•Free Body Diagrams (FBDs)
are tools used to analyze
physical situations
•FBDs show all the forces acting
on a single object
 As an example of a free-body diagram, consider a
30-newton book resting on a table that weighs 200
newtons. The book is on one corner of the table so
that its entire weight is supported by one leg.
 Because the table is in
equilibrium, the net
force acting on it must
be zero.
 The weight of the book
acts on the table. The
weight of the table acts
on the floor. The force
acting on the table is
the reaction to its
weight acting on the
floor.
 There are two ways to
handle positive and
negative directions in a
free-body diagram. One
way is to make all upward
forces positive and all
downward forces negative
 When you solve the
problem, if you have
chosen correctly, all the
values for each force are
positive.
 If one comes out negative,
it means the force points in
the opposite direction from
what you calculated.
What forces are acting on
objects?
•The object itself may be drawn
as a circle or box
•Draw and label all the external
forces on the object
• The only “rule” for drawing free-body
diagrams is to depict all the forces that
exist for that object in the given
situation.
A circus elephant falls off a tight rope.
Neglecting air resistance, draw a free
body diagram for the falling elephant.
A circus elephant falls off a tight rope.
Including air resistance, draw a free
body diagram for the falling elephant.
A book is at rest on a tabletop.
Diagram the forces acting on the book
A girl is sitting on a swing at the
playground, sketch a FBD of this
scenario.
•Free Body Diagrams are useful to
quickly see if there are unbalanced
forces on an object.
•If an object is in equilibrium, the
net force is zero.
•If the forces are unbalanced, then
acceleration occurs
Let’s calculate the magnitude and
directions of this FBD
Let’s calculate the magnitude and
directions of this FBD
Let’s calculate the magnitude and
directions of this FBD
Suppose three people are trying
to keep an injured polar bear in
one place. Each person has a
long rope attached to the bear.
Two people pull on the bear
with forces of 100 N each . What
force must the third person
apply to balance the other two?
The bear will not move if the net
force is zero.