Physics 20
DYNAMICS
Day 3
–
Friction!
N2L

Solving Problems
Make a F
net
statement for each of the following:
1) A rocket blasts off
from the earth’s
surface.
2) A rocket fires its
engine
while in space.
F
net
= F
A
+

F
g
F
net
= F
A
4) Runner accelerating
F
net
= F
A
+

f
3) Pushing a car along
F
net
= 0: F
A
=
f
(constant speed)
N2L

Solving Problems
Make a F
net
statement for each of the following:
1)
Pushing the
ball to the
floor in a
dribble
2) Atwood machine
F
net
= F
A
+ F
g
F
net
= F
g5
+

F
g3
3) Horses pulling a wagon
F
net
= 0: F
A
=
f
(constant speed)
Word Problems
•
A car accelerates from rest at 3.00 m/s
2
for 10.00 s until
it reaches it’s top speed at which point it continues for
another 20.0 s. How far does it travel?
D = v
i
t
1
+ .5t
1
2
+ v
a
t
2
D = .5(3)(10)
2
+ 30(20)
To find velocity of 2
nd
motion
v
f
= v
i
+ at
v
f
= 0 + 3(10)
v
v
= 30 m/s
D = 750 m The car travels 750 m.
QUESTIONS FROM LAST NIGHT’S
HOMEWORK???
Smile of the Day!
Daily Objectives:
1)
Explain, qualitatively and quantitatively, static and kinetic
force of friction
acting on an object.
(20 B1.5k)
Frictionless Driving…
3.5
1) Friction
•
Friction
is a force that
always opposes
motion. Frictional
force drops to zero
when an object stops
moving.
•
The
cause of frictional
force is electrostatic
attraction
Friction:
•
The coefficient of friction is a measure of the
stickiness of the surface
•
0 < µ < 1 usually.
•
There are different values for static,
µ
s
, and
kinetic friction
,
µ
k
(µ
k
<
µ
s
)
Determining the Magnitude of Frictional Forces
= force of static friction (N)
= coefficient of static friction
= normal force (N)
The magnitude of the force of static friction
changes
as you push harder to a maximum
value just before motion occurs.
Static Friction
–
Friction acting on an object that is
not moving
= force of kinetic friction (N)
= coefficient of kinetic friction
= normal force (N)
Kinetic Friction
–
Friction
acting on an object that is
moving
•
Friction is a building force. As you push on an
object the frictional force builds to its maximal
value.
•
The
static frictional coefficient
drops to the
kinetic frictional coefficient
when the object
starts to move (surfaces separate slightly). The
static coefficient is used when the object is
stationary and the kinetic coefficient is used
during motion.
Comparing the Magnitudes of Static
and Kinetic Friction
The magnitude of the force of kinetic friction is
never
greater than the maximum magnitude
of the force of static friction.
Material
Coefficient of
Static Friction
s
Coefficient of
Kinetic Friction
k
Copper on copper
1.6
1.0
Steel on dry steel
0.41
0.38
Steel on greased steel
0.15
0.09
Dry oak on dry oak
0.5
0.3
Rubber tire on dry asphalt
1.2
0.8
Rubber tire on wet asphalt
0.6
0.5
Rubber tire on dry concrete
1.0
0.7
Rubber tire on wet concrete
0.7
0.5
Rubber tire on ice
0.006
0.005
Curling stone on ice
0.003
0.002
Teflon™ on Teflon™
0.04
0.04
Waxed hickory skis on dry snow
0.06
0.04
Waxed hickory skis on wet snow
0.20
0.14
Synovial fluid
0.01
0.01
Normal Force:
•
The value of the Normal force changes from
horizontal to vertical to incline plane cases.
The normal force on a horizontal surface is
equal in magnitude to the weight, but on a
vertical surface is equal to the push holding
the object in place.
Normal Force:
•
i
) horizontal: F
N
=

F
g
•
pulled or pushed object: F
N
increased or decreased by
component of force
•
ii) incline plane: F
N
=
F
g
cos
θ
•
iii) vertical: F
N
=

F
applied
Example 3.17
Kinetic Friction Applies to Skidding Tires
When an object is skidding on a surface, it is slowing down, so the forces acting on it are
unbalanced
.
The only force acting horizontally is the force of kinetic friction.
Example 3.19
Example: Calculating
friction f = µF
N
1) If a force of 100 N is required to start a mass
of 100kg moving, what is the static coefficient
of friction?
2) If the coefficient of kinetic friction is
µ
k
=
0.235, what is the force of friction on a 1500
kg car moving at constant velocity?
Examples:
Be able to solve questions where you have to
calculate the frictional force which may also involve
kinematics formulas and N2L
•
A 1300 kg car, traveling at 60 km/h, slides to a
stop on a surface with a coefficient of kinetic
friction of 0.315. How far will the car travel?
Homework:
Worksheet: Friction
•
# 1
a,c
; 2, 4, 6, 8, 10, 12, 14
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