Download Chapter 5: “Frictional Forces”

Frictional Forces
Friction
• Two or more objects in contact moving past
each other will slow each other down. The
force which causes them to slow down is
friction.
Friction: the force that acts to resist the
relative motion of objects or materials in
contact.
 Friction is a force between surfaces that
depends on the materials in contact with each
other.
 SI unit: Newtons (N)
 Ex: Tires on the road, rubbing your hands
together, air resistance
• Forces are needed to overcome any
friction that may be present to get an
object moving.
• When friction is present, an object may
move with a constant velocity even if an
outside force is applied to it.
 If the NET FORCE is 0, there is NO
acceleration, but it can still be moving
Check Your Understanding
Why do sports shoes have more tread on
them compared to dress shoes?
Because more tread leads to more bumps,
which leads to more friction. Athletic
shoes need more friction to allow for
greater acceleration (rapid sprints, sudden
stops, and quick turns), whereas dress
shoes do not require the same sudden
change in motion.
2 Types of Friction
• There are 2 types of friction, and both
ALWAYS oppose motion
 Static Friction
 Kinetic Friction
Static Friction: the force exerted on
one surface by a second surface when
there is no motion between the two
surfaces
 Always with nonmoving objects; always larger
than kinetic friction
 More difficult to overcome at rest inertia
 SI unit: Newtons (N)
 Ex: trying to make a parked car move, trying
to move furniture
Kinetic Friction: the force exerted on
one surface by another surface when
the two surfaces rub against one
another because one or both surfaces
are moving.




Always with one object in motion
Always smaller than static friction
SI unit: Newtons (N)
Ex: a car rolling down the highway, walking
on concrete
Check Your Understanding
A car is broken down in the parking lot
and the driver tries to push the car off
to the side but can’t move it. What type
of friction is this?
Static friction. It won’t be considered
kinetic until the driver gets the car moving.
Check Your Understanding
If the friction forces between the road
and a car on a dry day are 4500 N and
5200 N, which is the frictional force
when the car is moving?
The 4500 N. Kinetic friction is always
smaller than the static friction because it
does not have to overcome the car’s initial
inertia.
Coefficient of Friction
• The friction between objects depends on the
materials the objects are made of
 Ex: rubber shoes have more friction on asphalt than
ice on asphalt
• A surface with more microscopic bumps will
have more friction
• Each surface-on-surface contact has its own
ratio
 The coefficient of friction
 Usually a decimal point
• Between 0 and 1
• No units!!!; just a number
• Just like with static and kinetic friction, the
coefficient of static friction is ALWAYS
bigger than the coefficient of kinetic
friction.
Check Your Understanding
Why is the coefficient of static friction
always bigger than the coefficient of
kinetic friction, even though it’s the
same material?
Because static friction has to overcome an
object’s initial inertia, thus making it more
difficult to move.
Free Fall and Air Resistance
• Air resistance is a type of friction because
it opposes movement.
• Air resistance diminishes the net forces
acting on an object in free fall.
 This is why an elephant and a feather will not
fall at the same rate on Earth; the air
resistance slows down the feather.
 The air resistance builds up quicker for the
small feather, thereby slowing it down quicker
• When the air resistance on an object in
free fall equals the object’s weight, the
object has reached its maximum speed, its
terminal velocity, and will have zero
acceleration.
– This is b/c the object is in equilibrium
– The net force is 0N, so the acceleration is 0
m/s2
• But it can still be moving at a constant speed
Terminal velocity: the maximum
speed a free falling object can obtain
due to air resistance; when the air
resistance against an object equals the
object’s weight
 The acceleration equals zero because it is
moving at a constant speed
 Ex: a piece of paper slowly falling in a
classroom full of air
Check Your Understanding
What is the acceleration due to gravity on
Earth in a vacuum?
 About 10 m/s2.
What is the acceleration of a piece of paper
floating towards the ground in a classroom
filled with air?
 Zero! It has already reached its terminal velocity
by the time it has left your hand. Because of the
small amount of weight of the paper, it does not
take long for the air resistance to equal the
paper’s weight.
Check Your Understanding
What is the net force acting on a piece of
paper in free fall in a room full of air?
 Zero! Since its acceleration is zero (terminal
velocity), its net force must also be zero
(remember Newton’s 2nd law of motion).