Download Frictional Force

Forces
Friction
The Lucky Cow
The driver of the car applies the brakes to avoid hitting the cow.
But how does this cause the car to slow down and stop? The brakes
cause the wheels to stop turning and to slide on the road surface.
This action produces a force that resists the forward movement of
the car. This force is called friction.
Forces
Friction
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Friction
All objects encounter
friction of some sort
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Friction
“How do we calculate the frictional force?”
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Friction
Types of Frictional Forces:
1.
2.
3.
4.
Static
Kinetic
Rolling
Fluid
Forces
Kinetic (Dynamic) Friction
Friction
It’s the friction encountered in motion.
Let's say you had a crate and you wanted to move it somewhere.
First, you have to overcome the static friction between the floor and
the crate to get it moving. Once you have it moving, you have to
keep it moving by overcoming the kinetic (moving) friction - which
is always much easier!
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Friction
Friction depends on:
1. The type of surfaces (u) “mu” - coefficient of friction
2. How hard the surfaces are pressed together (FN ) – Normal force
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Friction


Friction always opposes motion.
The applied force must be greater than the static frictional force
to move the block.
Forces
Friction
f s   s FN
static frictional
force
“Mu” coefficient
of static friction
Normal
force
f s (max)   s FN
maximum force
of static friction
fs max is the force you must
exceed to move the object.
Forces
Friction
f k  k FN
Kinetic
frictional force
“Mu” coefficient
of Kinetic friction
Once object moves.
 Use k
 fk is constant
 There is no “maximum fk.”
Normal
force
Forces
Friction

The coefficient of static friction is
f
s 
FN
where us= coefficient
FN= normal force
f = friction force
f
N
k 

FN
N
The coefficient of friction is dimensionless – no units.
The greater the us the more force needed to move the object.
Forces
Friction
The frictional force increases until it reaches a maximum
(in this case, 2.5N)
If we push harder still, (say, 2.6N), the block will move.
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Link
Friction
On the verge of slipping
FA
f
Sliding
Maximum Static Friction
Sliding
(Kinetic)
Friction
Friction, f
Applied Force, FA
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Friction
 Static friction, fs
 Kinetic friction, fk
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Friction
The object begin to move if the applied force
is larger than the max static friction.
Here the kinetic friction remains constant
no matter how large is the applied force.
Here, fl stands for the max static friction and
fk is the kinetic friction. It is slightly less than
the max static friction.
Forces
Friction
Approximate Coefficients of Friction
Forces
SUMMARY
Friction
Friction force, f, is proportional to normal force, FN.
f s  s n
f k  k n
us and uk are coefficients of friction
Frictional force is opposite the motion
Values of us and uk depend on surfaces
us and uk don’t depend on surface area
us and uk don’t depend on the velocity
Friction is due to the surfaces interacting
with each other on the microscopic level
(sliding over bumps, chemical bonds)
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Friction
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Friction
You push a giant barrel o’ monkeys setting on a
table with a force of 63 N. If k = .35 and s =.58,
when will the barrel have moved 15 m?
answer:
14.7 kg
Never, since this force won’t even budge it!
63 < 0.58 (14.7) (9.8)  83.6 N
Forces
Friction
Friction as the net force
A runner attempts to steal third
base. He’s running at a speed v
and has a mass m. The coefficient
of kinetic friction between his
uniform and the base pass is .
1. How far from second base
should he begin his slide in
order to stop right at the base?
Give the answer in terms of the
given quantities and any constants.
m, , and v.
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Friction
Measuring uk
A hockey puck is given an
initial speed of 20.0 m/s. It
slides 115 m before
coming to rest.
(a) Determine the coefficient of kinetic friction between
the puck and the ice.
(b) Could the coefficient of friction be larger than 1?
Forces
Friction
Experience might lead you to believe that if M was very
large, say 2000 Kg, m would not budge. This is where
friction comes in.
m
a
g
mM
Forces
Friction
A crate of mass 20 kg is sliding across a wooden floor. k
between the crate and the floor is 0.3. Determine the
strength of the net force acting on the crate. If the crate is
being pulled by a force of 90 N (parallel to the floor), find
the acceleration of the crate.
FN
.
fk
F
Fg
Fnet  F.  f k  90 N  59 N  31N
Fnet 31N
Fnet  ma a 

 1.6 sm2
m
20kg
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Friction
Problem: Forces are being applied to a box sitting on a surface
with friction. Will the box move horizontally (along the surface)?
F1=50N, F2=50N, Mass of the block 10kg, and us=0.4.
Forces
Friction
Measuring us
A brick is placed on an inclined
board as shown in the figure.
The angle of incline is increased
Until the block starts to move.
(a) Determine the static friction coefficient from the critical
angle, c, at which the block starts to move.
(b) What is us if the block starts sliding at C = 31°
Forces
Friction
If we slide or attempt to slide a body over a surface, the
motion is resisted by a bonding between the body and
the surface. The resistance is a force called the
frictional force, or simply friction. This force is directed
along the surface, opposite the direction of the intended
motion.

f
Forces
Friction
Block B weighs 711 N. The coefficient of static friction
between the table and the block is 0.25: assume the cord
between the knot and block B is horizontal. Find the
maximum weight of a block A for which the system will be
stationary.
Forces
Friction
Two blocks are connected by a cord over a frictionless
pulley. The mass of block A is 10 kg and the coefficient of
kinetic friction between and the incline is 0.20. The angle
of the ramp is 30o. Block A slides down the incline at
constant speed. What is the mass of block B?
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