① The mass of adults is about 65kg, and the mass of middle school students is smaller than that of adults, which should be about 50kg;

② The weight of middle school students is g = mg = 50k g×9.8n/kg = 490n；;

③ Barycenter diagram method: Make a line segment vertically downward from the barycenter, and make an arrow at the end of the line segment to indicate the direction.

So the answer is:

kg； 490; The schematic diagram is as follows: