1. it is not moving in any linear direction AND
2. it is not rotating..

If both of these conditions are met, then scientists and engineers say, "The body is in mechanical equilibrium." There are two other forms of eqilibrium called static and dynamic equilibrium. Static equilibrium means the body is moving at a constant velocity. Mechanical equilibrium is a subset of static equilibrium. Dynamic equilibrium is where a body is accelerating and moving at constant velocity at the same time. An example of this is a ball that is spinning while not moving linearly. Every spinning object experiences a centripetal acceleration. In the example the object is spinning but not experiencing any translational motion.

• The red car is in mechanical and static equilibrium.
• The blue car is only in static equilibrium
• The spinning "eight ball" is in dynamic equalibrium

This unit will only deal with mechanical equilibrium.

When you solved free body problems, in a previous unit, you were applying one of the conditions of mechanical and dynamic equilibrium by summing up the forces. This is called the first condition of equilibrium. It looked like this...

This YouTube video is here, http://youtu.be/PhOCrlQqI3A

When do you use the summing up the torques technique?

• Use summing up the forces and torques when the question asks how far away from a point a force is applied.This includes questions that ask...
• how far a person can walk on a board or beam.
• Or where can a weight be placed on beam
• Or where the lifting force needs to be placed. All these questions involve a force(s) and a distance. -and that's a torque.

Example 1: Using a torques to write a math sentence

Question Solution Video Solution Below is an extended free body diagram for a beam that is held horizontally. Each letter represents a force. Write the math sentence that sums up all the torques. Example answer: This is a YouTube video that can be found at, http://goo.gl/9gNQlI