# Would a moving scale read more than a scale at rest?

## Main Question or Discussion Point

Assuming there is a mass on a scale, and they move with the same velocity. I know mass increases for a moving object, but the scale's mass will also increase. So would the scale read any more than if they were both at rest?

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russ_watters
Mentor
No. This is the principle of relativity itself.

According to the scale the mass is at rest, so no the scale will not read any more.

jtbell
Mentor
Suppose the scale has a display that reads 1.0000 kg for a certain object, when you're at rest with respect to the scale (e.g. standing next to it in the laboratory). Now imagine running past the scale at 0.9c. Surely the reading is still 1.0000 kg.

Maybe the OP is thinking that if you run by the scale at 0.9c you would expect to observe a relativistic mass increase of the mass on the scale, the scale itself, and the Earth for that matter such that if the scale was a spring scale the indicator would shift reading due to additional compression between the mass on the scale and the Earth under the scale.

Something tells me that there may be a problem with using a spring scale here... similar to the way relativistic experiments need to exclude certain forms of clocks like pendulum clocks?