LESSON 3: STATICS
Introduction
In this lesson you will study a special case in mechanics wherein the net force and the net
torque on an object or systems of objects are both zero. These are the two conditions for
equilibrium. Statics is concerned with the calculation of forces acting on and within structures
that are in equilibrium.
Learning Outcomes
After successful completion of this lesson, you should be able to:
Study the conditions that are necessary for an extended body to remain stationary, both in
terms of its position in space and its rotational ability.
Express the conditions of equilibrium in the form of mathematical equations and apply them in
solving and analyzing variety of situational problems.
Discussion
3.1 First Condition of Equilibrium
If the vector sum of all the forces acting on the body is zero, then there will not be any change in
its state of motion, and the body is in a state of equilibrium under the action of these set of
forces.
It satisfies then that the sum of the x components (F1x, F2x, F3x . . . ) of all forces acting on the
object must be zero, and the sum of the y components (F1y, F2y, F3y, . . .) must also be zero.
These two equations are useful in calculating one or two unknown forces acting on an object if
all other forces acting on it are known. To use these equations correctly, here are some steps to
follow:
1. Make a drawing of the whole structure being considered.
2. Draw a separate force diagram known as the free body diagram for one part of the structure.
