The principle of the lever -- a simple teeter-totter

An object that rotates about a fixed point or about an axis because of a force being applied to it has torque.
Torque is proportional to the applied force and to the perpendicular distance at which this force acts from the given point or axis.


  • Objects in static equilibrium must satisfy the condition that all forces acting on its center of mass must add up (vectorially) to zero. 
  • This condition makes sure that all forces that are pulling or pushing the object from one side to the other are balanced. 
  • There is also another condition that must be met so that the object is in true static equilibrium. 
  • All torques acting on the center of mass of the object must add up to zero. 
  • This ensures that the object does not undergo any rotation.
In summary:

Conditions for static equilibrium:

  1. The sum of all translational forces = zero
  2. The sum of all rotational forces = zero

Solutions to Static Equilibrium Problems



From the Free Body Diagram:

(Taking torques about point A)