This is a question I already solved but was curious about something. There are two positive point charges of the same maginitude on the same axis. They are some distance appart with third point positive charge placed slightly off the midpoint between the two original charges. The question is to find the period of the SHM. In solving this I basically applied Coulumb's law as the restoring force. After looking at the answer the book gave I noticed it still had a variable for mass included. My question is why would mass even come into play for a system composed of point particles with electric force? If gravity is neglected, couldn't I just apply the the particle's charges in place of mass?