PHYS 2211 Module 11.4

Conditions for Static Equilibrium

Recommended Reading

11.4 Conditions for Static Equilibrium

Learning Objectives

By the end of this section, you will be able to:

  • Identify the physical conditions of static equilibrium.
  • Draw a free-body diagram for a rigid body acted on by forces.
  • Explain how the conditions for equilibrium allow us to solve statics problems.

First Equilibrium Condition

The first equilibrium condition for the static equilibrium of a rigid body expresses translational equilibrium:

Second Equilibrium Condition

The second equilibrium condition for the static equilibrium of a rigid body expresses rotational equilibrium:

Practice!

Practice 11.4.1

A 1 kg ball is hung at the end of a rod 1 m long.  The system balances at a point on the rod 0.25 m from the end holding the mass. What is the mass of the rod?
(a) 0.5 kg         
(b) 1 kg   
(c) 1.5 kg
(d) 2 kg
Practice 11.4.2

You decide to make yourself a desk, by attaching a wide shelf to the top of two legs. (A side view is shown.) When you put a book down on the end of the shelf, the force exerted ON the shelf at the attachment point labeled “A” is approximately
(a) Upwards
(b) Downwards
(c) To the right
(d) To the left
(e) Zero