Take the 2-minute tour ×
Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It's 100% free, no registration required.

I read the definition of work as $$W ~=~ \vec{F} \cdot \vec{d}$$ $$\text{ Work = (Force) $\cdot$ (Distance)}.$$
If a book is there on the table, no work is done as no distance is covered. If I hold up a book in my hand and my arm is stretched, if no work is being done, where is my energy going?

share|improve this question
4  
Good question! It's really a bit of biophysics, and it's not immediately obvious. –  Noldorin Dec 16 '10 at 16:37

8 Answers 8

While you do spend some body energy to keep the book lifted, it's important to differentiate it from physical effort. They are connected but are not the same. Physical effort depends not only on how much energy is spent, but also on how energy is spent.

Holding a book in a stretched arm requires a lot of physical effort, but it doesn't take that much energy.

  • In the ideal case, if you manage to hold your arm perfectly steady, and your muscle cells managed to stay contracted without requiring energy input, there wouldn't be any energy spent at all because there wouldn't be any distance moved.

  • On real scenarios, however, you do spend (chemical) energy stored within your body, but where is it spent? It is spent on a cellular level. Muscles are made with filaments which can slide relative to one another, these filaments are connected by molecules called myosin, which use up energy to move along the filaments but detach at time intervals to let them slide. When you keep your arm in position, myosins hold the filaments in position, but when one of them detaches other myosins have to make up for the slight relaxation locally. Chemical energy stored within your body is released by the cell as both work and heat.*

Both on the ideal and the real scenarios we are talking about the physical definition of energy. On your consideration, you ignore the movement of muscle cells, so you're considering the ideal case. A careful analysis of the real case leads to the conclusion that work is done and heat is released, even though the arm itself isn't moving.

* Ultimately, the work done by the cells is actually done on other cells, which eventually dissipates into heat due to friction and non-elasticity. So all the energy you spend is invested in keeping the muscle tension and eventually dissipated as heat.

share|improve this answer
    
Just like real scenario , what happens in solid objects like table. From where does the molecules get energy to remain in straight and hold up the weight of book –  LifeH2O Dec 16 '10 at 20:32
7  
For the table, the situation is different, because the molecules of the table aren't constantly "relaxing" and "contracting". Once you place the book onto the table, the atoms are pushed in a little bit (depending on how sturdy the table is) and settle into a new equilibrium due to electromagnetic and nuclear forces. –  Lagerbaer Dec 16 '10 at 22:42
3  
Unlike your arm, the table does not need to spend energy to hold up the book. It is a completely immobile system, as in: the table is a rigid object that needs no energy to stay still. You were correct when you said that you need to spend some energy to keep the book lifted, but that's because of how your muscles worked. You are not correct in saying that the table needs energy to keep the book lifted. All the table needs to do is apply Force, but applying force does not necessarily mean spending energy. –  Malabarba Dec 16 '10 at 22:45
    
A table has been crafted into a certain shape, restricted from the start by the bounds of nature, and so a table is known to be at rest when it is standing upright. It is comprised of molecular structures which have found their existence to be the most efficient when their molecules, when combined, provide enough weight distribution through its legs to carry its top. –  Danjah Dec 17 '10 at 21:03
6  
Let me just add that this is exactly the reason why rock climbers on steep walls must try to have their arms straight: Rather than hanging on bent muscles, which costs lots of energy, they hang on their body structure. Analog to the arm/table example. –  Lagerbaer Aug 11 '11 at 3:29

When contracted, the sarcomeres, the structure that actually do the work in a muscle, take turns doing the work. Only a third of them are engaged at any given moment.

This is because the sarcomere pumps blood as it contracts and relaxes, enabling it to get the energy it needs to do its work for longer periods. The temporary, superhuman strength some people experience may be some sort of override of this normal level of engagement.

This system doesn't have a different mechanism for holding a position, so the same thing goes on when trying to hold an object steady.

But if the muscle is contracted for a very long time and the energy in the blood being pumped becomes insufficient, sarcomeres will actually get stuck in their contracted position. This state doesn't require energy and the sarcomere will remain contracted until the load stops and normal circulation is restored.

I believe this is a survival mechanism that enables an animal to hang on, even when the load would otherwise be overwhelming.

It also can cause muscle stiffness when circulation through a muscle is impaired, a very common condition as people age.

share|improve this answer

Consider an analogy,

We get tired after STANDING for some time,without doing any work*. The reason behind this is same as the reason of why we dont do any work holding any object above our heads, but this case is easier to comprehend,

when we stand we r actually resisting the tendency of falling on the ground,muscles are holding on to the structure of our body so that we dont collapse on the ground like some non living thing,

these muscles have fibers which have have streached themselves ,which requires energy,

Similarly when we hold something above our head we r doing the same thing, resisting that collapsing tendency , which causes elongment in the muscles which requires energy.

share|improve this answer

The reason is that you need to spend energy to keep muscle stretched.

