I am having trouble deciding when to use which equation. If you're given the wavelength of a proton, let's say 100 fm, and have to find the kinetic energy of it, how would you do this? Here are my ideas: Idea 1: Use lambda = h/p, where p = sqrt(2Em). Idea 2: Use E=(pc)^2+(mc^2)^2 = (mc^2)^2+(h/lambda * c)^2 Then I'll use: K = E- E_o ==> (answer from idea 1 or 2) - mc^2 Which should I use? I personally think idea 2 is correct because it accounts for the relativisitic effects. thank you!