton unit is therefore :

Your muscles need to provide enough mechanical work (force) to counter the gravity effect that applies on the object. This force can be expressed in Joules (energy unit).

The dot product of force and distance is mechanical work. Thus, in SI units, a force of 1 N exerted over a distance of 1 m is 1 N·m of work. The Work-Energy Theorem states that the work done on a body is equal to the change in energy of the body. 1 N·m = 1 J (joule), the SI unit of energy.

So, your muscles produce mechanical work, and for that, they need energy. This energy comes from the breaking of Adenosine-Tri-Phosphate molecular bonds, the consuption of H2O (water) and O2 (oxygen). There is the whole digestion process before that but, ultimately, energy for the muscles is stored in those ATP molecules. Also your muscles will fatigue over time, because while they burn energy, they reject waste (lactic acid) that needs to be washed away by the blood flow, which requires a time during which the muscle is at rest. Else the wastes accumulate to the point that the muscle can't get any energy or O2 and becomes inefficient. Training can make the muscles more resilient (endurance, different of strenght that is equal to build muscular mass).

When the object is placed on a stand, the stand opposes a force that is equal to the force generated by gravity. Each action causes a reaction of equal strength.

I hopes it answers your question, more or less.

 

[Moach]

it's worth noting that while any weight is perceived as "at rest" - in physics terms, it's actually accelerating :shifty:

that's because the "natural" state of things is to fall (agree with gravity) - and anything that causes an object not to fall is somewhat "disagreeing" with gravity.... which means that by "not falling", an object is under constant acceleration

on earth's surface - that's exactly 1G worth of accel.... and you're accelerating UP, not down :uhh:


it's easier if you think of things in more spacey terms.... imagine you're on the ISS - things wheigh very little up there, right? - well, that's not because there's no graviity (common misconception), but because the ISS is in a state of free fall

so while the station and everything abord is "agreeing" with gravity - no acceleration (practically) is required to keep things at a relative state of "rest"....

if our wildest wishes came true and we miraculously found ourselves aboard it, we'd find it takes no muscular work to hold an object at any position from ourselves - be it atop our heads or however discomfortably you can handle it

as long as it's stopped, you don't have to bear its weight - so you won't get tired


so think of it like this - "falling" is the only true state of non-acceleration an object can be in (hence zero-G) - anything that makes an object "not fall", must then be opposite acceleration, right?


and acceleration = getting increasingly tired, as far as your muscles are concerned

 

[Rtyh-12]

"falling" is the only true state of non-acceleration an object can be in (hence zero-G) - anything that makes an object "not fall", must then be opposite acceleration, right?

Well, that's right, but there's also the equal acceleration in the opposite direction, so the object is actually not accelerating at all...

 

[RisingFury]

Your muscles need to provide enough mechanical work (force) to counter the gravity effect that applies on the object. This force can be expressed in Joules (energy unit).

:facepalm::facepalm::facepalm:

Work is not force, force is not work. Force cannot be expressed in Joules and force itself isn't mechanical work.

W = F * s, you only do work if move an object against a force - if you tried to lift an object, you do mechanical work. If you hold it at the same altitude, no work is done.

Joule is a unit of energy or work.
Newton is a unit of force.

---------- Post added at 14:19 ---------- Previous post was at 14:10 ----------

it's worth noting that while any weight is perceived as "at rest" - in physics terms, it's actually accelerating :shifty:

that's because the "natural" state of things is to fall (agree with gravity) - and anything that causes an object not to fall is somewhat "disagreeing" with gravity.... which means that by "not falling", an object is under constant acceleration

No, when an object is at rest in every moment, it's at rest. If an object is being supported by another object, it's not accelerating.

a = F / m, so when we take into account equal and opposite forces...
a = (F - F) / m = 0

it's easier if you think of things in more spacey terms.... imagine you're on the ISS - things wheigh very little up there, right? - well, that's not because there's no graviity (common misconception), but because the ISS is in a state of free fall

so while the station and everything abord is "agreeing" with gravity - no acceleration (practically) is required to keep things at a relative state of "rest"....

The astronauts are only at rest in the reference frame of the ISS. If you look in the reference frame of the ISS, than there is NO gravity. If you look from the reference frame from where there is gravity, then both the ISS and the astronauts inside are accelerating downwards - falling.

 

[Moach]

No, when an object is at rest in every moment, it's at rest. If an object is being supported by another object, it's not accelerating.

a = F / m, so when we take into account equal and opposite forces...
a = (F - F) / m = 0


(...)

