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FRICTION
WHAT CAUSES FRICTION?
We believe that intermolecular forces between two
surfaces form microscopic welds that must be broken for
an object to move.
As the object moves, these microscopic welds form and
break continuously.
WHAT DETERMINES THE STRENGTH
OF THE FORCE OF FRICTION?


materials in contact
normal force
FORCE OF FRICTION
Ff  μFn
COEFFICIENT OF FRICTION
The coefficient of friction is a number that indicates the
ratio of the magnitude of the force of friction (Ff)
between two surfaces to the magnitude of the normal
force (Fn - force perpendicular to these surfaces).
Ff - force of friction (N)
μ - coefficient of friction
Fn - normal force (N)
2 TYPES OF FRICTIONAL FORCES:
1. Friction can be STATIC, when the object is NOT
moving
2. Friction can be KINETIC, when the object is moving
An object at rest experiences more friction.
The force needed to start the motion of an object initially at rest is
greater than the force needed to keep it going at a constant
velocity. Static friction is usually slightly greater than the kinetic
friction.
2 TYPES OF COEFFICIENT OF FRICTION:
1. coefficient of static friction us
2. coefficient of kinetic friction uk
HOW TO SOLVE PROBLEMS THAT INVOLVE FRICTION
1. Write down your GIVENS
2. Draw a Free Body Diagram (FBD) – include all forces
acting on the object
3. Select positive direction (based on applied forces)
4. Solve for Fn using Fnet  ma (usually Fnety  0 )
5. Solve for Ff  μFn
6. Solve for a using Fnet  ma
Determining the coefficients of friction
for various surfaces can be done only
experimentally.
See Table 1 on page 170.
EXAMPLE 1
A 3.0kg block of wood sits on a
horizontal wooden floor. The largest
horizontal force that can be applied
to the block before it will start moving
is 14.7N. Once the block starts
moving, it only takes 8.8N to keep it
moving at a constant velocity.
a) Calculate the coefficient of static
friction for the block of wood and
the floor.
b) Determine the coefficient of kinetic
friction.
STATIC FRICTION PROBLEMS
EXAMPLE 2
Two sleds are tied together with a rope. The coefficient of static friction between each sled
and the snow is 0.22. A small child is sitting on sled 1 (total mass of 27 kg) and a larger child
sits on sled 2 (total mass of 38 kg). An adult pulls on the sleds.
a) What is the greatest horizontal force that the adult can exert on sled 1 without moving
either sled? (ans: 140N[fwd])
b) Calculate the magnitude of the tension in the rope between sleds 1 and 2 when the
adult exerts this greatest horizontal force. (ans: 82N)
EXAMPLE 3
A 4.0 kg block of wood sits on a table. A string is tied
to the wood, running over a pulley and down to a
hanging object. The greatest mass that can be hung
from the string without moving the block of wood is
1.8 kg. Calculate the coefficient of static friction
between the block of wood and the table.
EXAMPLE 4
The coefficient of static friction between a person’s
shoe and the ground is 0.70. Determine the
maximum acceleration of the 62.0kg person, if he
starts running on a horizontal surface from rest.
(ans: 6.9m/s2 [fwd])
KINETIC FRICTION PROBLEMS
EXAMPLE 5
The following forces are applied on a box: 18N [down], 22 N [right], 70 N [left], 13 N [up].
The box has a mass of 14kg and =0.12.
a) Draw a Free Body Diagram. (add Fg, Fn and Ff)
b) Calculate the force of friction acting on the box.
c) Calculate the acceleration of the box.
EXAMPLE 6
Two sleds tied together are pulled across an icy surface with an applied force of 150 N [E].
The mass of sled 1 is 18.0 kg and the mass of sled 2 is 12.0 kg. The coefficient of kinetic
friction for each sled is 0.20.
a) Calculate the acceleration of the sleds. (ans: 3.0m/s2[E])
b) Determine the magnitude of the tension in the rope between the sleds. (ans: 60N)
EXAMPLE 7
The truck’s brakes are applied so hard that the truck starts to skid on a dry asphalt road. If
the truck and its contents have a mass of 4.2 x103 kg, determine the force of kinetic friction
on the truck. (ans: 2.7x104N)
EXAMPLE 8
A 0.17kg hockey puck has a velocity of 19 m/s [F] when it suddenly hits a rough patch of
ice that is 5.1m across. Assume that the coefficient of kinetic friction between the puck
and the rough ice is 0.47.
a) Draw a Free Body Diagram.
b) Calculate the kinetic friction acting on the puck.
c) Determine the puck’s acceleration on rough ice.
d) Calculate the puck’s velocity as it leaves the rough ice
and returns to smooth ice. (ans: 18m/s)
EXAMPLE 9
A string is tied to a 3.2 kg object on a table and a
1.5 kg object hanging over a pulley. The
coefficient of kinetic friction between the 3.2 kg
object and the table is
0.30.
a) Calculate the acceleration of each object.
b) Determine the magnitude of the tension in the
string.
c) How far will the objects move in 1.2 s if the
initial velocity of the 3.2 kg object is 1.3 m/s
[right]? (ans: 2.4m)
HOMEFUN 4.2 and 4.3
read pg 168-171 questions pg 172 #1-7
read pg 173-177 questions pg 178 #1-10
EXAMPLE 10
A lawnmower (12kg) is being pushed by a horizontal force of
150.0N and a downward force of 40.0N.
a) If the kinetic coefficient of friction between the wheels
and the grass is 0.90, find the force of friction acting on the
lawnmower. (ans: 142N[bwd])
b) Determine the lawnmower acceleration.
(ans: 0.68m/s2 [bwd])
EXAMPLE 11
Two people are pushing on a crate of mass 50.0 kg.
One person pushes left with a force of 80.0N, the
other person pushes right with a force of 50.0N. The
coefficient of friction is 0.30. Find the acceleration of
the crate. The answer is tricky!! 
EXAMPLE 12
Your physics teacher is erasing the board with an eraser that has a
mass of 0.50 kg block. She slides the eraser up the chalkboard with
an applied force of 6.0N upward and 2.0N towards the board. If the
coefficient of kinetic friction is 0.40, calculate the acceleration of the
block. (ans: 0.60m/s2 [up])
TRY THIS!!!
A hockey puck of mass 0.30kg is sliding along ice at 6.0 m/s (assume frictionless). It
encounters a rough patch of ice with a coefficient of kinetic friction of 0.10.
a) How long will it take for the puck to stop? (ans: 6.1s)
b) How far will it travel? (ans: 18m) motion equation