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Lesson 4.4
Everyday Forces
Essential Question: What are some
of the everyday forces?
• How do you know that a bowling ball weighs
more than a tennis ball? Imagine holding one
ball in each hand.
What is weight?
• A measure of the gravitational force exerted on
an object; its value can change with the
location of the object in the universe.
 = 
 = −9.81 m
2
s
• If an astronaut weighs
800 N (180 lb) on Earth,
he would weigh only
about 130 N (30 lb) on
the moon.
• Weight depends on
location because it is
equal to the magnitude
of the force due to
gravity.
• Imagine a television set at rest on a table. We
know that the gravitational force is acting on
the television. Why doesn’t the television
continue to fall forward toward the center of
the Earth?
What is the normal force?
• A force that acts on a surface in a direction
perpendicular to the surface.

• The normal force is always perpendicular to
the contact surface but is not always opposite
in direction to the force due to gravity.
• In the absence of other forces, the normal force
is equal and opposite to the component of the
force of gravity that is perpendicular to the
contact surface.
How do you calculate the magnitude of
the normal force?
 = 
• The angle is the angle between the normal
force and a vertical line and is also the angle
between the contact surface and a horizontal
line.
What causes things to slide or to stick?
• Friction causes our applied forces to work
harder!
• Friction opposes the applied force.
• There is static friction and kinetic friction.
What is static friction?
• The force that resists the initiation of sliding
motion between two surfaces that are in
contact and at rest
• Static friction is abbreviated with 
• The force of static friction is always equal to
and opposite in direction to the component of
the applied force that is parallel to the surface.
• As the applied force increases, the force of
static friction also increases.
• If the applied force decreases, the force of
static friction also decreases.
• When the applied force is as great as it can be
without causing the object to move, the force
of static friction reaches its maximum value,
,
What is kinetic friction?
• The force that opposes the movement of two
surfaces that are in contact and are sliding over
each other. 
• The magnitude of the net force acting on the
object is equal to the difference between the
applied force and the force of kinetic friction.
• Kinetic friction is always less than the
maximum static friction.
• When two surfaces are
stationary with respect
to each other, the
surfaces stick together
somewhat at the contact
points.
• This adhesion is caused
by electrostatic forces
between molecules of
the two surfaces.
In free-body diagrams…
• The force of friction is always parallel to the
surface of contact.
• The force of kinetic friction is always opposite the
direction of the motion.
• To determine the direction of the force of static
friction, use the principle of equilibrium. For an
object in equilibrium, the frictional force must
point in the direction that results in a net force of
zero.
A push below the horizontal.
A pull above the horizontal.
The force of friction is proportional to
the normal force.
• It’s easier to push a
chair across the floor at
a constant speed than to
push a heavy desk
across the floor at the
same speed.
• Experimental
observations show that
the magnitude of the
force of friction is
approximately
proportional to the
magnitude of the normal
force that a surface
exerts on an object.
What does friction depend on?
• Many factors including:
– The relationship between
normal force and the
force of friction
– The composition and
qualities of the surfaces
in contact.
How can we calculate friction?
• We can approximate friction.
• The relationship between normal force and the
force of friction provides a good
approximation for the friction between dry, flat
surfaces that are at rest or sliding past one
another.
What is the coefficient of friction?
• The ratio of the magnitude of the force of
friction between two objects in contact to the
magnitude of the normal force with which the
objects press against each other
• The coefficient of friction is represented by the
symbol, μ
How do you find the coefficient of
kinetic friction?
• The ratio of the force of kinetic friction to the
normal force

μ =

How do you find the coefficient of
static friction?
• The ratio of the maximum value of the force of
static friction to the normal force.
,
μ =

