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-A force that opposes
motion
-Acts parallel to the
surfaces in contact.
Cause of Friction
Friction Force vs. Weight
Cause of Friction: the
Ff
microscopic roughness between
surfaces…like two gears locking
together.
W
walking
tires
brakes
sand on icy streets
If it is not moving, then there is no
friction until an applied force acts on
the object! What if it is moving?
Ff
F
Friction can also cause heat!
1. Sliding Friction – occurs when two
solid objects slide over each other.
1. Static Friction – not moving
Static > Kinetic
2. Kinetic Friction - moving
So… it takes more force to start
moving an object
Force vs. Time
Maximum Static Friction
Constant Kinetic Friction
F
t
 Factors that affect Sliding Friction:
a. The normal Force acting on an object. FN
(Fg only equals FN in a special circumstance, ie. lab)
b. The coefficient of Friction ( µk < µs,
unitless) The texture of the surfaces. The
stickiness between surfaces.
General Formula for Friction:
friction force = coefficient of friction x normal force
or
Ff = µFN
Static - µs
Kinetic - µk
Which object would have the
greatest normal force?
So which would experience the
greatest friction force? (assume all
blocks are moving)
Which would have greater friction
force… sand paper or a waxed floor?
Why?
What about surface area? Which would
have a greater friction force?
Friction force does not depend on the
area of contact. Same FN and µ!
2. Rolling Friction – (< sliding)
wheels and ball bearings
3. Fluid Friction – (< sliding)
occurs when a solid object moves
through a gas or a liquid substance
Liquids
Gases
Hammer
Feather
Now for the problems…
just like before…
draw your FBD
Then:
ΣFx = ma
but now we need:
ΣFy = ma
Ff = µFN
Blockhawk Stephen Hawking is moving a 100 kg
block from one side of his laboratory
to the other. To accomplish this he ties
a rope to the block and then ties it to
his wheelchair such that the rope is
pulled horizontally to the floor. If the
coefficient of sliding friction is 0.35,
what force must Mr. Hawking apply
to the rope to move at constant speed?
Ans: 343 N
Warm Up
1. A 30 N block has a coefficient of friction
of 0.26. What force is required to slide it
along a level surface at constant velocity?
2. A 35 kg crate requires a force of 110 N
to slide at an acceleration of 2 m/s2. What
is the coefficient of friction?
In the figure below the coefficient of friction is
0.25. Find the tension in the rope then find the
acceleration of the 18 N block.
A crate having a mass of 60 kg falls horizontally off
the back of a truck which is traveling at 30 m/s.
Compute the coefficient of kinetic friction between
the road and the crate if the crate slides 45 m on the
ground with no tumbling along the road before
coming to rest. Assume that the initial speed of the
crate along the road is 30m/s.
30 m/s
Ans. 1.02
Atwood Again…
A cord running over a pulley connects
two objects. The coefficient of friction
between the object and the table is
0.33. Find the acceleration of the
system and
7 kg
the tension of
the rope.
5 kg
Angle Block Problem:
A 40 kg lawnmower is being pushed at a
constant speed along the ground. The
angle at which the force is being applied
is 35º below the horizontal. What force
will keep the lawnmower moving at a
constant velocity if the coefficient of
kinetic friction is .3?
How does pulling a block at an upward
angle change the normal force?
…friction force?
F
FN
y
x
Fg
How does pulling a block at a
downward angle change the normal
force? …friction force?
FN
x
y
Fg
F
If all the boxes are the same, which
requires a greater force to pull? Why?
F
F
F
Warm-up
A 925 N crate is being pulled across a
level floor by a force of 325 N at an angle
25° above the horizon. The coefficient of
kinetic friction between the crate and the
floor is 0.25. Find the acceleration of the
crate.
325 N
25°
925 N
Answer: 1.034 m/s2
What if we’re on an incline?
If you were sledding down a hill would mass affect your acceleration?
There are always at least
two forces acting upon any
object that is positioned on
an inclined plane - the
force of gravity and the
normal force. The force of
gravity (also known as
weight) acts in a downward
direction; yet the normal
force acts in a direction
perpendicular to the
surface.
FN
F
30º
Fg-y
Ff
Fg
Fg-x
- Rotate the x & y axis
- Break Fg into x and y components
- Angle between Fg and y is equal to the angle of the incline
Inclined to be Friction
Old Grandma Jones is trying to load a
wooden crate full of apples into her
truck. To accomplish this, she plans to
slide the crate up along a board. If the
crate has a mass of 50 kg and the board
is at a 30º incline, what is the maximum
force she must apply parallel to the
board to move the crate at constant
velocity into her truck if the coefficient of
friction is .25?
That
DEAD
That
Darn
Cat!Cat!
David is speeding down a hill (incline) when he
notices a cat in the road. He immediately
switches from the gas pedal to the brakes in
order to stop the car. The slope of the incline is
20º and the car has a mass of 1500 kg.
a. Draw a force diagram showing all the forces acting
on the car.
b. Determine the acceleration of the car if the
coefficient of friction between the road and the
tires is 0.55.
c. If the cat is 25 m away, and David is going 10 m/s
when he slams on the brakes, does he hit the cat?
Quiz part 2
1. The block given an initial velocity of 10 m/s is sliding
down the hill and will come to a stop due to friction
(µk=.7). Hint (The only force acting on the block down the hill is Fgx)
a. Draw a free body diagram of the block.
b. Write the Newton’s 2nd law equations.
c. Find the acceleration of the block
d. How far will it travel
before it stops?
Quiz: Part 3
Atwood Again…
A cord running over a pulley connects
two objects. The coefficient of friction
between the object and the table is
0.33. Find the acceleration of the
system and
7 kg
the tension of
the rope.
5 kg
The brakes are slammed on a speeding
truck and it skids to a stop. If the truck
were heavily loaded so it had twice the total
mass the skidding distance would be:
a) the same
b) 1 ½ times as far
c) twice as far
d) four times as far
Answer: a) the same
Twice the mass means the skidding
tires will bear against the road with
twice the force, which results in twice
the friction. Twice as much friction
acting on twice as much mass
produces the same deceleration and
hence the same stopping distance.