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?
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 surfacearea (in contact) mean more friction?
Friction is not dependant of surface area in contact, or surface smoothness.
Friction depends on the two types of surfaces in contact.
“Sticky” surfaces “slippery” surfaces
rubber tire and asphalt rubber tire and wet asphalt
rubber and wooden flooring leather (shoes) and wooden flooring
glass and glass 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)
It opposes motion.
It always acts parallel to the surface.
It cannot cause motion!
usingFREE BODY DIAGRAMS to show forces
What force is going to cancel the weight?
NORMAL FORCE IS THE FORCE OF A SURFACE ON AN OBJECT.
IT IS ALWAYS DIRECTED PERPENDICULAR TO THE SURFACE.
The resulting force causes
Fnet = Fa - f
Fnet = ma
Once you get it moving, it is easy to keep moving. Why is that?
The constant K is called the “coefficient of kinetic friction”
µK 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)
µkinetic = Fapplied / Fnormal
The constant µS is called the “coefficient of static friction”
µS is discovered by increasing Fapplied until the
block is at the brink of starting to slide.
Fapplied – µstatic (Fnormal) = ma
if: ma = 0(no motion)then: Fapplied = µstatic (Fnormal)
µstatic = Fapplied / Fnormal
static friction kinetic friction
Material on Material
steel / steel0.60.4
add grease to steel0.10.05
metal / ice0.0220.02
brake lining / iron0.40.3
tire / dry pavement0.90.8
tire / wet pavement0.80.7
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
Temperature of road/tires