A Question
Chong Wenhoo says:may i noe how much is the energy expended in the person's arm muscle to carry that load for 1 hour, standing stationary in that position?
Gayshot says:
erm, zero
Chong Wenhoo says:
then how come the arm will ache?
Chong Wenhoo says:
eventually will tremble and shake like hell, that arm
Gayshot says:
hmmm, u need to exert a force
Chong Wenhoo says:
AH U DUNNO RIGHT
Gayshot says:
but no work is done
Chong Wenhoo says:
yah so i'm asking u, how much energy is expended
Gayshot says:
by conservation of energy, net change in system energy = zero
Gayshot says:
theorectically. but den hor in reality
Gayshot says:
the person will lose heat energy due to friction all these etc
Gayshot says:
so he loses energy. slowly.
Chong Wenhoo says:
why is it that if the load is heavier his arm lose energy faster?
Gayshot says:
got friction greater ma, the load will tend to slip from ur hand.
Gayshot says:
haha. i dunno. i specualting. [<---hahahaha spell wrongly hahahaha]
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(some lines edited to save space)
So...help please?
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10 Comments:
ey well i THINK:
if u talk about KE in object, then its zero. But it'll have GPE cuz its some distance frm the earth's surface
as for the arm, i think my lecturer got say something about having tension in the arm's muscles la. Anyway if u think about it, ur arm is exerting a constant upward force to overcome the weight of the object held. U use energy to exert that force, since the muscles have to be tensed up to overcome the extension caused by the weight of the object. As for how much energy is used.. duno. Biomechanics anyone?
4:18 PM
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4:41 PM
aye, i think u all crazy lor... Work done is defined as force applied * displacement in that direction... it has nothing to do with whether u feel tired or not... different context, different definitions, cannot link together...
if u dunno wat i toking abt, imagine pushing sth from point A to B and den back frm B to A, wat is the work done? ZERO. Y? displacement = 0. (Edchong :"DEN Y THE HELL I FEEL TIRED?") simply becos physics never say when work done is zero u wun feel tired, it juz says when work done is zero, displacement is zero/the force applied to bring abt that displacement is zero.
Den u say but W.D. is in joules, energy expended is in joules also, where does the energy i expend go to? well, clap clap, beats me... i'm not a bio student...
But aniwei wat i'm trying to say is u shouldn't equate feeling tired(or energy expended) to doing work (some aguas feel tired even when they dun do anything...)
and there is indeed energy expended when u carry the 1 kg load... where it goes to? the energy is used to sustain ur muscle contraction...
4:44 PM
I think I understand what u(triceo) are saying, but I think you never read what I posted properly. The thing is, neither csq or me used the phrase "work done" or the concept of work done in any of our sentences at all.
7:55 PM
A to B and den back frm B to A, wat is the work done? ZERO. Y? displacement = 0.
I believe that is wrong. Work done is a scalar quantity although it is calculated from 2 vectors. Here we are talking about 2 different forces (the force to move from a to B, and the force to move from B to A) , so the total work done is actually Force1 * (B-A) + Force2 * (A-B)
Yay. Hope I'm not wrong.
8:02 PM
Ok i try to answer. You pull your penis hard. Pain a not? To hold the the thing you have to exert a for equal to the weight of the object. By Newton's Third Law the object exerts an equal and opposite force on your hand. The object pulls on your hand or specifically the muscles on your hand. So your hand pain lah. Your muscle keep on getting pulled where got no pain. pain then tremble lah, that's the body's natural reaction.
In terms of energy (I try to bullshit), when the thing pulls on your hand, your muscle cells are continually dying and being replenished. The muscle cells have a higher rate of metabolism to replace or repair the dead or damaged cells. More heat energy is produced and lost, so your hand tremble n feel damn shiok.
8:14 PM
haha...heat heat heat. the universal solutionto any thing that principle of conservation of energy ... though i tink it is the correct ans.
How this heat comes about is debatable. maybe it comes because ur body needs to form or break certain chemical bond to generate force. Think enthalpy change. Coincidentally ATP which releases energy needs energy to get it going. ( high activation energy )
Of cos i dunno wat realli happens in the body. For those of u that know, maybe entropy plays a part too... even friction between surface also lose energy. so hor, heat is loss in a lot of ways as ur hand attempts to create this force.
It may be possible to view the hand as a spring. The elastic potential energy in the stretched /contracted muscle has ability to generate force and do work, but it only obtains its energy from chemical processes with the body. so ur body exhaust important chemicals to produce this energy which is slowly leached away in the form of thermal energy. So yar, the energy in u get less n less.
9:44 PM
Gayshot says:
but no work is done
as to weichung's workdone is a scalar, oh yar hor... hahaha
10:42 PM
triceo: ok my mistake. Sorry. But the point is that he never used work done to explain anything to me. And u assumed that we were talking about using work done to explain it, when neither of us were.
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korlyn: eh u want your physics notes back?
11:50 PM
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