We need it to calculate the how big it is, it’s mass, or even volume to get the measurements of lots of objects
<span>On the scale the only external forces are the man's weight acting downwards and the normal force which the scale exerts back to support his weight.
So F = Ma = mg + Fs
The normal force Fs (which is actually the reading on the scale) = Ma + Mg
But a = 0
So Fs = Mg which is just his weight.
Fs = 75 * 9.8 = 735N</span>