Problem: a. Calculate the energy in eV of an electron with a wavelength of 1 fm. b. Make the same calculation for a neutron. Solution (so far): a. λ=h/p=(hc)/(pc)=(1240 MeV fm)/(pc)=1fm so, pc=1240 MeV E=√[(pc)^2+E_0^2] =√[(1240 MeV)^2+(.511MeV)^2] ∴E=1.24 GeV This is the same answer as the back of my book, so I'm assuming this is the correct method of solution. However, I do the same thing for the neutron and my answer does not agree. b. E=√[(1240 MeV)^2+(940 MeV)^2] ∴E=1560 Mev My book says the correct answer is 616 MeV. I don't see how an energy like that is even possible. Solving the following for pc, E^2=(pc)^2+E_0^2 pc=√[E^2-E_0^2] When you plug in the "correct" answer of E=616 MeV you get, pc=√[(616 MeV)^2-(940 MeV)^2] You certainly cannot take a square root of a negative number and get a meaningful answer. Any suggestions?