Lecture Written by: Prof. Martha Selby

Authored for presentation by: Prof. Martha Selby

Revised by: Lex Jacobson

HTML Documentation by: Larry Genalo Jr.

Date last updated: 7/24/96


The study of forces acting on bodies.

3 Branches of Mechanics:

  1. Statics
  2. Dynamics
  3. Strength of Materials


The study of rigid bodies that are in equilibrium.


A "push" or "pull" exerted by one body on another, such as:
A person pushing on a wall
Gravity pulling on a person


A quantity possessing only a magnitude such as mass, length, or time.


A quantity that has both a magnitude and direction such as velocity or force.


Force is a vector quantity, therefore a force is completely described by:
  1. Magnitude
  2. Direction
  3. Point of Application

Types of vectors used in statics:

Vector Addition - the parallellogram law.

Resolution of forces into components.

The net effect of a number of forces on one point can be the same as the effect of one force.

Free Body Diagram

A free body diagram is a sketch of the body and all the forces acting on it.

3 steps in drawing a free body diagram:

  1. Isolate the body, remove all supports and connectors.
  2. Identify all EXTERNAL forces acting on the body.
  3. Make a sketch of the body, showing all forces acting on it.


A body is in equilibrium if the sum of all the external forces and moments acting on the body is zero.

Steps in solving a statics problem.

  1. Draw a free body diagram.

  2. Choose a reference frame. Orient the X & Y axes. (Most often X is chosen in the horizontal direction and Y is chosen in the vertical direction.)

  3. Choose a convenient point to calculate moments around.

  4. Apply the 3 equilibrium equations and solve for the unknowns.


Two children balance a see-saw in horizontal equilibrium. One weighs 80 pounds, and the other weighs 60 pounds and is sitting 4 ft. from the fulcrum. Find the force the fulcrum applies to the beam and the distance to the fulcrum to the 80 lb. child. (Neglect the mass of the see-saw.)