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
This Is What Rock n' Roll Looks Like depth about the genre mix in "This Is What Rock n' Roll Looks Like": Although the song's title references the physical appearance of what rock and roll looks like, Porcelain Black revealed in an interview with AOL's "NoiceCreep" that rock and roll is not dependent on the physical appearance of someone. She stated: "This Is What Rock n' Roll Looks Like" received generally mixed reviews by critics, most of whom were surprised with Porcelain Black's outgoing persona and the pop production by RedOne. Gerrick D. Kennedy of "Los Angeles Times" complimented the song's production by stating that it could have