Measurements of the far-field radiated energy in very simple elastodynamic fault models is presented, and the scaling of the radiated energy with moment and rupture length is examined. The models produce a complex sequence of events having a wide range of sizes as a result of a frictional-weakening instability. Thus, radiated energy from a broad range of sizes of events can be measured. Using conservation of energy, I am able to measure the far-field energy very accurately and efficiently. I study a range of frictions, from velocity weakening to slip weakening, in order to examine the effects of the physics of the rupture source on the radiated energy. Examining the scaling of radiated energy as a function of moment and rupture length, I find differences for slip-weakening as compared to velocity-weakening friction. I find distinct differences in how the apparent stress scales with moment and also how the apparent stress divided by the stress drop scales with moment for the different frictions. Most dramatically, the apparent stress divided by the stress drop is significantly smaller for slip weakening relative to velocity weakening. This suggests that measurements of radiated energy versus moment and rupture length in earthquakes, combined with forward elastodynamic modeling, can be used to constrain possible source physics.