Why must the lamina swing freely in physics​

Unless the lamina is able to move freely under the influence of gravity, you do not know where to draw that line.

Explanation:

Allowing it to swing freely is the only way to ensure that the center of gravity (CG) is below the pivot point. The purpose of the exercise is to find a line containing both the pivot point and the CG. Unless the lamina is able to move freely under the influence of gravity, you do not know where to draw that line.

when insulating materials rub against each other, they may become electrically charged . electrons , which are negatively charged, may be 'rubbed off' one material and on to the other. the material that gains electrons becomes negatively charged. the material that loses electrons is left with a positive charge.