-- The girl's mass is 55 kg.
-- Her weight is (m·g) = (55 kg)·(9.8 m/s²) = 539 N .
-- The downward force on the bottom layer of atoms on the soles of her shoes is 539 N.
-- The 'supporting' force of the floor on those same atoms must be <em>EXACTLY 539 N upward</em>, resulting in a NET force of zero.
-- If the upward force of the floor is not exactly equal to her weight, then the NET force on the bottom of her feet is not zero, and the bottom of her feet must be accelerating, either up or down.
In order for the bottom of her feet to remain connected to the rest of her, the entire girl must likewise be accelerating vertically.
If she is NOT, then we know that the net force on the bottom of her feet is zero, and the upward supporting force of the floor must be exactly equal to her weight.
We could have realized this immediately if we trusted Newton, who told us long ago that every action produces an <u><em>equal and opposite</em></u> reaction.
