1. Static Equilibrium
Question: What is the direction of a third force, so that the shoe does not go anywhere?

2. Static Equilibrium
When forces acting on an object which is at rest are balanced, then we say that the object is in a state of static equilibrium.
The resultant of these forces equals zero. That is, the vector sum of the forces adds to zero. .

3. Static Equilibrium
Each dog's force has been resolved into its x- and y-components.
A vector diagram showing only the components of each dog's force.
The addition of a third force would place the shoe into a state of static equilibrium.

4. Condition of Static Equilibrium
For an object to be at rest, or static equilibrium (translational only),
the sum of the forces has to be zero:
∑F= a.k.a. Fnet=0

5. Static vs. Dynamic Equilibrium
Static equilibrium principles are of most interest to people who want to determine that their structures are going to stay where they put them. 
This car is in dynamic equilibrium because it is moving at constant velocity. There are horizontal and vertical forces, but the net external force in any direction is zero.