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

[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.

[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".

[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.