An electron and a proton are released from rest in space, far from any other objects. The particles move toward each other, due to their mutual electrical attraction.

(a) When they meet, is the kinetic energy of the electron greater than, less than, or equal to the kinetic energy of the proton?

(b) Choose the best explanation from among the following:

I. The proton has the greater mass. Since kinetic energy is proportional to mass, it follows that the proton will have the greater kinetic energy.

II. The two particles experience the same force, but the light electron moves farther than the massive proton. Therefore, the work done on the electron, and hence its kinetic energy, is greater.

III. The same force acts on the two particles. Therefore, they will have the same kinetic energy and energy will be conserved.