Free Body Diagrams
Made by the Learning Center, 2009
Free-body diagrams are diagrams used to show the relative magnitude and
direction of all forces acting upon an object in a given situation. The free-body diagram
above depicts four forces acting upon the object. Objects do not necessarily always
have all forces acting upon them. There will be cases in which the number of forces
depicted by a free-body diagram will be one, two, or three, or more. There is no hard
and fast rule about the number of forces which must be drawn in a free-body diagram.
The only rule for drawing free-body diagrams is to depict all the forces which
exist for that object in the given situation. Thus, to construct free-body diagrams, it is
extremely important to know the various types of forces. If given a description of a
physical situation, begin by using your understanding of the force types to identify which
forces are present. Then determine the direction in which each force is acting. Finally,
draw a box and add arrows for each existing force in the appropriate direction; label
each force arrow according to its type. If necessary, refer to the list of forces and their
description in order to understand the various force types and their appropriate symbols.
Example of a free body diagram:
Fnorm
Fapplied
Ffriction
Fgravity
Types of Forces

Frictional Force- The friction force is the force exerted by a surface as an object
moves across it or makes an effort to move across it. There are at least two
types of friction force - kinetic and static friction. The friction force always
opposes the relative motion of an object.

Gravitational Force- The force of gravity is the force with which the earth, moon,
or other massively large object attracts another object towards itself. By
definition, this is the weight of the object. All objects upon earth experience a
force of gravity which is directed "downward" towards the center of the earth. The
force of gravity on earth is always equal to the weight of the object as found by
the equation: Fgrav = m * g

Normal Force- The normal force is the support force exerted upon an object
which is in contact with another stable object. It will be perpendicular to the
surface.

Applied Force- An applied force is a force which is applied to an object by a
person or another object. If a person is pushing a desk across the room, then
there is an applied force acting upon the object. The applied force is the force
exerted on the desk by the person.

Tension Force- The tension force is the force which is transmitted through a
string, rope, cable or wire when it is pulled tight by forces acting from opposite
ends. The tension force is directed along the length of the wire and pulls equally
on the objects on the opposite ends of the wire.
Example Problems:
1. A book is at rest on a table top. Diagram the forces acting on the book.
2. A skydiver is descending with a constant velocity. Consider air resistance. Diagram the forces
acting upon the skydiver
3. A car is coasting to the right and slowing down. Diagram the forces acting on the car.
4. A rightward force is applied to a book in order to move it across a desk at constant velocity.
Consider frictional forces. Neglect air resistance. Diagram the forces acting on the book.
Answers:
#1
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