Download 5.1 Friction Intro and Static Friction

5.1 Friction Intro and Statics
SWBAT describe what factors affect
the frictional force.
SWBAT calculate the static friction
between two surfaces.
Read and outline p. 135-139
A phonebook is at rest on top of a table. If it’s
mass is 2 kg, what is the normal force acting on
the book?
After answering this question in your notebook, answer the following:
The pages of phone book A and B are
interleaved (shuffled together). How
much force does it take to pull them
apart? Do you think you can do it???
Definition: Friction is the force exerted by a
surface as an object moves across it or makes an
effort to move across it.
1. Do all surfaces exert the same frictional force?
2. What other factor can increase frictional force?
We measure a surfaces’ “roughness” using the
coefficient of friction.
μ (mu) is the symbol of the coefficient of friction.
– μ does not have units.
– μ is a scalar quantity.
The larger the μ, the rougher the surface.
(when μ=0, the surface is frictionless)
Where can we find the coefficients of friction?
Typical Metal Surface
Polished Metal Surface
Wood Surface
Porcelain Surface
1. Do all surfaces exert the same frictional force?
– No, we use μ to differentiate between surfaces.
2. What other factor can increase frictional force?
Which will have a larger force of friction?
(Which will be harder to move?)
Fa= 5 N
Fa= 5 N
Which will have a larger force of friction?
(Which will be harder to move?)
Fa= 5 N
Fa= 5 N
Which will have a larger force of friction?
(Which will be harder to move?)
Fa= 5 N
1. Do all surfaces exert the same frictional force?
– No, we use μ to differentiate between surfaces.
2. What factors can increase the force of friction?
– Changing the normal force acting on the object.
How are these factors related to the frictional force?
Frictional Force = (coefficient of friction) (Normal Force)
Where is the formula for friction?
A force that resists the initiation of sliding motion
between two surfaces that are in contact and at
rest.
Static Friction (Ffs) acts
opposite the applied force (Fa)
As Fa increases, Ffs increases
until FfsMAX is reached and the
block starts to move.
FfsF
fs
FF a
a
Friction always acts
parallel to the surface
of contact
A 5 kg wooden block is at rest on a wooden
table.
a) How much force is required to start the block
moving?
b) What would the frictional force be if the
applied force is 10 N?
Zip the monkey wants to move a stationary 20 kg crate
of bananas across a horizontal floor. The material of
the crate and the floor are unknown, but it takes 10 N
to start the crate moving (the maximum static friction
is 10 N). What is the coefficient of static friction
between the crate and the floor?
A 10 kg copper block is at rest on a steel table.
How much force is required to start the block
moving across the table?