Don't forget tomorrow's HMWK: p. 123, #19-24, 27.
Also announced: Midterm will be next Monday and will cover content (more broadly than other tests) from the class so far this semester from measurement and scientific method to forces.
Today started with a question; if surface area is changed, friction doesn't change (on the average, over multiple trials). WHY? It seems like we learned that microwelds happen and cause friction where objects touch, so it makes sense for surface area to be a factor... and it is, sort of. MICROSCOPIC surface area is what counts, and it depends on the surface types and the force pushing the surfaces together. When surface area is changed on the macroscopic (what we can see) level, say by putting a box on its end, the weight of the object doesn't change... the pressure per unit area is greater, causing the box's surface to compress any tiny bumps within it, and microscopic surface area in contact actually remains the same as it was when the box sat normally. A student said his answer well today; "the microwelds in an area are more concentrated."
Today's notes:
Friction
Force that opposes sliding motion
Friction:
n Affects objects in contact
n Opposes sliding forces up to a maximum
n Depends on the surfaces involved and the force pushing them together
What is friction caused by?
n Tiny bumps in all material come into contact
n Microwelds are formed
n Small attractive forces “bond” materials
n More force pushes materials closer together
n Leads to more microwelds
Types of Friction: Static Friction
n Surfaces not moving relative to one another
n Static Friction is the strongest type of friction.
Kinetic Friction
n Once in motion, objects don’t form as many microwelds.
n Results in lower frictional resistance
n Easier to keep something sliding than to start it.
Rolling and Friction
n Reduces sliding friction involved greatly
n Application – bearings – small cylinders or balls that surfaces roll over
n (wheels as well)
Calculating Frictional Force
n Frictional force =
force pushing surfaces together • coefficient of friction
A coefficient of friction is just a ratio (%) of force between surfaces that gets transformed into friction
Coefficients of friction:
μs = coefficient of static friction
μk = coefficient of kinetic friction
μ = greek letter mu (lower case) pronounced “myeou”
Practice problem 1:
n A 4N book is being pulled across a surface with a μk of 0.29. Find the frictional force.
Practice Problem 2
n A 10N textbook requires 8N of force to get it to start moving, and 6N to keep it moving. Find μs and μk
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