Friction
• Friction- a force that opposes an impending or actual
motion.
Static Friction
• Resists initial motion.
• Coefficient of static friction determines how much it
resists and depends on the materials.
• μs
• As you apply more force to a block, the block will
ultimately break loose and move.
• It happens when you reach the threshold of motion.
• The ToM exceeds the maximum possible value of the
static friction
3 Insights to Static Friction
• Ff is proportional to the normal force.
• While proportional, it depends on the surface.
• Ff is independent of the apparent size of the contact
area between the two solid surfaces. (The surprising
one).
Ff = μsFN
Example
• A climber stands on the rock face of a mountain. The
soles and heels of her boots have a static friction
coefficient equal to 1.0.
a. What is the steepest slope she can stand on without
slipping?
b. Assuming she has a static friction coefficient
between pants and rock 0.3, what happens if she
sits down to rest?
Hint: tanθ= sin θ/cos θ
• Static friction is WHY WE CAN MOVE
• It’s the driving force
• Grab a book and a protractor (any book in your bag
will do).
• Take your calculator (or mine---slackers) and place it
on the cover.
• Raise it until the calculator starts to slide. What’s the
angle?
• Calculate the coefficient of static friction between
your calculator and your book.
Kinetic Friction
• Definition: the retarding force exerted on a sliding
body in contact with a surface.
• If in uniform motion (i.e, equilibrium), it will be
equal and opposite to the applied force.
• Kinetic friction is generally less than static.
F f = μk F N
Causes of Friction
• The force that holds atoms and molecules together is
electromagnetic.
• Without this force, there’d be nothing. Not you, not
me, not textbooks…etc.
• Only when atoms are close to each other will they
experience an appreciable force attractive force.
(adhesion)
• Short range force
• Negligible at distances equivalent to only about 4-5
atomic diameters. (1-2 x 10-10 m)
• Common objects may look smooth, but are jagged on
a microscopic level.
• Static friction arises out of the need to rip apart the
areas of bonded contact.
• To move the two surfaces, the welds must be torn.
• Keeping the surfaces separate helps reduce adhesion
and friction.
• Ex: baby oil, grease
• Adhesion is lessened if the two surfaces are already
moving…that’s why μs > μk