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Forces and Newton’s Laws Part 3:
Everyday Forces of Weight, Normal,
and Friction
Review
• What do we already know?
• Write out Newton’s 1st, 2nd and 3rd Laws
• 1st Law: An object at rest stays at rest and an
object in motion stays in motion unless a force is
applied.
• 2nd Law: The net force on an object is equal to
its mass times its acceleration. Fnet = ma
• 3rd Law: Forces have equal magnitudes and
opposite directions.
Everyday Forces
• Everyday forces are
those that we can
observe in our
surroundings.
• They happen everyday
- whether we observe
them or not!
• There are three that we
will learn about:
Weight, Normal Force
& Friction.
Weight
• Weight is the magnitude
of the gravitational force
exerted on an object.
• Weight is how much
gravity pulls on our mass.
• Weight is an object’s
mass times the
acceleration of gravity.
• Equation: W = m•g
(Unit is Newtons)
• If you go to the moon will
your mass or weight be
different?
Example #1- Weight
• An object has a mass
of 55kg. Calculate
the objects weight:
– On Earth - g = 9.81
m/s2
– On Venus - g = 8.87
m/s2
– On Mercury - g = 3.59
m/s2
• Where does the
object weigh the
most? The least?
Normal Force
• Normal Force, FN, is
one that acts on an
object, perpendicular
to the surface the
object is on.
• The object must be
on a surface.
• Newton’s 3rd Law
pair to the Weight.
Why the “Normal Force”…
• And not the
Perpendicular Force?
• One definition of
“Normal” is
perpendicular.
• The FN is ALWAYS
PERPENDICULAR to
the SURFACE.
Example #2 - Normal Force
• A book with weight
100N is resting on a
table.
• What is the Normal
Force exerted on the
book by the table?
• Hint: The Normal
Force and Weight are
3rd Law pairs
Friction
• Friction is a force
that opposes an
applied force
• Oppose: to actively
resist
Friction continued…
• Friction exists when an
object is moving - called
Kinetic Friction
• Kinetic - resulting from
motion
• Friction attempts to keep
an object from moving
when a force is applied called Static Friction
• Static - lacking motion
Example #3 - Friction
• Rub your hands together. What happens?
• This heat is a result of Friction. Friction
opposes your hands motion, and this
causes energy to be changed into heat.
• Watch Miss Probst attempt to move the
block. Is she applying a force? Why is it
not moving?
Activity - Part 1
• You will identify objects in the
classroom that exhibit the everyday
forces.
• Follow along in your activity sheet.
Be sure to find an object for each
quesiton!
Free Body Diagrams: What are
they?
• A free body diagram
is a simple sketch that
shows the forces
acting on an object
• Free body diagrams
do not show the
forces the object
exerts on something
else!
How to use Free Body
Diagrams?
• Before you can draw a
free body diagram you
must:
– Identify all forces acting on
the object
– Know what direction those
forces act
• Free body diagrams help
us to visualize the forces
acting on an object.
• This is important because
we will need to “Sum the
forces” for Newton’s 2nd
Law problems!
Drawing Free Body Diagrams
•
Steps for drawing free
body diagrams:
1. Identify all the forces
acting on your object
2. Draw a simple sketch of
your object
3. Forces are drawn from the
center of the object
(objects center of mass)
4. Forces are drawn in the
direction they act
5. Forces are labeled with
correct magnitude if you
know it
Example #4 - Free Body
Diagram
• You have a calculator
sitting on the table in
front of you. Draw a
free body diagram to
illustrate the forces on
the calculator.
• Follow the steps in
your note taking guide
to solve!
Example #4 - Free Body
Diagram Continued
• Now you begin to
push the calculator
across the table to a
friend. Draw the free
body diagram to
illustrate the forces on
the calculator.
• Follow the steps in
your note taking guide
to help!
Activity - Part 2
• For each of the objects you found in
Activity Part 1 you will draw the Free Body
Diagram.
• Follow the steps in your note taking guide
to help.