Free Body Diagrams

1
Free Body Diagrams

• Allows the analysis of the dynamics of an object
  or group of objects.

2
Free Body Diagrams

• In general follow these steps
• Sketch the system and its surroundings
• Enclose the system with a boundary (draw a box
  around the object(s) considered to be the
  system.)
• Shrink the system to a single point or small box
  at the center of a coordinate system with one of
  the axes parallel to the direction of motion.
• Represent all relevant forces with a labeled
  vector. Draw only forces that cross the
  boundary drawn in 2 above.
• Indicate which forces (if any) are equal in
  magnitude to other forces by drawing hash marks
  on the force vectors.

3
Example Tractor pulling a log with a rope at
constant velocity
What are the dynamics of the log?
4
1. Sketch the system and its surroundings
5
2. Enclose the system with a boundary (draw a box
around the object(s) considered to be the
system.)
6
3. Because the shape of an object is unimportant,
shrink it to a point
7
3. Place the point at the center of a set of
coordinate axes with one of the axes parallel to
the direction of motion
y
x
8
Work your way around the boundary line drawn in
step 2 and identify all the external forces that
cross the boundary.

• Gravity
• Normal
• Rope Tension
• Friction

y
x
9
Construct qualitative force vectors to represent
these forces. Draw forces that are equal in
magnitude the same length.
y
FN Normal
FT Tension
fk Friction
x
Fw mg

• Gravity
• Normal
• Rope Tension
• Friction

10
Indicate which forces (if any) are equal in
magnitude to other forces by drawing hash marks.
y
FN Normal
FT Tension
fk Friction
x
w mg

• Gravity
• Normal
• Rope Tension
• Friction