Download Document

Homework
• P. 146 30, 33, 34, 35, 37
What is Friction?
 Why is there Friction?
Surface roughness
Electronic interactions at the atomic level
Friction is caused by the temporary
electrostatic bonds created between two
objects in contact with one another.
Examples of Friction
- Desirable
- Undesirable
What causes friction?
• Why is there Friction?
Surface roughness
Electronic interactions at the atomic level
 Friction is caused by the temporary electrostatic
bonds created between two objects in contact with
one another.
• Examples of Friction
- Desirable
- Undesirable
Examples of Friction
- Desirable
- Walking
-
- Driving
- Braking
Undesirable
- Engine Efficiency
- Coasting
- Pushing a heavy object
Why would I want to change friction?
- How would I do it?
Friction & Applying Newton’s 2nd
Law
System
Chapter 6.2
Friction
• How does friction affect the motion of
objects?
– It can slow an object down
like the friction between
the tires and the road.
– It is responsible for
increasing the speed of an
object like a car.
– It is also responsible
for objects being able to
change direction.
Static Friction
• Static Friction:
– The resistive force that keeps an object from
moving.
Fground-on-crate
Fforward
Ffriction
Fnet = Fforward – Ffriction
Fforward
Ffriction
System
Fgravity
Since the crate is not accelerating, Fnet = 0
Fforward = Ffriction
Note: As long as the crate does not move, Fforward = Ffriction
Kinetic Friction
• Kinetic Friction:
– The resistive force that opposes the relative
motion of two contacting surfaces that are moving
past one another.
– Since the crate will initially accelerate, Fnet > 0.
Fground-on-crate
Ffriction
Fforward
Ffriction
System
Fgravity
Fforward
Fnet
Fnet = Fforward – Ffriction
Note: If the crate moves at a constant
speed, then Fforward = Ffriction and Fnet =
0.
An Important Term
• APPLIED FORCE
– Usually whatever is pushing or pulling
– NOT the same as Net Force
Determining the Frictional Force
For people who had a lot of wrong ideas about Physics the
Greek alphabet sure gets used a lot!
• The force of friction is proportional to the normal
•
•
•
•
•
force and a proportionality constant ( - pronounced
mu) called the coefficient of friction. FN
For static friction:
– 0 < Ff, static < sFN
For kinetic friction:
Ff
– Ff, kinetic =  kFN
Note: FN = the force normal (perpendicular) to
the frictional force on the object.
 is dimensionless
Ff, static > Ff, kinetic
Frictional Force
• For static friction:
– 0 < Ff, static < sFN
• For kinetic friction:
– Ff, kinetic =  kFN
Determining the Frictional Force
•  (the coefficient of friction) is usually in the range
of 0<=  <= 1, but this is not always the case
Material 1
Material 2

Tire, dry
Road, dry
1
Tire, wet
Road, wet
0.2
Rubber
Steel
1.6
Teflon
Teflon
0.04
Ice
Wood
0.05
Glass
Metal
0.5 - 0.7
Chromium
Aluminum
Chromium
Aluminum
0.41
1.3
Determining the Frictional Force
• Sketch a graph of Fs vs applied force
• Sketch a graph of Fk versus applied force
• Sketch a graph showing the transition from Fs
to Fk
Ff versus applied force
Ff versus applied force
The Normal Force
• The normal force is a force that opposes the
Earth’s gravitational attraction and is
perpendicular to the surface that an object
rests or is moving on.
– For a horizontal surface, FN = Fg = mg.
– For a surface that is not perpendicular to gravity,
FN = Fgcos
FN

The Normal Force
FN
Fg
FN

cos = adj/hyp
F
g
FN = Fg = mg
FN = Fg cos = mg cos
Example 2: Determining Friction
(Balanced Forces)
•
Assume that the man in the figure is
pushing a 25 kg wooden crate across a
wooden floor at a constant speed of 1 m/s.
–
How much force is exerted on the crate?
FN
Fforward
Ff
System
Fg
Diagram the Problem
+y
FN
Fforward
Ff
System
FN
Fg
y-direction: FN = Fg
x-direction: Fnet = Fforward - Ff
Fforward
Ff
Fg
+x
Since the crate is moving with constant speed,
a = 0, Fnet = 0, and Fforward = Ff
State the Known and Unknowns
• What is known?
o Mass (m) = 25 kg
o Speed = 1 m/s
o Acceleration (a) = 0 m/s2
o k = 0.2 (wood on wood)
• What is not known?
o Fforward = ?
Perform Calculations
• y-direction:
o Fg = FN = mg
• x-direction: a = 0
0
o Fnet = Fforward – Ff
o Fforward
o Fforward
o Fforward
o Fforward
=
=
=
=
Ff
kFN; Fforward = kmg
(0.2)(25 kg)(9.8 m/s2)
49 N
Example 3: Determining Friction
(Unbalanced Forces)
Assume that the man in the figure is pushing a
25 kg wooden crate across a wooden floor at a
speed of 1 m/s with a force of 49 N.
•
–
If he doubled the force on the crate, what would
the acceleration be?
FN
Assume
Constant
speed
Fforward
Ff
System
Fg
Diagram the Problem
+y
FN
Fforward
Ff
System
FN
Fg
Fforward
Ff
Fg
+x
y-direction: FN = Fg
x-direction: Since a > 0, Fnet = Fforward - Ff
State the Known and Unknowns
• What is known?
o Force = 98 N
o Mass (m) = 25 kg
o Speed = 1 m/s
o k = 0.2 (wood on wood)
• What is not known?
oa?
Perform Calculations
• y-direction:
o Fg = FN = mg
o Fnet = 98N – 48N
• x-direction: a > 0
o Fnet = Fforward – Ff
o ma = 49N
o ma = Fforward – Ff
o a = 49N/25kg
o ma = Fforward – kmg
o a = 2.0 m/s2
o a = Fforward – kmg
m
o a = (98N)/(25kg) – (0.2)(9.8 m/s2)
o a = 2.0 m/s2
Key Ideas
• Friction is an opposing force that exists
between two bodies.
• Friction is proportional to the normal
force and the coefficient of friction;
static or kinetic.
• The force required to overcome static
friction is greater than that required to
overcome kinetic friction.