Question

For equilibrium, the normal forces act in which direction in the free body diagrams if they are constructed for the friction part calculations?

(a) Vertically Upward

(b) Vertically Downward

(c) Horizontally Right

(d) Horizontally Left

The question was posed to me in an international level competition.

My question is taken from Problems Involving Dry Friction topic in section Friction of Engineering Mechanics

Answer 1

Correct option is (a) Vertically Upward

The explanation is: As the loads are being acting in the downward direction. Thus to make the forces balance, the normal forces act in the vertically upward direction. As we know that when there is no lubricating fluid present between the surfaces in contact, the dry friction occurs. This friction magnitude is taken out from these normal forces.