Download Physics 207: Lecture 2 Notes

Chapter 5
Assignment:
 Problems p 105 – 107
 #’s 15-17 and 19-21
Friction a force that opposes motion
What do you think friction depends on?
It depends on the two surfaces that are in contact.
Not all surfaces have the same amount of friction.
Do rough / smooth surfaces have more friction?
A rough surface will have less overall contact with an object.
More or less friction?
Smooth surfaces will have more contact with an object.
More or less friction?
It depends on the size of the object.
Does more mass mean more friction?
Does more surface area (in contact) mean more friction?
Friction...

Friction is caused by the “microscopic” interactions
between the two surfaces. Your book refers to it as
“electromagnetic bonding”
Friction is not dependant of surface area in contact, or surface smoothness.
Friction depends on the two types of surfaces in contact.
“Sticky” surfaces
rubber tire and asphalt
rubber and wooden flooring
glass and glass
“slippery” surfaces
rubber tire and wet asphalt
leather (shoes) and wooden flooring
paper and a desktop
FRICTONAL COEFFICIENT ( µ )
(mu, “mew” 12th character of the ancient Greek alphabet)
A number (coefficient) that is given to two materials that helps
determine the frictional force between them.
µ will always be less than 1 (one) for us in this class.
(sticky surfaces can be more than 1)
friction

How does it work?
It opposes motion.
It always acts parallel to the surface.
It cannot cause motion!
using FREE BODY DIAGRAMS to show forces
N
F applied
F friction
mg
FREE BODY DIAGRAMS
Vector arrows are used to represent forces.
What force is going to cancel the weight?
NORMAL
FORCE
F applied
F friction
m
Weight (mg)
NORMAL FORCE IS THE FORCE OF A SURFACE ON AN OBJECT.
IT IS ALWAYS DIRECTED PERPENDICULAR TO THE SURFACE.
What if Fapplied is larger than the frictional force?
The resulting force causes
an acceleration.
F normal
F applied
F friction
m
Weight (mg)
Fnet = Fa - f
Fnet = ma
Remember Newton’s first law (inertia) and the
level of force needed to move a refrigerator;
Once you get it moving, it is easy to keep moving.
Ugh!
Why is that?
Sliding Friction
The constant K is called the “coefficient of kinetic friction”
µ K is discovered by pulling with a constant Fapplied
as the object slides at a constant speed.
Fapplied – µkinetic (Fnormal) = ma
if: ma = 0 (no acceleration)
then: Fapplied = µkinetic (Fnormal)
or:
µkinetic = Fapplied / Fnormal
Static Friction...
The constant µ S is called the “coefficient of static friction”
µ S is discovered by increasing Fapplied until the
block is at the brink of starting to slide.
Fapplied – µstatic (Fnormal) = ma
if: ma = 0
then: Fapplied = µstatic (Fnormal)
(no motion)
or:
µstatic = Fapplied / Fnormal
Some coefficients of friction
static friction
s
Material on Material
steel / steel
add grease to steel
metal / ice
brake lining / iron
tire / dry pavement
tire / wet pavement
0.6
0.1
0.022
0.4
0.9
0.8
kinetic friction
k
0.4
0.05
0.02
0.3
0.8
0.7
More on friction :
Friction between road and tires is traction or adhesion
It takes nearly 25% longer to stop on wet
asphalt as dry
four times longer to
stop on dirt road as dry asphalt.
Wet snow is almost impossible to start,
stop or turn a vehicle on (inertia)
Coefficient of friction
– A. Dry asphalt is .78
– B. Wet asphalt is .60
– C. Dirt road is .20
– D. Dry snow is .20
– E. Wet snow is .10
Types of Traction
There are three types of traction. Each are to
overcome inertia. (rest, motion, straight line)
– A. Acceleration traction
– B. Braking traction
– C. Cornering traction
Things That Can Affect Traction.
A.. Tires - Type, Wear, Inflation
B.. Road Surface - Asphalt., Concrete, Gravel, Dirt
C.. Condition of road – Surface, Rain, Snow, Ice
More Factors that affect Traction:
 Weight of vehicle
 Type of vehicle
 Bank of road/curve
 Speed of vehicle
 Driver response
 Temperature of road/tires