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### Can we use formula E=mv^2 to calculate the total energy?

Posted: **Thu Oct 19, 2017 12:30 pm**

by **Yizhou Liu 3L**

We know that E=pv and p=mv, which means total energy should equal to mv^2. In the photoelectric experiment, we also know that E=work function+(mv^2)/2. However, work function doesn't equal to (mv^2/2), which means this equation is wrong. Does anyone know where I get wrong? Thank you.

### Re: Can we use formula E=mv^2 to calculate the total energy?

Posted: **Thu Oct 19, 2017 1:14 pm**

by **Curtis Tam 1J**

I think it could be used but if the "velocity" of the work function is not given (I haven't seen a problem that gives velocity as part of a work function) then maybe it's not valid. I also know that photons have no rest mass so I don't know what you would put for "m".

### Re: Can we use formula E=mv^2 to calculate the total energy?

Posted: **Thu Oct 19, 2017 2:24 pm**

by **Kevin Liu 3G**

I don't think that E and Ek are the same values so you can't exactly make the two equal. Ek is 1/2mv^2 whereas E is the energy needed to eject an electron. Also, for the questions relating Ek = E - work function most of the time Ek is 0 because the velocity is 0 meaning that the energy needed to eject an electron = work function.