Download Kinetic friction - Ridgefield School

Friction:
Friction: from book, ever
present resistance to motion
whenever two materials are
in contact with each other.
Friction: (ME) Two
surfaces rubbing together
and their stickiness
Causes Frictional Forces
It’s time to experience
friction so rub your hands
Friction: Two surfaces rubbing
together and their stickiness.
Always direction opposite of motion.
• frictional force: A force that slows motion, or prevents
motion from starting.
• Friction: frictional forces arise at the contact points
between the molecules of the different bodies. Contact
force that is resistance to Motion.
•ALWAYS OPPOSES MOTION, i.e, in opposite
direction of motion. Friction increases as contact
pressure increases, i.e., more weight means more
friction. Usually dissipated as heat or usually generates
heat.
•SURFACE AREA DOES NOT CHANGE FRICTION.
* Friction Force is Usually lowercase “f ” usually in
italics.
Frictional Forces
• frictional force: A force that slows motion, or
prevents motion from starting.
• 2 kinds – Kinetic friction – MOTION
•
slows objects down.
•
occurs during Motion.
-Static friction – REST
-prevents motion from starting.
occurs before motion, when still,
not moving, or at REST.
Which is bigger?????
Frictional Forces
• Which is bigger?????
Question: Does a car stop faster when the tires
are locked and sliding or when rolling?
• 2 kinds – Kinetic friction – MOTION
•
Sliding tires
-Static friction – REST
-Rolling tires
New Symbol
• μ – greek letter “Mu”
• Coefficient of friction.
– μs – coefficient of static friction
– μk – coefficient of kinetic friction
• Coefficient of friction: Decimal between 0.0 and
1.0, unitless. Decimal percent of force in the
normal direction. (Mu is percent of weight that
turns into friction force.)
• Coefficient of friction: μ where f = μ FN
• Usually just given in the problem, depends on
materials used and their surface conditions.
Static and Kinetic Frictional Forces
Kinetic Frictional Force: friction force that occurs while sliding.
The magnitude fk of the kinetic frictional force is given by:
fk = k FN
where  k is the coefficient of Kinetic friction, and FN is the magnitude of
the normal force.
Normal Force: Force that is 90 degrees to the surfaces. (Normal is 90)
degrees.
Kinetic frictional forces occur when the object is moving;
it acts to slow down the motion!
Frictional forces are independent
of the area of contact between
the surfaces! (talk about sides of block then race tires)
Static Friction, ƒs
fs £ m s N
• Just enough force to
keep object at rest.
• s is coefficient of
static friction
• N is the normal force
f
F
Kinetic
Friction, ƒk
f k = mk N
mk < ms
• k is coefficient of
kinetic friction
• Friction force opposes
direction of motion
• N is the normal force
f
F
Coefficients
of Friction
f £ ms N
f = mk N
ms > mk
Instructions for demo on next slide:
Place light mole box on table and slide.
Place brick on table and slide.
List mu static = .4, mu k = .3
Mass of light box is 2kg.
Mass of brick is 10 kg.
Find Friction resistance force when static
and when in motion on board.
Find both friction resistance and Reaction Force.
Same for Static but Not the same thing when in accelerating.
Newton’s 3rd Law:
Reaction Force Diagram
F
FR
F = Push by fingers = 90 N
FReaction = Push Back = -90 N
No exception, F always = -FR
Note: Reaction force is sometimes all friction
and sometimes not.
