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Where does the energy come from to support things?

by venton
Tags: energy, support, things
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venton
#1
Jan23-11, 01:38 PM
P: 36
Very basic question, I am almost embarrased to ask it...
If I had to hold say a clock or a wall unit above the ground it would take energy for me to do so. I would need to eat food to get energy to support the clock in my arms, and if I ran out of energy I would drop it.

If I put a nail in the wall and hung the clock on it, the wall supports it. For years on end.

But where does the energy come from to support the clock?
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russ_watters
#2
Jan23-11, 02:14 PM
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Don't feel embarassed, this is a very common misunderstanding people have.

It takes you energy to hold something because you're a biological being and your muscles constantly burn energy in order to generate a force. That's just how we work. So if you use your muscles to support something, you'll burn energy.

But force and energy aren't the same thing. In fact, energy is force times distance. So on other situations, just applying a force doesn't necessarily require an input of energy. A laptop computer sitting on a table is a good example. There's force, but no energy.

You might say that this makes a human body inefficient. If you hold up a laptop, there's an input of energy, but no output. But, if you think about it, I'm sure you can devise ways of holding up a laptop that use very little energy. Hold up a laptop at arms length and you'll burn quite a bit of energy holding it up - and you probably won't be able to hold it for more than a few seconds. Rest it on your legs while lying on your couch and you can hold it indefinitely with no noticeable change in your body's energy input.
cepheid
#3
Jan23-11, 02:16 PM
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If you just set a heavy object on a table, then the force of gravity on it (its weight) is exactly cancelled out by the contact force on it from the table. So there is zero net work done on the object, and no change in its energy. The object can remain there indefinitely, without requiring a continuous transfer of energy from the table to the object.

If you try to do the same thing by holding up the object, then I think the reason why you cannot do so indefinitely is biological -- it has to do with the way muscles work. I'm not an expert, but I think that for a muscle to remain in a contracted position when there is a load on it, the muscle actually must rapidly contract and relax, continuously. So there is a continuous conversion of chemical energy in your body into mechanical energy of the movement of your muscles, even for this situation in which you are not doing any work, and the object's energy is not changing.

As for where the support comes from in the first situation: fundamentally, all contact forces between objects are electrostatic in nature -- they arise because of the repulsive force between the atoms that make up those objects.

venton
#4
Jan23-11, 02:23 PM
P: 36
Where does the energy come from to support things?

Informative answers, thanks. So it seems there has to be movement for energy to be used.
For example, if the nail which held the clock on the wall was too thin, it may bend and in doing so generate heat. But it is only because the nail moves when it bends that energy as heat is output?
K^2
#5
Jan23-11, 02:46 PM
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Not so much generated as transfered. Work is transfer of energy, and it is a force applied over a displacement along the direction of displacement. A clock hanging on a nail has potential energy due to gravitational field. If the nail bends, the clock does work on the nail, transferring some of its energy to the nail. And yes, most of it becomes heat.
WarpedWatch
#6
Jan26-11, 09:31 AM
P: 38
Quote Quote by venton View Post
Informative answers, thanks. So it seems there has to be movement for energy to be used....
On the microscopic level, the cells in your muscles are moving even when a limb is holding something in static position. Those muscle cells take turns contracting and relaxing, so that each cell can recover its ATP. Therefore on the microscopic level, lots of work is being done (force acting through a distance) even though on the macroscopic level it doesn't look like anything is happening. A wall that holds up a clock has nothing moving on a microscopic level, so no work is being done and therefore no energy is required.
venton
#7
Jan26-11, 09:44 AM
P: 36
I see, that makes sense about the cells doing work.
I am still having trouble with the idea that the wall does no work. It just seems that it should take some energy from somewhere to resist the clock dropping for all those years. I initially wondered if this energy was coming from the gravity on the other side of the world pushing back against the clock.
WarpedWatch
#8
Jan26-11, 11:47 AM
P: 38
Quote Quote by venton View Post
....
I am still having trouble with the idea that the wall does no work. It just seems that it should take some energy from somewhere to resist the clock dropping for all those years. ....
The wall provides a force to the clock, but that force does not move the clock, so work is not performed. Work = force times the distance the force is exerted.

It's important to note the difference between force and work or energy. Force can perform work or generate energy only by moving something. But a force by itself can not be converted into energy.

Think of it this way: if you call a football player a "force of nature" but he only sits on the couch and drinks beer all day, he's not going to make any goals. No matter how big he is or how awesome his "force" might be, he's worthless if he just sits there on his butt. He might as well be a physicist at that point.
venton
#9
Jan26-11, 12:16 PM
P: 36
I am turning this on it's side to get away from gravity (so vertical = horizontal), and I think I am at the root of my question...
Imagine a car driving very slowly forward. I put my hands on it to try to stop it, but can't, even though I burn energy trying.
The powered car meets a stationery car, and it pushes it along. The stationery car has to start it's engine and drive against the other car to stop it - using energy to resist it - if it runs out of fuel it gets pushed again. So, both cars are burning energy, but there is no distance involved, whilst they are resisting each other.

But, if the powered car meets a solid brick wall, it stops, even though the car is pushing against it.

It seems logical to me, that because I, and the other car had to use energy to try to stop the car, the wall must need to draw the energy from somewhere to resist it also - but where?
russ_watters
#10
Jan26-11, 02:00 PM
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Quote Quote by venton View Post
It seems logical to me, that because I, and the other car had to use energy to try to stop the car, the wall must need to draw the energy from somewhere to resist it also - but where?
The wall draws its energy to stop the car by causing the earth to rotate away from the car.
WarpedWatch
#11
Jan26-11, 05:08 PM
P: 38
Quote Quote by venton View Post
...
Imagine a car driving very slowly forward. I put my hands on it to try to stop it, but can't, even though I burn energy trying....
Maybe you won't stop it, but your exertion will indeed slow it down.



