The system of two charged point particles traveling side by side at arbitrary velocity less than the speed of light and accelerating slowly is analyzed within classical electromagnetism as to the connection between the electrostatic potential energy of the system and the inertial mass. The total external force necessary to accelerate the system is calculated, and it is found that the change of system mass associated with the electrostatic potential energy of the two particles is accounted for exactly by the electromagnetic force of one charged particle on the other during the acceleration. The example provides a striking illustration and detailed mechanism for understanding the mass‐energy idea associated with special relativity.

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