How do you calculate the force of
friction?
• If the value of μ and the normal force on the
object are known, then the magnitude of the
force of friction can be calculated directly.
 = μ
Coefficients of Friction
• The coefficient of kinetic friction is always
less than or equal to the coefficient of static
friction.
• Coefficient of friction ():
– Determined by the nature of the two surfaces
– s is for static friction.
– k is for kinetic friction.
– s > k
A 24 kg crate initially at rest on a horizontal floor
requires a 75 N horizontal force to set it in motion. Find
the coefficient of static friction between the crate and
the floor.
Once the 24 kg crate is in motion, a horizontal force of
53 N keeps the crate moving with a constant velocity.
Find μ , the coefficient of kinetic friction, between the
crate and the floor.
A 25 kg chair initially at rest on a horizontal floor
requires a 165 N horizontal force to set it in motion.
Once the chair is in motion, a 127 N horizontal force
keeps it moving at a constant velocity.
a) Find the coefficient of static friction between the
chair and the floor.
b) Find the coefficient of kinetic friction between the
chair and the floor.
http://www.youtube.com/watch?v=Vpq877lvG_
M
Inclines

Ff
FN
mg cos 


mg 
mg sin 


Tips
•Rotate Axis
•Break weight into components
•Write equations of motion or
equilibrium
•Solve
A 91 kg refrigerator is placed on a ramp. The
refrigerator begins to slide when the ramp is raised to an
angle of 34°. What is the coefficient of static friction?
A box is sitting on a ramp. The ramp makes an
angle of 40° with the horizontal. The box has a
mass of 20.0 kg. What is the coefficient of static
friction between the box and the ramp?
A block of wood is sitting on an inclined plane.
One end of the plane is raised until the wood
block slides down. The wood block slides down
when the plane has an angle of 30 degrees to the
horizontal. What is the coefficient of friction?
A museum curator moves artifacts into place on various
different display surface. Find , and 
Moving a 145 kg aluminum sculpture across a
horizontal steel platform
A museum curator moves artifacts into place on various
different display surface. Find , and 
Pulling a 15 kg steel sword across a horizontal steel
shield
A museum curator moves artifacts into place on various
different display surface. Find , and 
Pushing a 250 kg wood bed on a horizontal wood floor
A museum curator moves artifacts into place on various
different display surface. Find , and 
Sliding a 0.55 kg glass amulet on a horizontal glass
display case
A student attaches a rope to a 20.0 kg box of books. He
pulls with a force of 90.0 N at an angle of 30.0° with the
horizontal. The coefficient of kinetic friction between
the box and the sidewalk is 0.500. Find the acceleration
of the box.
Two students are sliding a 225 kg sofa at constant speed
across a wood floor. One student pulls with a force of
225 N at an angle of 13° above the horizontal. The other
student pushes with a force of 250 N at an angle of 23°
below the horizontal. What is the coefficient of kinetic
friction between the sofa and the floor?
A student pulls on a rope attached to a box of books and
moves the box down the hall. The student pulls with a
force of 185 N at an angle of 25.0° above the horizontal.
The box has a mass of 35.0 kg, and μ between the box
and the floor is 0.27. Find the acceleration of the box.
The student moves the box of books up a ramp inclined
at 12° with the horizontal. If the box starts from rest at
the bottom of the ramp and is pulled at an angle of 25.0°
with respect to the incline and with the same 185 N
force, what is the acceleration up the ramp? Assume that
μ = 0.27.
A 75 kg box slides down a 25.0° ramp with an
acceleration of 3.60 m s2 .
a) Find μ between the box and the ramp.
b) What acceleration would a 175 kg box have on this
ramp?
A box of books weighing 325 N moves at a constant
velocity across the floor when the box is pushed with a
force of 425 N exerted downward at an angle of 35.2°
below the horizontal. Find μ between the box and the
floor.
What is air resistance?
• A form of friction.
• Whenever an object
moves through a fluid
medium, such as air or
water, the fluid provides a
resistance to the object’s
motion.
• For a falling object, when
the upward force of air
resistance balances the
downward gravitational
force, the net force on the
object is zero. The object
continues to move
downward with a constant
maximum speed, called
the terminal speed.
What are the Four Fundamental
Forces?
• Electromagnetic
– Caused by interactions
between protons and
electrons
– Produces friction
– Act over long range
• Strong nuclear force
– The strongest force
– Short range
• Weak nuclear force
– Short range
• Gravitational
– The weakest force
– Act over long range
These are all field forces.
• What are some of the everyday forces?

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