Question

A challenge guise;
a square box held with a person's fingers pushing downward one quarter of the way along the top of the box and with their thumb pushing sidewards on the bottom of the side face of the box. Friction between the person's fingers and the box is obviously very important if the box is not to fall.Determine the required coefficient of friction to keep the box from falling. (required figure has been attached: checkout)

Answer 1

check out the image

Answer 2

Assuming, Box will not deform..

Mu = Co-efficient of friction..
Ft = Frictional force..
Fa = Fb = Normal force at particular points..

also Mu @B = Mu @A...