Newton’s 2rd Law: ΣF = ma
Free Body Diagram in X direction
(Box is in motion)
F
ΣF = ma
fk
fk = Kinetic Friction = k FN
F = Push
Net Force = ΣF = ma
ΣF = Fpush + fk = ma
Can also find acceleration:
a = ΣF/ m
Newton’s 2rd Law: ΣF = ma
Free Body Diagram in X direction
(Box is in motion)
F
ΣF = ma
fk
fk = Kinetic Friction = k FN
F = Push = 90N
= -30N
Net Force = ΣF = ma
ΣF = Fpush + fk = ma
ΣF = 90N - 30N = 60N
Can also find acceleration
a = ΣF/ m gives a = 60N / 10 kg = 6 m/s2
Static Friction Force
Kinetic Friction Force
μs = 0.4
fs = μsFN
μk = 0.3
fk = μkFN
Box fs = 0.4 x 20 N = 8 N
Brick fs = 0.4 x 100 N = 40 N
Box fk = 0.3 x 20 N = 6 N
Brick fk = 0.3 x 100 N = 30 N
Also known as breaking force:
= the force that will begin
movement
= maximum Static Friction
Force
= μsFN
Static and Kinetic Frictional Force
Example
A sled is traveling 4.00m/s along a horizontal stretch of snow.
The coefficient of kinetic friction is k = 0.0500. How far
does the sled go before stopping?
Static and Kinetic Frictional Force
Formula
1) FN= mg = weight
2) ΣF= fk , fk = k FN
3) ΣF= max = k FN
Known Variables Unknown Variables
4)
max = k mg
ax =
k = 0.0500
vo = 4.00 m/s
x =
5) (/m=>) ax= k g
vf = 0.00 m/s
6) ax = .05 (9.80m/s2)
7)vf2=vo2+2axx. Find x
Static and Kinetic Frictional Force
Formula
1) FN= mg = weight
2) ΣF= fk , fk = k FN
3) ΣF= max = k FN
Known Variables Unknown Variables
4)
max = k mg
ax = 0.49 m/s2
k = 0.0500
vo = 4.00 m/s
x =16.3m
5) (/m=>) ax= k g
vf = 0.00 m/s
6) ax = .05 (9.80m/s2)
7)vf2=vo2+2axx. Find x
Static and Kinetic Frictional Force
Formula
Did we need to know the mass of the 1) FN= mg = weight
sleder? No.
2) ΣF= fk , fk = k FN
Why? It cancels out in the ax
equation.
3) ΣF= max = k FN
Real Life Ap: This applies to car
max = k mg
tires in accidents. By measuring the 4)
length of skid marks, they can
5) (/m=>) ax= k g
calculate the speed a car was going
before an accident. k of a tire is the 6) ax = .05 (9.80m/s2)
same for all cars since it does not
7)vf2=vo2+2axx. Find x
depend on car mass or surface area
Static and Kinetic Frictional Forces
Static Frictional Force:
Reaction force to anything trying to
start motion.
•Equal and opposite to applied force.
•DOES NOT EXCEED
THE APPLIED FORCE,
but is equal to it.
Static and Kinetic Frictional Forces
Static Frictional Force:
Reaction force to anything trying to
start motion.
Equal and opposite to applied force,
until reaches maximum value and
motion starts.
friction “breaks” when F is great
enough and motion begins.
Static and Kinetic Frictional Forces
Static Frictional Force Breaks at a certain value:
fs = s FN
fs = force of static friction
s = coefficient of static friction
FN = Normal force
Static and Kinetic Frictional Forces
Static Frictional Force Breaks at a certain value:
fs = s FN
fs = force of static friction
s = coefficient of static friction
FN = Normal force
s is a given value. It depends on the object and
the surface.
Static and Kinetic Frictional Forces
Static Frictional Force:
fs = force of static friction
s = coefficient of static friction
FN = Normal force (usually weight)
Normal force is usually just the
weight of the object.
FN = Mass* 9.80 m/s2
IMPORTANT!!!!!
If the surface is not horizontal use trig.
Multiply by cos of the angle of incline.
Notes on friction
Almost always:
μ s > μk
It is easier to keep an object moving than it is to
start from rest. Think about pushing a car.
Both are almost always less than 1. If it was
greater than one, it would be easier to pick the
object up and carry it than it would be to push it
across the flat surface (something like velcro)