Free Body Diagrams

Introduction

• A free-body diagram (FBD) is a visual representation used to model and analyse the forces acting on an object. It helps in understanding how different forces affect the motion or equilibrium of an object.

Components of a Free Body Diagram

• In a free-body diagram:
  • The object being analysed is typically represented as a box or a dot.
  • Forces acting on the object are represented as thin arrows originating from the centre of the box or dot.
  • Each arrow represents a specific force, and its direction indicates the direction of the force.
  • The length of the arrow may be used to represent the magnitude (size) of the force.
  • Forces may be labelled to indicate their type (e.g., tension, friction, gravity).

Purpose of Free Body Diagrams

• Free body diagrams are essential for:
  • Identifying and visualising the forces acting on an object.
  • Understanding the forces' directions and magnitudes.
  • Analysing the net force and its effect on the object's motion or equilibrium.

Drawing Free Body Diagrams

• Free body diagrams do not need to be drawn to scale, but it can be useful if they are for accurate analysis.
• It is important to label each arrow with the appropriate force it represents and indicate the force's direction.
• The type of force involved may also be shown in the diagram.

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Free Body Diagrams

Examples of Free Body Diagrams

Weight and Reaction Force for a Resting Object:

• Situation: A box rests on a table.
• Diagram: Two arrows pointing in opposite directions, one acting upwards (reaction force from the table) and one acting downwards (weight of the box).
• The object is represented as a box or dot.

Weight, Upthrust, Thrust, and Air Resistance for an Accelerating Speedboat:

• Situation: A boat rests on water.
• Diagram: Four arrows of different lengths representing different forces, such as weight (downwards), upthrust (upwards), thrust (forward), and air resistance (opposite to the boat's motion).
• The object is represented as a box or dot.