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 acting on a body may be best defined as equal to the rate of increase of momentum, i. e. by the equation

or

By the substitution of the previous equations presented in this article we obtain:

which are the desired transformation equations of force. These equations, which have here been derived from the principles of non-Newtonian mechanics, are those which were chosen by Planck to agree with electromagnetic considerations.

Field around a Moving Charge.

As an application of these transformation equations, we may calculate the force with which a point charge in uniform motion acts on any other point charge, merely assuming Coulomb’s