Equilibrium of a Rigid Body

Activity Overview

In this activity, students investigate equilibrium of a rigid body by applying forces from 1, 2, 3 or 4 spring scales. As the students move the scales and change the forces they carry, the spring scale extensions allow them to see that the Principle of Concurrent forces is always satisfied (unless all of the forces are parallel).

Learning Objectives

By the end of the activity, students should be able to:

  1. Read the force in a spring scale
  2. Trace the line of action of spring scale forces on a large sheet of paper
  3. Convert those lines of action into vectors and add them graphically (or algebraically, if given supplementary instruction)
  4. Demonstrate that a single, non-zero force cannot act on a body
  5. Show that if 2 forces act they must be equal, opposite and collinear
  6. Illustrate by example, that if 2 or more forces act on a rigid body, their vector sum must be zero
  7. Demonstrate that the lines of action of any 3 non-parallel forces on a rigid body must cross at a common point (The Principle of Concurrent Forces)
  8. By counter-example, demonstrate that the Principle of Concurrent Forces does not hold when more than 3 forces are applied to a rigid body
  9. And, if the Contract was used,

  10. Discuss how contracts can facilitate attainment of collective goals

Tips for Using the Activity

Students should be encouraged to enjoy the Activity.

They should not be rushed or under pressure to obtain the “correct answer” or a high grade.

Instead, they should be encouraged to explore, and figure out how forces on a rigid body must interact for equilibrium to exist.

Equipment Design

The equipment was designed to be simple and intuitive to operate, informative to the students, and durable.

Additional Information

See the Downloadables section for more information on this activity.