1. The problem statement, all variables and given/known data An electron collides elastically with a hydrogen atom that is initially at rest. Assume all the motion occurs along a straight line. What fraction of the electron's initial kinetic energy is transferred to the atom? (Take the mass of the hydrogen atom to be 1840 times the mass of an electron.) 2. Relevant equations KE = (1/2)mv^2 uapp = vsep p = mv 3. The attempt at a solution I used the formulas above, and came up with 3 equations: p-cons u1x = v1x + 1840v2x u1x = v2x - v1x KE u1x^2 = v1x^2 + 1840v2x^2 But i keep trying and can't use them to find any one velocity.