Quote Quote by venton View Post
...So, both cars are burning energy, but there is no distance involved, whilst they are resisting each other....
Cars have clutches which often have a fluidic interface or some other kind of interface where energy is getting turned into heat from friction of parts rotating against each other. The force of friction is acting over the distance of the circle, the effective circumference of the clutch mechanism.

Russ is correct about the ultimate energy absorber being the rotational energy of planet earth itself.
cepheid
#12
Jan26-11, 07:27 PM
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Forget the engine. Let's say a moving car hits a stationary car. Momentum has to be conserved, but kinetic energy is not necessarily conserved (it could be converted to other forms of energy).

Let's say the moving car collides with the stationary car, they stick together, and afterwards they are moving at a slower speed. Let's say furthermore that their combined kinetic energy is now less than the initial kinetic energy of the moving car. The relevant question is NOT "where did the energy come from to slow down the first car?" The relevant question is, "where did the kinetic energy that the first car initially had, (and is now lost) GO?" Answer:

- Some of it was used to do work to deform the cars
- Some of it was converted into sound
- Some of it might have been converted into light
- Some of it was converted into heat

What do we mean when we say that some of the energy was converted into heat? It means that the "ordered" motion of the car (all particles that make up the car have a net movement in the same direction) is converted into disordered, random motions of those particles. So, the random disordered component of motion that the particles had increases. The energy associated with this disordered motion of the constituent particles is called thermal energy.

The idea that much of the initial kinetic energy would be converted into thermal energy is almost certainly true in the "car hits a brick wall" scenario as well.
venton
#13
Jan27-11, 12:10 AM
P: 36
I was trying to avoid the crash scenario and just imagine the car 'pushing' slowly against me, other car, or wall.
If it was a rocket pushing against the wall, I see that the rotational energy of the planet is resisting (because the rocket is not connected to the ground).
But regarding the car, I think the following thought experiment shows it is not the planet's rotation which is resisting....

Imagine a giant right angled bookend. The car drives onto the base of the bookend and meets the 'wall' of the bookend. Place the whole bookend with car onto ice (so there is little/no friction with the earth). The car starts pushing against the bookend (which it itself is on). The bookend does not move, all that happens is that the car's energy is turned into heat from the tyres spinning and heat in the clutch etc. It is as if the force is reflected directly back at the car.
This is unlike a rocket pushing against the bookend, where it would clearly push it over the ice as 'work'.
I'm not sure what that means with regard to the clock on the wall.. is the clock the 'rocket' above, or the 'car'?
russ_watters
#14
Jan27-11, 06:46 AM
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Quote Quote by venton View Post
I was trying to avoid the crash scenario and just imagine the car 'pushing' slowly against me, other car, or wall.
If a car is just pushing against the wall, it generates no energy because nothing is moving. It's creating static forces between the car and the ground and the car and the wall.

Yes, there are situations where, like the human body, a car can be 0% efficient.
WarpedWatch
#15
Jan27-11, 08:47 AM
P: 38
Quote Quote by venton View Post
...The bookend does not move, all that happens is that the car's energy is turned into heat from the tyres spinning and heat in the clutch etc. It is as if the force is reflected directly back at the car.
This is unlike a rocket pushing against the bookend, where it would clearly push it over the ice as 'work'...
I applaud your imaginative thought experiments. However, you are still confusing the concept of force with the concepts of energy and work. Energy can be converted into work. Work performed on a system can store energy in that system. But a force by itself can not be converted into work or energy unless it makes something move. Your intuition tells you that a force must burn energy because your own intuitive experience is derived from the use of your muscles. But muscles are actually tiny very complex machines that are taking turns contracting and expanding like rubber bands, and all that microscopic movement is burning up chemical energy. Solid matter, like brick and metal, does not need to microscopically twitch like muscles to maintain a static force. On a molecular level, solid substances maintain their shapes because the charged particles within them (electrons, protons) maintain a balance of repulsive and attractive forces generated by their electric charges.

Maybe a quick way to visualize the difference between force and work is to imagine a spring. A force (like gravity) by itself will do no work on the spring. But when you use that force to compress the spring some distance, then you have done work on the spring, and you have stored energy in the spring. In fact the more "springy" the spring is, the more energy you might be able store in it. If the spring were as inflexible as granite, for example, then its compression would be microscopic, and because the distance moved would be so small, then the amount of energy stored in that granite spring would be very small, too.

I'm guessing that your hang-up with this Force vs. Work thing stems from referencing your bodily experiences. In this situation your common sense is leading you astray. The great thing about studying physics is that it can help you overcome these sorts of common sense attitudes and break down problems into mathematical definitions that allow you to effectively dissect what's happening so you can make calculations that you can apply to real-world problems. One of the first things you learn in physics is to develop what's sometimes called a "free-body diagram" of forces. It's a great way to untangle your intuition about objects.
QuantumPion
#16
Jan27-11, 09:12 AM
P: 778
If you hold an object in your hands while your elbows are supported on a table, you could hold the object pretty much indefinitely because the weight of the object is being supported by your bones. This is analogous to hanging an object on a wall with a nail. The crystal structure of the metal in the nail or the calcium of your bones can withstand the stress indefinitely. On the contrary, trying to hold up an object at arms length would be like trying to hang an object on the wall with a rubber band. Your muscles provide a chemical reaction to hold your shoulder joint up, but this requires constant energy expenditure, like holding a car stationary at the top of a hill by revving the engine with the clutch partially engaged.
venton
#17
Jan27-11, 02:40 PM
P: 36
Thanks for the tips everyone, I am going to have to abandon my intuition and start reading some basic physics books.


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