The astronauts are only at rest in the reference frame of the ISS. If you look in the reference frame of the ISS, than there is NO gravity. If you look from the reference frame from where there is gravity, then both the ISS and the astronauts inside are accelerating downwards - falling.

yes well, it's all relative isn't it?


but back in the reference frame which is more relevant to the original question - an object rested on yoour hand is still undergoing acceleration, if not it would be falling...

and that acceleration over time implies the "work" your muscles must put up, and thus - get tired from it (which for some happens faster than for others)

 

[N_Molson]

:facepalm::facepalm::facepalm:

Work is not force, force is not work. Force cannot be expressed in Joules and force itself isn't mechanical work.

Hey, please consider that given the question, that was an attempt to give a "simplified" explanation. Now obviously that's not a course of physics, so you would be kind to avoid the triple-facepalm, thanks. A "This is a little more complex than that" would have been a lot less offensive (and I would have gladly added a "thanks" to your complement of explanation).

From what I understood from the original question, a bunch of equations isn't going to help a lot.

We need to get a little more civil, because, honestly, I told myself before posting "I'm going to be shot down because this is a far too simple explanation". That gets a little irritating, sometimes. :dry:

 

[RisingFury]

but back in the reference frame which is more relevant to the original question - an object rested on yoour hand is still undergoing acceleration, if not it would be falling...

and that acceleration over time implies the "work" your muscles must put up, and thus - get tired from it (which for some happens faster than for others)

Again, wrong.

Force of gravity F = m * g, force of your hand on the object F = -m * g, total force F = m * g - m * g = 0, so no acceleration.

Acceleration over time doesn't imply anything. Force multiplied by distance traveled is work. Your muscles spend energy because for them that's the only way of generating the force. Kinda like a rocket that hovers in the air. It's not gaining or losing any potential energy, but it still has to burn fuel, because that's how it produces force.

Hey, please consider that given the question, that was an attempt to give a "simplified" explanation.

Simplified explanation should still be correct. You made the mistake of trying to explain physics without knowing the underlying concepts.

So here's the simplest answer I can think of:
- Gravity exerts force on an object.
- To counter the force, muscles use energy, which is inherent in their construction.

 

[Moach]

Again, wrong.

Force of gravity F = m * g, force of your hand on the object F = -m * g, total force F = m * g - m * g = 0, so no acceleration.

Acceleration over time doesn't imply anything. Force multiplied by distance traveled is work. Your muscles spend energy because for them that's the only way of generating the force. Kinda like a rocket that hovers in the air. It's not gaining or losing any potential energy, but it still has to burn fuel, because that's how it produces force.

well, from a classical mechanics standpoint, then indeed, no acceleration is taking place - but from what Eistein says, free fall IS an inertial state of rest - look: http://en.wikipedia.org/wiki/Universality_of_free_fall#Development_of_gravitation_theory


by that logic - whatever holds an object in place against gravity, is the sole provider of a constant acceleration, without which, the object would proceed to "not accelerate" towards the ground

Whenever an observer detects the local presence of a force that acts on all objects in direct proportion to the inertial mass of each object, that observer is in an accelerated frame of reference

in other words - it's not the object that's falling, it's you that is not


and mind the "quotes" - i did not mean "work" as a unit of measure, but as a literal reference to the physical effort involved there

 

[Grover]

basically, any use of the muscles strains them. they tear suprisingly easily, but never all the way across, rather a series of small tears running in the same direction as the muscle. when the muscle repairs these, they grow bigger to compensate for the work they do (adjusting to your normal lifestyle)

its these tears that make it ache after a long time, and the lack of energy (as mentioned above) that make them tired as they start to respire anaerobically (i think) to produce the energy they need

 

[roser777]

Thanks everybody, really very interesting discussion.

The discussion so far suggest that Gravity exert force on the muscles. To counter that force the muscles burn energy derived from breaking of ATP molecules. The muscles get tired first becouse of the decreased levels of energy released over time and secondly becouse of series of tearing of muscles in the process and their repairing and growing bigger to compensate for the work they do (adjusting to your normal lifestyle) Ita is this tearing of the muscles that make it ache after a long time. The reduced level of energy at subsequent stages make them tired.

It suggest that tiring and aching of muscles is mainly due to biological reasons and the time element has no effect so far as application of laws of physics is concerned in this case.
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