Simple physics confusion

In physics, a system does work only when applying a force through a distance. In your example, you are applying a force, but through a distance of zero. Therefore, in terms of physics, you are doing no work. On a biological level, though, you are expending energy. Our bodies are built in such a way that merely keeping the muscles taut to apply a force, requires energy. Not being a doctor, I am unsure of the precise chain of events, but this energy undoubtedly ends up as heat.

Good question. This is a fundamental example of the consrvation of energy and of course the same amount of energy you put into lifting your briefcase must exist in other forms after the lifting.

Lifting a briefcase is a pretty simple action. The energy goes into two main places.
1) Most of the energy goes into "Potential Energy". Potential energy means that as the briefcase is now higher up in the air, it has more potential to fall faster when you drop it. Potential Energy can be calculated as (mass * height * gravitation_acceleration).

2) A small amount of energy will go into heat, both from your muscles and from moving through the air.

Now for potential energy... the beauty of physics is that the potential energy is conserved (i.e. the same amount of energy you originally put into the system) for anything you do in the future.

The briefcase is now higher in the air. Let's say you now drop it. Well, even though you don't add any additional energy, the briefcase will go faster and faster (gaining) kinetic energy. The amount of kinetic energy when it reaches the original position will be the same amount of energy you originally put in to raise it (minus small losses to heat and friction).

Now if at this point there is a floor the briefcase is going to crash. At this point the energy will be changed to heat, sound and a little bit of light. The amount of energy in the heat and sound and light that are released in the collision will be exactly the same amount you put in originally (pretty cool huh?).

If you keep following the process, the sound will hit a surface and turn to heat as will the light... so eventally you will get pretty nearly all heat.

But at each step in the process... and when all is said and done, the amount of energy, by law, will always equal the amount of food you burned to lift your briefcase.

Eric, he specified that he was holding the briefcase still.

Thanks Ebrown, you contributed some useful and interesting points but unfortunately those 2 segments were the 2 out of 3 that I already understood. What I don't understand is why it's required to exert force to hold it in place since that seems to defy physics as I understand them.

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Brandon, Ah, I see... it's because of the joint action of our body that the force must be provided to hold our body in position. whereas a solid statue would not be forced to expend any energy whatsoever.

This is why holding large weights on your head (and thus applying minimal torque or pressure upon our joints and muscles to bend out of shape) is significantly easier than holding large weights in your hand...

I should have considered things on a molecule by molecule basis (in which most phyics become clear) rather than considering the human body as a solid object which was my original problem. Thankyou for your help.

A non physics addition, then. Gravity acts the same as acceleration. A body at rest (sea level, 40deg latitude, etc.) has an acceleration of 1g. Your effort is offsetting that acceleration. Just as riding a bicycle up a hill at a constant speed represents a constant acceleration.

Well, ebrown and Brandon are here. Now Adrian can come along shoot holes in my little analogy, too.

Antibuddha,

From a physics perspective, you are making things more complicated than they need to be. You don't need to consider things on a molecule by molecule basis.

Brandon's initial answer is the key to your question and I think the basis of your confusion. The presence of a force does not mean a change in energy. As Brandon stated a force only results in a change of energy if there is motion across a distance.

Forget this whole muscle thing for now. It is needlessly complicated and has nothing to do with the basic question.

Consider a book on a table. The book is not falling even though gravity is pulling on it. This means that the table is putting a force on the book equal to its weight.

In this example there is a signifcant force, but there is no transfer of energy. There is no heat generated, no change in motion... nothing. Until it is upset, this book and table configuration will continue ... with a constant force but no change in energy... indefininately.

So the point is-- a change in energy happens when there is force over a distance. If there is no motion there is no change in energy.

OK Now, for completeness lets consider the muscle thing...the difficulty you have holding up a book for any amount of time.

For muscles (and only for muscles) it takes energy to exert a force.

Thisis is a biological problem with how your muscles work and has nothing to do with the basic physics involved. Remember our humble table can do this job just fine indefinitely with no expenditure of energy.

Incidently, energy is conserved even when you are just holding a book. In this case the energy is not going into the book (because the book is neither moving nor gaining potential energy).

When you use your muscles to hold up a book, all the energy you use turns into heat. Your muscles heat up when used.

Roger,

It seems you are talking about General Relativity. Even us physics nerds only invoke GR when it is necessary In this case Newtonian Physics suffices just fine. (Of course Newtonian physics is a subset of General Relativity, but why make things much more complex than needed.)

You are making the same mistake I address above by implying that "effort" (and I assume you mean expenditure of energy) is required to keep a body under the influence of gravity at rest.

The book on a table shows that this is clearly not true. I will withdraw this objection if under your definition, a table makes an "effort" to hold up the things on it. But, this doesn't sound right to me.

No, I don't intentionally change definitions to suit myself. The objection stands.

Trust me... I do. That's just me. Though I only need to consider the few interactions that do occur between each type of molecule in each type of situation and then multiply it by billions. It's not as bad as it sounds, I already know most of the biological interactions.

Having done so I now understand the situation.

That's what I had to do to understand the stupid "can you deliver more than X ammount of force to a taut piece of tissue paper with a punch?" question that I thought was extremely pointless and confusing in physics.



No, I understood after his initial answer. Sorry if my reply didn't establish that, I addressed each of your posts seperately as though I only heard it on its own... a bad, (and kind of strange) habbit I pick up on forums.



I can't really understand situations without including all parts of it (to a reasonable degree... I mean I can understand a book resting on a table without considering complex geothermal shifts in china's influence on american weather, but still, I understand systems better than I understand components).



The weight of the briefcase pulls down upon the hand which thus places a two way tension on the striated muscle inside (one pulling from its fixed attachment to the shoulder, the other the force upon the hand) thus stretching it as any stretchable material would against two forces. If you held the elbow flexed then your lower arm would be resisting torque and the upper arm resisting the downward pull of the lower arm.

In order to resist this distortion the actin continually pulls on the myosin with chemical bonds. This molecular movement is powered by ATP being converted to ADP (or ADP to AMP in extreme usage). This power is recharged first from Creatine Phosphate for a short time at extreme power (sufficient for very heavy weights), when this runs out the strength decreases potentially causing large ammounts of pain from muscle strain as the muscle can no longer resist distortion instinctively causing people to let go.

After the creatine phosphate the body recharges it from the water-glycogin pool. This reaction is anaerobic but produces lactic acid, buildup of which triggers your brain to experience soreness within the muscles, the burning sensation which is felt for a while.

Then finally when the glycogen runs out it switches to Aerobic reaction which triggers the panting and puffing and produces even less energy thus dropping strength once more and potentially causing muscle strain again.

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See, I get it

But thankyou everyone for your help, it finally sunk in...