[quote=aonin;16663]Hey guys, i was just wondering what would happen if a person was on a spacecraft traveling at 0.9c was standing on a set of scales that measured his mass, went past a stationary observer.

Would the stationary observer see the reading on the scales increase due to mass dilation?

[/quote

The scale would be measuring the inertial force which is being applied to the pan of the scale. The inertial force is related to the inertial mass by F = dP/dt where P = mv. This F and thus the m would be different for each observer. You can't really use a "scale reading" since the scale is calibrated to be read from the rest frame of the scale.

topsquark - see

[0709.0687] On the concept of relativistic mass and the reference material which relates the relative percentages of what is being printed in newer relativity texts.

re - "Scales do not actually measure mass, they measure weight. "

That is incorrect. If a scale is being accelerated then it's measuring inertial force, not weight. Weight requires a gravitational field since it is related to passive gravitational mass. These quantities, passive gravitational mass and inertial mass, are defined differently but are postulated to have the same value. See article I wrote which is linked to above.