Mass in special relativity what causes these particles to be termed "massless." However, even massless particles have a relativistic mass, which varies with their observed energy in various frames of reference, The invariant mass is the ratio of four-momentum (the four-dimensional generalization of classical momentum) to four-velocity: and is also the ratio of four-acceleration to four-force when the rest mass is constant. The four-dimensional form of Newton's second law is: The relativistic expressions for "E" and "p" obey the relativistic energy–momentum relation: where the m is the rest mass, or the invariant mass for systems, and "E" is the total energy. The equation is
Jerk (physics) Jerk (physics) In physics, jerk is the rate of change of acceleration; that is, the time derivative of acceleration, and as such the second derivative of velocity, or the third time derivative of position. According to the result of dimensional analysis of jerk, [length/time], the SI units for its magnitude are m/s (or m⋅s); this can also be expressed in standard gravity per second ("g"/s). Jerk is a vector, and (as with "acceleration") there is no distinct term to denote its scalar magnitude (more precisely, its norm, as, e.g., there is "speed" for the norm of the velocity vector). Jerk