# Question on conservation of energy -- Pulling a piece of iron off of a magnet

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yungman
TL;DR Summary
How to explain magnet
This is not homework, just a question of curiosity. Something that came to my mind last night.

Say I have a magnet and a piece of iron. Say I use it to do weight training exercise. I stick the magnet on the wall and stick the iron on the magnet. Say it takes 100lbs of force to pull the piece of iron away. So for exercise, I keep pulling it off, then stick it back on, then do it again as exercise.

I am doing work. If the magnetic stay as strong over long period of time( say it's a natural magnet). Meaning I can keep doing this exercise forever. That means the magnet has infinite amount of energy that will not be used up!

Is my assumption wrong that the magnet can last forever? Or there are other reasons?

Thanks

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If your 100lb weight were on the floor and you picked it up, does that mean that the Earth has "infinite energy" ? because you can drop it then pick it up, again ?

russ_watters and malawi_glenn
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permanent magnets are not "permanent"

yungman
If your 100lb weight were on the floor and you picked it up, does that mean that the Earth has "infinite energy" ? because you can drop it then pick it up, again ?
You drop it and pick it up, you do work. I don't know how to answer about the earth. I am not a physics major, just studied 2 semesters of general physics in college 50 years ago!

yungman
permanent magnets are not "permanent"
But it will last a long time. You mean it store a LOT of energy in the piece of magnet.

Say you have a piece of rare Earth magnet, you really only need a small piece to generate 100lbs worth of force.

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You can also, as mentioned, lift a block of mass of the ground "infinetly" many times.
Does that mean that there is an infinite amount of gravitational energy stored in the block?

yungman
You can also, as mentioned, lift a block of mass of the ground "infinetly" many times.
Does that mean that there is an infinite amount of gravitational energy stored in the block?
No, the energy is the earth, not the block. Just like the piece of iron has no energy, it's the magnet.

To be honest, I don't really know. Like I said, the little bit of my knowledge was from 50 years ago on Physics 1a and 1b! It's just came into my mind. I appreciate you try to get me to look it up, honestly, I don't even know where to start, don't have books or anything, just a wild thought. Can you just tell me, it's not for anything else, I likely just move on. I just happen to be here in the EE sub-forum here lately answering a question...which is my career before I retired.

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No, the energy is the earth, not the block. Just like the piece of iron has no energy, it's the magnet.
is energy an object?

topsquark
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The point is, you apply a force to lift the weight, so you put energy into the combined Earth + weight + gravitational field. So that has more energy than before. If you drop the weight, what force acts? Do you think Earth + weight + gravitational field will have more or less energy once the weight has fallen compared to before it fell?

Similarly with the magnet. You put in the energy when you separate the magnets. What does the work when they come together again?

The whole thing is analogous to a battery. You can always get some energy out, as long as you put some in first.

topsquark, Lnewqban and malawi_glenn
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I am doing work. If the magnetic stay as strong over long period of time( say it's a natural magnet). Meaning I can keep doing this exercise forever. That means the magnet has infinite amount of energy that will not be used up!
Look at the system as a whole. What is pulling the weight against the force of the magnet? You are. You are providing the energy. This energy is part of the magnet-weight-person system, in the form of potential energy, which is converted into kinetic energy when the weight is released, and then finally into thermal energy and a few other types upon impact against the magnet. Then you come along and pull it away, adding more energy into the system. When viewed this way, it is immediately obvious that the energy being expended can't be sustained forever because it comes from the chemical potential energy of your body, and your body is part of the system.

topsquark, malawi_glenn, DennisN and 1 other person
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Gonna eat at some point. Proper nutrition is paramount to increasing strength and muscle mass. Do not neglect intake of fluids either, nothing beats plain water.

Gold Member
The energy is coming from you, the person doing the exercise. All of the energy is accounted for in the position of the weights you are lifting, the work you do, and the heat you give off. There is no energy contribution from the Earth itself, the weights themselves, or the magnet.

topsquark, russ_watters and vanhees71
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The energy is coming from you, the person doing the exercise. All of the energy is accounted for in the position of the weights you are lifting, the work you do, and the heat you give off. There is no energy contribution from the Earth itself, the weights themselves, or the magnet.
There is an energy sink if you allow the weights to slam down on the Earth's surface or onto the magnet that is attracting them. Those will not be perfectly elastic collisions.

russ_watters
Ahzmandius
Two things to clarify here:

1. Energy conservation is not about the amount of energy, but what happens to the energy over time. Mathematically -> dE/dt = 0.
2. Energy conservation does only apply in isolated systems (neither a transfer of energy nor a transfer of matter is allowed)
In particular, the second point is violated in your example, so the energy conservation is not applied here if you do consider the magnetic field + the iron system only.

Gold Member
MHB
...nothing beats plain water.
Clearly you have never encountered chocolate milk.

-Dan

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Clearly you have never encountered chocolate milk.

-Dan
Sugar. Not good for you. Drink 3 liters chocolate milk everyday if you want

vanhees71 and topsquark