What exaclty is Mechanical Energy? Or more specificaly potential energy (the subset)

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Because i'm a bit confused...
I know mechanical energy is potential energy and kinetic energy but I'm not sure of the exact definition of potential energy.
Is potential energy (the subset of mechanical energy) the following?
  • Gravitational potential energy
  • Elastic potential energy
  • Chemical potential energy
  • Electric potential energy
  • Nuclear potential energy
as it shows on Wikipedia - http://en.wikipedia.org/wiki/Potential_energy[/PLAIN] [Broken]

or is it just the following?
  • Gravitational potential energy
as the are saying to me on Yahoo Answers - http://answers.yahoo.com/question/index?qid=20120618063654AASSBFN
 
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  • #2
PhanthomJay
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Wiki gives a few good examples. Potential Energy is energy stored in a body or system by virtue of its position within a force field.
 
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yes but is it just GPE or is it others as well? That's the big question here. Because you could say chemical energy is energy due to the position of charges, right?
 
  • #4
Drakkith
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yes but is it just GPE or is it others as well? That's the big question here. Because you could say chemical energy is energy due to the position of charges, right?
Potential energy in general is all of it. You can have chemical potential energy, nuclear potential energy, etc.
 
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Now we're getting somewhere! So does that mean chemical energy is mechanical?
 
  • #6
Drakkith
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Now we're getting somewhere! So does that mean chemical energy is mechanical?
I don't believe so. Chemical processes are described by quantum effects, so I don't think you would say they are mechanical. But I don't really know honestly.
 
  • #7
PhanthomJay
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By definition, mechanical energy is the sum of the potential and kinetic energies of a body or system. Unfortunately, the term 'mechanical' energy is used in a completely different context, as in electrical energy converted to mechanical energy by a motor, or mechanical energy converted to electrical energy by a generator. Don't confuse the two definitions.

By the first definition, where Mechanical Energy is the sum of the potential and kinetic energies, then Chemical Potential Energy is only part of the total mechanical energy of a system.
 
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OK so people normally (and unfortunately) use the term "mechanical energy" for something else. So then what is the real term for this "something else" for which it is normally used? Its a sort of organized kinetic energy... PhanthomJay?
 
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my previous question still applies.
 
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  • #11
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Potential energy means it will begin to move spontaneously in a particular direction if it is permitted to, but before it can move it may need a push, it may have to be released from captivity. Gravitational - it's on the shelf but with a nudge it will fall. Electrical PE - a charge may be confined in some way but if you remove the confinement then it will be spontaneously attracted or repelled somewhere. Elastic PE, a stretched spring is on a hook but it you slide it off the hook it will go boing. Chemical PE - a spark puts the dynamite "over the edge" and then a lot of motion will occur.

It's considered mechanical energy because all kinds of PE are related to conservative forces, never related to nonconservative forces. The energy expended to put something somewhere can be recovered, and the form in which it is recovered is motion.
 
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  • #12
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OK i get it, chemical energy IS a type of mechanical energy. But now i'm wondering why so many people deny that, see my "yahoo answers" replies:


I-copeland
Chemical energy = mechanical energy?
because chemical energy equals potential energy right?
and everyone knows potential energy is mechanical...

Prasanta
No!
Potential energy is energy due to height.
There is mechanical power but not mechanical energy.
Chemical energy does not equal potential energy as defined above.

Norrie
From wherever you obtained the data for the question, it's complete and utter rubbish.
Mechanical Energy is a combination of Potential and Kinetic energies and generally
produces Work, Force and/or Power.

Carte Blanche
Chemical energy may equal potential energy under certain conditions...but is not always
equal

Steve4Ph...
The term mechanical energy is generally used when talking about individual objects - it
changes when an object's speed or position changes.

The term chemical energy is used to refer to energy associated with chemical reactions -
rather than energy changes due to speed/position changes of individual objects.

It's just a convention. Anyway, physicists tend not to use the term chemical energy.
 
  • #13


!!! Here I restrict my discussion to classical physics domain.
Potential Energy is defined for conservative forces or field. These conservative forces may be gravitational force, spring force ( obeying Hooke's Law), electrostatic forces (I believe all type of bonds explained in chemical reactions are essentially electromagnetic forces. Also inter atomic and inter molecular forces are the same).

Following law is always applicable (classical Physics):
Under the action conservative forces ONLY, sum of "kinetic energy and potential energy" of the system is always constant. Now it is your prerogative that you call it (the sum of Kinetic & Potential energy) what.
 
  • #14
PhanthomJay
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OK so people normally (and unfortunately) use the term "mechanical energy" for something else. So then what is the real term for this "something else" for which it is normally used?
that is the term used.
Its a sort of organized kinetic energy... PhanthomJay?
I don't know how organized it is, but yes, mechanical energy, when used in this context, is energy of motion, and thus a form of kinetic energy. Ahh, definitions, we need a wordsmith on board....
 
  • #15
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OK so...

What other forces are there apart from conservative forces then?
 
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  • #16
PhanthomJay
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expletive deleted . You are not the wordsmith I was looking for.
What other forces are there apart from conservative forces then?
Non- conservative forces. Gravity and spring forces are conservative. Friction, applied, and other contact forces are non conservative.
 
  • #17
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So your saying PJ that there are two different "mechanical energies" that people refer to?
 
  • #18
PhanthomJay
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There are two distinctions to which I refer. I can't speak for all people.
Suppose that a motorized pulley is used to move a crate up an incline. The mechanical energy (definition 2) of the motor/pulley causes work to be done on the system which in turn increases the mechanical energy (definition 1) of the system. Well, what do you think, people, and what do you think, I-c?
 
  • #19
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By definition, mechanical energy is the sum of the potential and kinetic energies of a body or system. Unfortunately, the term 'mechanical' energy is used in a completely different context, as in electrical energy converted to mechanical energy by a motor, or mechanical energy converted to electrical energy by a generator. Don't confuse the two definitions.

By the first definition, where Mechanical Energy is the sum of the potential and kinetic energies, then Chemical Potential Energy is only part of the total mechanical energy of a system.
So are you saying here that because it is used out of context it is used to mean something completely different, so therefore mechanical energy has two meanings?
 
  • #20
PhanthomJay
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So are you saying here that because it is used out of context it is used to mean something completely different, so therefore mechanical energy has two meanings?
When i think of Chemical energy, I think of mixing chemicals together; when i think of Electrical energy, I think of powerlines; when i think of Nuclear energy, it's the bomb; when i think of Mechanical energy, I think of engines and machines. Each of these types of energy, and others I have not listed, are capable of producing changes to the kinetic and potential energies of a system, that is, changes to the mechanical energy of a system.
 
  • #21
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so one type of chemical energy is mechanical and the othe is not...
 
  • #22


f****d up s**t. OK so...

What other forces are there apart from conservative forces then?
Non conservative forces (dissipative) like friction, viscous etc.

Work done against conservative forces is stored in one way or other. Work done against electrostatic force is stored as electrostatic potential energy, Work done (at constant velocity) against gravitational force is stored as gravitational potential energy, I am trying talk as simple as possible.
But work done against friction is dissipated in the form of heat.

Please read the basic concept of potential energy from some good book. This will help you.
 
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  • #23
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So is chemical energy mechanical and non-mechanical?
 
  • #24
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Ok makes sense, what others are thier apart from friction?

Oh and my previous q still applies:
so one type of chemical energy is mechanical and the othe is not...
 
  • #25


So is chemical energy mechanical and non-mechanical?
Mechanical word might have coined in mechanics but if I have to chose between mechanical & mechanical and non-mechanical then I will go with mechanical.
 

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