Physics Help Forum E=mc2 gives us potential energy.

 Special and General Relativity Special and General Relativity Physics Help Forum

 Oct 13th 2017, 12:54 AM #1 Senior Member     Join Date: Feb 2017 Posts: 203 E=mc2 gives us potential energy. E=mc2+1/2 mv2 If we interpret this formula then as we know E= potential energy + kinetic energy. 1/2 mv2 gives us kinetic energy. So mc2 must be potential energy if the formula is correct. Is this right?
 Oct 13th 2017, 02:13 AM #2 Senior Member   Join Date: Apr 2015 Location: Somerset, England Posts: 995 No, relativistic energy is not potential energy, you need a potential field for that. look here Relativistic Energy Edit You need to understand that potential energy is not related to the motion (or velocity) of a body. A body may be moving at constant velocity but its potential energy may be changing (or may not) Or it may not be moving at constant velocity (ie acelerating), yet its potential energy may or may not be changing. So the short answer is that any expression energy as a function of velocity will not be potential energy. Note that it may not (all) be kinetic energy either. The reference I gave shows that energy due to relativistic motion manifests itself as mass. topsquark likes this. Last edited by studiot; Oct 13th 2017 at 05:06 AM.
 Oct 22nd 2017, 02:02 AM #3 Senior Member     Join Date: Feb 2017 Posts: 203 For the layman. I think I understand now, so this is for the layman: Observe E=mc2. Here c is the velocity of light. So if it is velocity we know that if an object has kinetic energy it has velocity. So if the equation E=mc2 was potential energy then c2 would not have been a part of the equation. In a formula of potential energy we would not see velocity since potential energy means object is at rest.
 Oct 22nd 2017, 07:02 AM #4 Senior Member   Join Date: Apr 2017 Posts: 403 In a way 1/2 mc2 could be called potential energy , it's the energy contained in the mass , the mass has the potential/ability/possibility of releasing it .... I've never understood the form of this E=1/2 m c2 equation .... why does it mimic so closely the kinetic energy equation???... 1/2m v2 Why does the speed of light appear in this equation ....
Oct 22nd 2017, 07:17 AM   #5
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 Originally Posted by avito009 I think I understand now, so this is for the layman: Observe E=mc2. Here c is the velocity of light. So if it is velocity we know that if an object has kinetic energy it has velocity. So if the equation E=mc2 was potential energy then c2 would not have been a part of the equation. In a formula of potential energy we would not see velocity since potential energy means object is at rest.
No, as studiot said before, an object may be moving or not moving and have potential energy. An object sitting at the top of a hill, not moving relative to the hill) has more kinetic energy than the same object, sitting at the bottom of the hill, not moving. And an object moving on a horizontal plane, at any speed has the same potential energy at that same object sitting still on that plane.

Oct 22nd 2017, 11:51 AM   #6

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 Originally Posted by oz93666 I've never understood the form of this E=1/2 m c2 equation .... why does it mimic so closely the kinetic energy equation???... 1/2m v2
First off you have written the total energy wrong. The 1/2 is not there.

You might be interested in a thread I just responded to, here.

-Dan
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Oct 22nd 2017, 07:58 PM   #7
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 Originally Posted by avito009 E=mc2+1/2 mv2
That expression is incorrect since mv^2/2 is not the kinetic energy of a object moving at speeds comparable to the speed of light. The correct expression is

K = m_0 c^2(g - 1) where g = Lorentz factor. The derivation is on my website here: Work-Energy Theorem

 Originally Posted by avito009 If we interpret this formula then as we know E= potential energy + kinetic energy. 1/2 mv2 gives us kinetic energy. So mc2 must be potential energy if the formula is correct. Is this right?
Not in general. However there are instances when it is correct. E.g. some of the energy that the nucleus of an atom has comes from electrical potential energy. Another example is that of a compressed spring. If you compress a spring then it has more potential energy. There is an increase in that mass of the spring as a result.

Oct 22nd 2017, 09:43 PM   #8
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 Originally Posted by topsquark First off you have written the total energy wrong. The 1/2 is not there.
Silly me ...how could I make such a mistake ....

Thanks for that link , but what I'm looking for is an explanation in simple english .... no formulas ...

Why does the speed of light occur (squared) in the equation equating matter to energy ???

Oct 22nd 2017, 10:01 PM   #9

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 Originally Posted by oz93666 Silly me ...how could I make such a mistake .... Thanks for that link , but what I'm looking for is an explanation in simple english .... no formulas ... Why does the speed of light occur (squared) in the equation equating matter to energy ???
In SR time is viewed a bit differently than in Classical Physics. As time and distance are fundamentally related to each other we need a way to put time on the same footing as a distance...we need a conversion factor between the two. This factor is the speed of light: c. (ie. ct has units of distance.) Why that conversion factor is c requires some knowledge of the Lorentz transformations. I'm sorry, this tells you very little but I don't know how else to explain it without going into the Math.

If it helps, the system of units that most Relativists and Particle Physicists use sets c = 1 so the equations don't directly use it. (This forces the speed to have the domain $\displaystyle 0 \leq v < 1$.)

-Dan
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 Oct 23rd 2017, 09:21 PM #10 Senior Member   Join Date: Apr 2017 Posts: 403 Energy stored in a capacitor = 1/2 C V squared Energy stored in an inductor = 1/2 L I squared Energy stored moving mass = 1/2 m v squared Energy stored in a stationary mass = m c squared ????

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