Free Body Diagrams
Free Body Diagrams
• Free-body diagrams are used to show the
relative magnitude and direction of all forces
acting on an object.
How to draw free body diagrams
Step One:
Draw a box to
represent the object.
Label the mass of the
object.
100 kg
How to draw free body diagrams
Step Two:
Draw arrows to
represent the forces.
• The size of the arrow
represents the magnitude
of the force.
Normal
Friction
100 kg
Gravity
• Arrows must move out
from the center of the
object.
Applied
• Label the value of the
force or label the type of
force.
Common Forces
• Fn = Normal Force- how much force an object has
to counteract gravity.
• Fw = Weight Force- How much force the weight of
an object puts on the earth.
• Ff = Friction Force- How much force friction causes
on an object.
• F1 or F2 = Any push or pull force
Forces
• There can be any number of forces acting on
an object, it doesn’t always have to be 4.
Example 1
• A book is at rest on a table top. Diagram the
forces acting on the book.
1 kg
Example 1
• A book is at rest on a table top. Diagram the
forces acting on the book.
1 kg
Normal
1 kg
Gravity
There is a Normal Force (pushing up) &
a Gravitational Force (pulling down)
Example 2
An egg is free-falling from a nest in a tree.
Neglect air resistance. Draw a free-body
diagram showing the forces involved.
There is only a Gravitational Force
(pulling down)
Gravity
Example 3
A flying squirrel is gliding (no wing flaps) from a
tree to the ground at constant velocity. Consider
air resistance. Draw a free-body diagram
showing the forces involved.
Example 3
A flying squirrel is gliding (no wing flaps) from a
tree to the ground at constant velocity. Consider
air resistance. Draw a free-body diagram
showing the forces involved.
Air Resistance
Gravity
Example 4
A rightward force is applied to a book in order
to move it across a desk. Consider frictional
forces. Neglect air resistance. Construct a freebody diagram.
100 kg
Example 4
A rightward force is applied to a book in order
to move it across a desk. Consider frictional
forces. Neglect air resistance. Construct a freebody diagram.
Normal
Friction
100 kg
Gravity
Applied
Example 5
• A car runs out of gas and is coasting down a
hill.
Example 5
• A car runs out of gas and is coasting down a
hill.
Gravity
Net Force
We can use free body diagrams to help us solve
force problems.
1) Draw the free body diagram with forces.
2) Label the forces with the appropriate
value.
3) Add the forces like vectors.
Net Force (Example 1)
• What would be the resultant force?
1200 N
800 N
Net Force (Example 2)
• What would be the resultant force?
500 N
250 N
500 N