Download FORCES: Weight, The Normal Force, and Friction

FORCE DUE TO FRICTION
What do you think?
• How would you describe friction?
▫ How is friction related to: heat, energy,
force, velocity
Review: Weight and Mass
• Mass is the amount of matter in an object
▫ Kilograms, slugs
• Weight is a measure of the gravitational force on an
object
▫ Newtons, pounds
▫ Depends on the acceleration of gravity
• Weight = mass  acceleration of gravity
▫ W = mag where ag = 9.81 m/s2 on Earth
▫ Depends on location
 ag varies slightly with location on Earth
 ag is different on other planets
Review: Normal Force
• Force on an object
perpendicular to the surface
(Fn)
• It may equal the weight (Fg),
as it does here
• It does not always equal the
weight (Fg), as in the second
example
▫ Fn = mg cos 
Static Friction
• Force that prevents motion
• Abbreviated Fs
• How does the applied
force (F) compare to the
frictional force (Fs)?
• Would Fs change if F was
reduced? If so, how?
• If F is increased
significantly, will Fs
change? If so, how?
• Are there any limits on
the value for Fs?
Kinetic Friction
• Force between surfaces that opposes movement
• Abbreviated Fk
• Does not depend on the speed
• Using the picture, describe the
motion you would observe
▫ The jug will accelerate
• How could the person push the
jug at a constant speed?
▫ Reduce F so it equals Fk
Calculating the Force of Friction (Ff)
• Ff is directly proportional to Fn (normal force)
F f   Fn

Ff
Fn
• Coefficient of friction ():
▫
▫
▫
▫
Determined by the nature of the two surfaces
s is for static friction
k is for kinetic friction
s > k
Typical Coefficients of Friction
• Values for  have no units and are approximate
Classroom Practice Problem
• A 24 kg crate initially at rest on a horizontal floor
requires a 75 N horizontal force to set it in motion.
Find the coefficient of static friction between the
crate and the floor.
▫ Draw a free-body diagram and use it to find:
 the weight
 the normal force (Fn)
 the force of friction (Ff)
▫ Find the coefficient of friction.
• Answer: s = 0.32
Classroom Practice Problem
• A student attaches a rope to a 20.0 kg box of books.
He pulls with a force of 90.0 N at an angle of 30.0˚
with the horizontal. The coefficient of kinetic friction
between the box and the sidewalk is 0.500. Find the
magnitude of the acceleration of the box.
– Start with a free-body
diagram.
– Determine the net force.
– Find the acceleration.
• Answer: a = 0.12 m/s2