# Potential and kinetic energies.

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#### lovebunny

Do the equations of the potential (mgh) and kinetic energies (.5mv^2) represent a mass? Someone said that the pot and kin energies do not represent a mass. Maybe a physicists could explain if the potential and kinetic energies represent a mass.

#### topsquark

Forum Staff
Do the equations of the potential (mgh) and kinetic energies (.5mv^2) represent a mass? Someone said that the pot and kin energies do not represent a mass. Maybe a physicists could explain if the potential and kinetic energies represent a mass.
The comment was in regard to a massless photon and that $$\displaystyle E = h \nu$$ is not a kinetic energy despite having the same unit as kinetic energy.

mgh and (1/2)mv^2 represent the gravitational potential energy and kinetic energy of a massive object. The m is right there in the equation.

-Dan

#### lovebunny

I am so so so sorry since I thought extremely wrongly that you stated before that the pot and kin energies do not represent a mass. So, you have not explain how a massless light particle can be represented with the units of the kinetic energy that includes the units of the mass. Please do not get upset since you did not personally derive Planck's energy element. Planck derived the energy element so please do not get mad and lets have a nice conversation regarding physics. Its not this the purpose of science is that one can have a logic and friendly discussion to advance the understanding of physics for physicists. If that is not possible then please ignore my post and let other people take part in this conversation since I will try my best to convince them that nothing I post has to do with them personally and that this is a simple conversation that no one will remember since no one will remember or look at don't get so upset since getting upset is bad for your heart and I would not want anything bad to happen to you.

#### topsquark

Forum Staff
I am so so so sorry since I thought extremely wrongly that you stated before that the pot and kin energies do not represent a mass. So, you have not explain how a massless light particle can be represented with the units of the kinetic energy that includes the units of the mass. Please do not get upset since you did not personally derive Planck's energy element. Planck derived the energy element so please do not get mad and lets have a nice conversation regarding physics. Its not this the purpose of science is that one can have a logic and friendly discussion to advance the understanding of physics for physicists. If that is not possible then please ignore my post and let other people take part in this conversation since I will try my best to convince them that nothing I post has to do with them personally and that this is a simple conversation that no one will remember since no one will remember or look at don't get so upset since getting upset is bad for your heart and I would not want anything bad to happen to you.
A simple derivation: The momentum of a light wave is defined as $$\displaystyle p = \frac{h}{\lambda}$$. From then from SR we get that $$\displaystyle E = pc = \frac{hc}{\lambda}$$. Better derivations exist but this one gets the job done.