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Hi.

I've just re-read a high school introduction about SR. It introduces the relativistic mass. I know that this concept isn't used anymore in modern formulations of SR, but observations should be the same in all formulations.

They make the following thought experiment: An electric tram and a battery powered motorbike both drive over a scale at speeds close to ##c##. They then argue that only the tram will appear heavier on the scale since it receives its kinetic energy through electricity from the power lines from above, it's an open system. The motorbike though is a closed system, it transforms the energy saved in its battery to kinetic energy, so its total energy and hence mass remain constant.

Is this correct? I remember that the relativistic mass is a subtle issue and one needs to distinguish between longitudinal and transverse mass (which they don't in this text). Wouldn't we need transverse mass here (since gravity works perpendicular to the direction of motion)?

I've just re-read a high school introduction about SR. It introduces the relativistic mass. I know that this concept isn't used anymore in modern formulations of SR, but observations should be the same in all formulations.

They make the following thought experiment: An electric tram and a battery powered motorbike both drive over a scale at speeds close to ##c##. They then argue that only the tram will appear heavier on the scale since it receives its kinetic energy through electricity from the power lines from above, it's an open system. The motorbike though is a closed system, it transforms the energy saved in its battery to kinetic energy, so its total energy and hence mass remain constant.

Is this correct? I remember that the relativistic mass is a subtle issue and one needs to distinguish between longitudinal and transverse mass (which they don't in this text). Wouldn't we need transverse mass here (since gravity works perpendicular to the direction of motion)?