Drawing Free-Body Diagrams

Drawing Free-Body Diagram

Lesson Notes

Goal

• To draw a free-body diagram that shows the types of individual forces that act upon an object, their direction, and their relative strengths

What is a free-Body Diagram?

    A special type of diagram that represents all the forces acting upon an object. Arrows represent the forces. The force arrows labeled according to type, pointing in the direction of the force, have a length that is in proportion to their relative strength (longer arrows are stronger forces.)

Types of Forces

    Our video on the topic of Recognizing Force Types is most thorough source of information on the topic of identifying the types of forces that act upon objects. A brief summary of those force types is presented here. While useful, this summary lacks the thoroughness that is required to adequately identify force types. We recommend our video.

• `F_{grav}`      Always present
• `F_{tens}`       Force from string, rope, wire, cable, etc.
• `F_{sprng}`   Force from compressed/stretched spring
• `F_{fric}`       Two surfaces sliding across each other
• `F_{air}`         Object moves through surrounding air
• `F_{norm}`    Results from two surfaces pressing against each other
• `F_{app}`         A person pushing/pulling on an object

Method for Drawing Free-Body Diagrams

The task of drawing a free-body diagram involves

1. Identifying the types of forces acting on the object
2. Determining the direction of those forces
3. Deciding upon the relative size of opposing forces

1. Drawing a force diagram with arrows having a labeled force type and the proper size relative to any opposing force.

Examples:

• A book is at rest on a table

Forces present:

    `F_{grav}` (down)

    `F_{norm}` (up)

• Draw the free body diagram for a book is attached to string and hanging from the ceilling.

Forces present:

    `F_{grav}` (down)

    `F_{tens}` (up)