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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 in contact.
 Friction is a force between surfaces that depends
on the materials in contact with each other and
how much force is pushing the objects together
 SI unit: Newtons (N)
 Ex: Tires on the road, rubbing your hands together, air
resistance
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 between two
surfaces when there is no motion between the two
surfaces
 Always with nonmoving objects; always larger than
kinetic friction


Takes more force to overcome
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 between
two surfaces when they rub against one another
because one or both surfaces are moving.
 Always with at least one object in motion
 Always smaller than static friction

Does note take as much force to overcome
 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 (Kinetic friction)and 5200 N
(static friction), 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.
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