Since all motion is relative to an inertial frame of reference, is a bodies kenetic energy only relevant when the observer is the intertial frame of reference? (i.e. would the kinetic energy be different for another observer and if so does this violate any of the principles of energy conservation or relativity? This is not for a homework question this is just out of interest. Me and my physics teacher think this is an interesting topic. Because motion is relative, is speed simply just an illusion created by an observer? If you a traveling at the speed of light in an empty universe (you would be stationary using urself as an inertial frame of reference) so would it be possible to speed up even faster. Since there would be nothing to say your speed is relative to, then speed really is an illusion. Please answer any of the questions asked or contiue the converstation by adding ideas. Any arguments or criticisms to my statements above are welcome.