An electron collides elastically with a stationary hydrogen atom. The mass of the hydrogen atom is 1837 times that of the electron. Assume that all motion, before and after the collision, occurs along the same straight line. What is the ratio of the kinetic energy of the hydrogen atom after the collision to that of the electron before the collision? (hydrogen / electron) Momentum is conserved, so m1v1i +m2v2i = m1v1f + m2v2f I can rearrange this to get v2f = [ 2m1 / (m1 + m2) ] v1i So this pretty much gives me the ratio I need (final hydrogen in comparison to initial electron). Since m2 = 1837(m1), I get v2f = [ 2m1 / m1 + 1837m1 ] v1i Canceling out mass, I get v2f = [ 2 / 1 + 1837 ] v1i v2f = [ 2 / 1838 ] v1i So the hydrogen has more than 900 times the velocity as the electron does? How does this go into KE? This is where I fall apart. Any help is appreciated.