Question

Can force be conserved?


I know this might sound crazy but here is a analogy of what I mean:

When a magnet is charged with electricity to "magnetize" it. A force is applied to create that magnetic field. Is it possibile for that same magnet to have a stronger/greater force then the inputed force to create it?

This is the easiest example I had in mind!

Answer 1

@Jemurray3 , @experimentX , @myininaya Help!

Answer 2

Force is always "conserved" in a closed system. Newton's 3rd law.

Answer 3

If you stay classical, that is.

Answer 4

you can have stronger/greater force then the input force

you CANNOT have greater energy then the input energy

Answer 5

Forces are fields ... fields can be conservative ... but never heard that field itself is conserved.

Answer 6

Well the way I interpreted the question is that for a given system, can\[\sum\vec F=\vec 0N\]And it can, so long as\[\sum\vec F_{external}=\vec 0 N\]Since internal forces cancel by Newton's 3rd law.

Answer 7

seems like D'Alembert's principle

Answer 8

if I interpreted Q correctly, I would say energy is conserved. You applied more energy to align those micro magnetic dipoles. Hence more dipoles are aligned to give stronger magnetic field.

Answer 9

@experimentX how could you say forces are field ...? think of conservative forces

Answer 10

mathematically, forces are vector fields ... and some fields are conservative while some may not. conserved and conservative have different meanings.

Answer 11

Yes, -GRAD U

where grad is gradient and u is potential energy

Answer 12

When a magnet is being magnetized isn't there a source that causes that? That source would most likely be a force. When that force is applied the magnet creates a magnetic field. Would that field/force is the same as the input or possibly greater? Hope you all are getting this.

Answer 13

Is it possibile?

Answer 14

In a spring mass system, if you apply greater force, you will be repelled by greater force. do you call force is conserved?

Answer 15

A conservative force, such as gravity, means that if I move in a loop (i.e. I go up and down the stairs), then there is no net change in energy. Then consider friction. If a cube could magically propel itself, and it slid back and forth one time across a table with friction, would it gain back the energy it expelled to move?

Answer 16

@experimentX You're starting to get this! I mean the same concept applied in the law of conservation Ein = Eout is it every applied in force? I know this might sound stupid, but seriously! When I use a force on a object as input Fi would the output force be equal? CAN THE OUPUT force ever be greater then inputed force? Thats what I mean.

Yea its really weird to think about but I was just thinking...

Answer 17

If I'd understood your problem then ... I think it might be
you can't say forces are conserved.
Input (Electric Field) + Output (Magnetic Field) = constant <-- i don't think it should be like this

Force is not a characteristic of a system. Force is rather function of coordinates or position. The energy is characteristic of system. So, Energy remains conserved. Force is an interaction. (i guess)

Answer 18

@Hope99 Why are you ignoring N's 3rd Law (or me)?

Answer 19

Depending on ur definitions of "in" and "out," it literally is Fin= - Fout!!!

Answer 20

@vf321 Im not. Im saying all that I have to say then well se you're final comments.

Answer 21

to make sure that all of you understood what I mean. + Im not "ignoring" anyone.

Answer 22

So can you tell me where you're at right now? 'Cause to me it seems like we're mainly fretting over the definition of "conserved"

Answer 23

the net force on a system (even if you have to use some coordinate transformation) is zero ... but do you call "conserved" ?

Answer 24

@experimentX "Force is not a characteristic of a system. Force is rather function of coordinates or position. The energy is characteristic of system. So, Energy remains conserved. Force is an interaction. (i guess)"

What do you mean? Could you clarify a bit more using the same example (magnet)?

Answer 25

You know ... let's say .. you applied huge field ... but put the ferromagnetic material far away ... what do you think would happen?

Answer 26

there isn't strong magnetization.

Answer 27

@experimentX, look.

Lets say we have a bar magnet that's magnetic field is rated 1500Gauss. The magnetic field/force was created from the material's of the bar magnet being magnetized. Now, how are they magnetized? By electricity. A form of energy that is applying a certain force on the bar magnet to make it fully charged! A force was used to make that magnet "magnetized".

Now what I'm trying to say and @vf321 seem's to have answered it already is: CAN THE MAGNETIC FIELD/FORCE GENERATED BY THE BAR MAGNET BE "GREATER/STRONGER/MORE" FORCE THEN THE INPUTED FORCE from the electricity? Based on newton's 3rd laws it's equal Fin=Fout. I guess.

However, what's amazing is the electricity's force is just a short one. However, the bar magnet's magnetic field/force can last for 100's of years if super cooled and taken care of.

Hope you all are getting this. Sorry I didn't give much information yesterday because I was tired.

Answer 28

That electrical force that is applied on the bar magnet to charge it and make it "magnetized" somewhat conserved or something. Because as I said the electrical force would last for only a bit. However, the magnetic field/force could last for a very very very long time! Amazing! Its as if the magnet is a force storage device :P

Answer 29

i wouldn't exactly say the force is stored. rather some potential energy is stored ... i guess. I don't exactly know how though.

Answer 30

Bar magnets don't use electricity (unless you're talking on a molecular level). There is no circuit delivering energy in the electromagnetic sense.

If you had a solenoid inductor, for example, then it's magnetic field is powered by the movement of electrons inside the wire, which, by the Biot-Savart law, cause a magnetic field to form inside (and outside) the solenoid.

Answer 31

However, there's no force "going in" you could look at the current, which does have a direct relationship with the magnetic field for an inductor - \(B = \mu_0 \frac{NI}{L}\) - for magnetic field B, constant mu0, number of turns of the solenoid N, current I, and inductance L.

Answer 32

The only force I can imagine is the following:
\[\vec J = \lim_{A\rightarrow 0}\frac{I}{A}\]\[\vec E = \rho\vec J\]\[\vec F_E=q\vec E\]
So by force, do you mean the force that every electron is propelled by inside the conductor?

Answer 33

Thanks, I feel that I pretty much got the answer. Yea force & energy really a complicated matter.

Answer 34

thanks all!

Answer 35

so what did you end up concluding?