The first thing you need know is that the work $W=F \Delta x$ is the energy transfer between objects. Hence, there are no work done on the book when it is put on the table because there are no movement.

When your arm muscle is stretched, however, it consumes energy continuously to keep this state so you feel tire very fast. This energy comes from the chemical energy in your body and most of them are converted into heat and lost to the surrounding. In this situation, no energy is transferred to the book, so no work is done.

You can feel the different energy consumption when your arm is stretched in different angle. A particular case is that you put the book on your leg when you sit on a chair so your muscle is relaxed and the energy spent is less.

There are also a special type of muscle, smooth muscle, requires very little energy to keep its state so that it can always keep it stretched and you won't get tire:

Tonic smooth muscle contracts and relaxes slowly and exhibits force maintenance such as vascular smooth muscle. Force maintenance is the maintaining of a contraction for a prolonged time with little energy utilization.

share|improve this answer
4  
The default state of muscle is relaxed (i.e. long), and the active state is contracted (i.e. shortened). Remaining relaxed does not require energy beyond that to keep the cells alive; contracting does. –  dmckee Jan 29 '11 at 17:51

Energy is being expended maintaining it in position. Earth's gravity is applying a force downwards, the book is being accelerated down gravitation force.

A force is being applied to the hand and arm which must be resisted and thus energy expended.

The arm and book are not a closed system.

share|improve this answer
1  
Err, no, the book isn't being accelerated down. –  Billy ONeal Dec 16 '10 at 14:51
1  
It's trying to accelerate, but it's not succeeding. This acceleration cannot account for the distance in the equation for $W$. –  Malabarba Dec 16 '10 at 16:22
1  
@Nim: and what if the arm wouldn't move at all? Do you think then you wouldn't tire? –  Marek Dec 16 '10 at 16:52
3  
@Nim: so what? The table is also providing force to oppose gravity if there is a book upon it. Do you think it is doing work? –  Marek Dec 16 '10 at 17:17
4  
This is an example of an "obvious" answer that is completely wrong. Obvious because everyone knows how hard it is to hold things up for a long time. Completely wrong because it misunderstands the physical definitions of "work" and "energy" and conflates them with "force". Read mbq's answer for a pop-sci level explanation of how bio-physics connects the everyday understanding of "holding stuff up is hard" with the physics understanding of "work". –  dmckee Jan 29 '11 at 17:56

Perhaps an analogy is in order. Lets hold up the book by using an electromagnet (say we put a piece if steel under it ). If the coils were made of superconducting material it would take no energy input to maintain the position/field strength. But if we use ordinary wire, ohmic loses within the coil must be made up for by externally supplied electrical energy.

share|improve this answer

This is about how your muscles work -- the're an ensemble of small elements that, triggered by a signal from nerves, use chemical energy to go from less energetical long state to more energetical short one. Yet, this obviously is not permanent and there is spontaneous come back, that must be compensated by another trigger. This way there are numerous streches and releases that in sum gives small oscillations that create macroscopic work on the weight.

share|improve this answer
    
Exactly. This is a good explanation of the biophysics of a not-so-obvious phenomenon. –  Noldorin Dec 16 '10 at 16:37
    
Right, this question is more of a biological question than a physical one. You can always rest your arm on something and miraculously, the book remains in position without any work needed. –  Christian Mar 31 at 19:07

When a physicist talks about work, they are using the word in the technical sense of the equation you quote. To a biologist, though, work might be defined as energy expended to carry out a task. In your example, your arm will not naturally stay in the position described. Your body (mostly your muscles) must expend energy to hold your arm (and the book) in a set position, unsupported by anything but your own physiology.

So, by the biologist's definition, your muscles are doing work to hold up the book and your arm (muscle fibers are contracting and relaxing based on a host of chemical processes at the cellular level). But by the physicist's technical definition, no work is being done.

share|improve this answer
3  
That doesn't really address the question of where the energy is going. –  Mark Eichenlaub Dec 16 '10 at 13:21
    
The energy is going where most energy goes: into heat and entropy. –  Jeremy Dec 16 '10 at 13:42
    
So the physicist's definition of work treats motion too small to be seen with the naked eye as if it were zero? I don't think that is right. –  Trevor Wilson Jun 30 at 6:37

protected by Qmechanic Nov 19 '13 at 7:19

Thank you for your interest in this question. Because it has attracted low-quality answers, posting an answer now requires 10 reputation on this site.

Would you like to answer one of these unanswered questions instead?

Not the answer you're looking for? Browse other questions tagged or ask your own question.