Free Body Diagrams

1
Free Body Diagrams

• Pictures and Notes

2
There must be a simple way to show all the forces
on an object visually.

• Free-body diagrams

3
Free Body diagram

• Object represented by dot
• Forces represented by arrows
• Each arrow points away from dot
• Each arrow is labeled (Fg, Fa, Ff)

4
Labeling Forces

• By letter and subscript(s)
• The letter will usually be F (for Force) but can
  be other values
• The subscript is meant to define the specific
  type of force or its origin, such as
• Fg (force of gravity)
• Ff (force of friction
• FBob (force applied by Bob)

5
Which forces are shown on the diagram?

• Only represent the forces that are applied to the
  object,
• NOT forces the object applies to other things or
  the surrounding environment

6
Direction of Motion arrow

• Generally all arrows represent forces applied to
  an object.
• If object is moving, it is good to draw an
  additional arrow separate from the diagram which
  indicates the direction of motion
• This info is used to determine the direction of
  certain force values like friction, air resistance

7
The forces on a Kitty Cat
8
Fground
Fgravity or Fweight
9
What would be the free body diagram of ..

• What are the forces on the dog?

10
Answer
Direction of motion arrow (DoM)
Reactive force of ground
Applied force of ground pushing paws forward
Force of leash
Friction force
Weight of dog