Changes

Since speed is relative, it follows that two observers in different inertial reference frames may disagree on the mass and kinetic energy of a body. Since all inertial reference frames are treated on an equal footing, it follows that mass and energy are interchangeable.

Since speed is relative, it follows that two observers in different inertial reference frames may disagree on the mass and kinetic energy of a body. Since all inertial reference frames are treated on an equal footing, it follows that mass and energy are interchangeable.

There is a logical difficulty, however, to an increase in relativistic mass.  Such increase would only exist in the direction of motion, and the rest mass would remain intact with respect to a force applied in a direction orthogonal to velocity.  Neither mass nor energy is a vector, and the notion of the mass of an object having different values depending on the direction of an applied force is illogical.  In recent years most physicists have shifted away from Einstein's original reliance on relativistic mass and his suggestion that mass increases.  Instead, most physicists today teach that  

In recent years most physicists have shifted away from Einstein's original reliance on relativistic mass and his suggestion that mass increases.  Instead, most physicists today teach that  

:<math>F=\frac{d}{d\tau} p</math>  

:<math>F=\frac{d}{d\tau} p</math>