An electron and a proton, initiall sepatated by a distance d in air are released from rest simultaneously. The two particles are free to move. When they collide, are they (a) at the midpoint of their initial separation, (b) closer to the initial position of the proton or (c ) closer to the initial position of the electron?

Explanation

Because of Newton's third law, the forces exerted on the electron and the proton are equal in magnitude and opposite in direction. For this reason, it might seem that the particles meet at the midpoint. The masses of the particles, however, are quite different. In fact, as the nass of the proton isabout 2000 times greater than the mass of the electron, the proton's acceleration `(a=F//m)` is about 2000 times less than the electron's acceleration. As a result, the particles collide closer to the initial position of the proton. More specifically, they collide at the location of the center of mass of the systen, which remains at rest throughout the process because there is no external force.