Friction
What is Friction?
It is the opposing
force which acts
between any two
surfaces in contact
when there is a
relative motion
between them.
Cause of friction
Friction arises due the interlocking of
elevations and depressions in the two
surfaces in contact.
simulation
Friction is self adjusting force
Free Body Diagram
Normal
Force FN
Applied
Force F
Friction
Force ff
Gravity
Force Fg
Fg = mg
FN = F g
ff = F
Static Friction
The opposing force
that comes into play
when one body
tends to move over FN
the surface of F
fs
another.
Fg
f s FN s
s coefficient of static friction
Simulation
Limiting Friction
It is maximum value of force required to start
the motion.
Laws of limiting Friction
The magnitude of limiting friction between any two
surfaces in contact is directly proportional to the
normal reaction between them.
Direction of the force of limiting friction is always
opposite to the direction of motion.
Limiting friction depends on material and nature of
surfaces in contact.
Limiting friction does not depend on area of contact.
Kinetic Friction
Once the Force of Static
Friction is overcome, the Motion
FN
Force of Kinetic Friction
F
is what slows down a
fk
moving object!
Fg
f k FN k
k coefficient of kinetic friction
Types of Friction
To initiate motion of the box
the man must overcome the
Force of Static Friction
Upon sliding, the baseball
player will come to a
I better be complete stop due to the
safe Ump!! Force of Kinetic Friction
Static & Kinetic Friction Coefficients
Material Coefficient of Coefficient of
Static Friction S Kinetic Friction S
Rubber on Glass 2.0+ 2.0
Rubber on Concrete 1.0 0.8
Steel on Steel 0.74 0.57
Wood on Wood 0.25 – 0.5 0.2
Metal on Metal 0.15 0.06
Ice on Ice 0.1 0.03
Synovial Joints in 0.01 0.003
Humans
Static VS. Kinetic Friction
Ideal Result
What are the forces acting on the block
sliding down a rough inclined plane?
N
f
mg
Application Analysis
An empty cart is being
rolled across a
warehouse floor. If
the cart was filled, the
force of kinetic friction
between the cart and
the floor would
1. Decrease
2. Increase
3. Remain the same
Application Analysis
Sand is often placed on
an icy road because the
sand:
1. Decreases the coefficient
of friction between the
tires of a car and the road
2. Increases the coefficient
of friction between the
tires of a car and the road
3. Decrease the gravitational
force on a car
4. Increases the normal force
of a car on the road
Rolling is easier than sliding. Why
?
Methods to reduce friction
To reduce the amount of friction, apply a
lubricant between two surfaces.
Motor oil, wax, and grease are examples.
Methods to reduce friction
Streamlining
Methods to reduce friction
Use of ball bearings
Methods to reduce friction
Friction can also be reduced by rolling,
rather than sliding, an object.
Methods to reduce friction
Smoothening or polishing the surfaces .
How does hovercraft functions?
Friction is necessary evil.
Friction is called necessary evil, because we
can not do with out it , at the same time it is
an evil because it involves unnecessary
waste of energy and wear and tear.
Review
Friction -It is the opposing force which acts
between any two surfaces in contact when
there is a relative motion between them.
Static friction -The opposing force that comes
into play when objects are at rest.
Limiting friction-It is maximum value of force
required to start the motion.
Review
Kinetic or dynamic friction - It is the friction
which acts when the body is in motion.
Methods to reduce friction
Smoothening
Lubrication
Ball bearing
Streamlining
