Download FORCES - Mr. Maloney

Friction and
spring force
Physics
Mr. Maloney
Objectives
You will be able to …
 qualitiatively describe friction and what factors
contribute to it.
 differentiate between static and kinetic friciton.
 describe and use a simple model of friction.
 describe and use the Hookean model of a
spring.
 use models of friction and springs as part of
force problems.
© 2002 Mike Maloney
Friction
 Friction
is the force that opposes the
motion between two surfaces.
 Friction always acts in a direction opposite
the direction of motion, and is always
parallel to the surfaces.
© 2002 Mike Maloney
Friction Defined (wikipedia)
Friction is not a fundamental force, as it is derived from
electromagnetic force between particles.
It cannot be calculated from first principles, but instead
must be found empirically (in the lab).
When contacting surfaces move relative to each other, the
friction between the two surfaces converts kinetic
energy into thermal energy, or heat. Contrary to earlier
explanations, kinetic friction is now understood not to
be caused by surface roughness but by chemical
bonding between the surfaces.
© 2002 Mike Maloney
Static Friction
 Static
Friction is the force that
opposes the start of motion.
 It is the friction that is at play before you get
something moving.
 If your applied force is greater than the
maximum value of static friction, the object
will move.
© 2002 Mike Maloney
Sliding (Kinetic) Friction
 Once
something starts moving, the friction
decreases.
 The friction between surfaces that are in
motion relative to one another is called
SLIDING FRICTION (kinetic friction).
 i.e., your car stops faster if it is not
skidding.
© 2002 Mike Maloney
Measuring Friction
 What
do you think friction depends on?
 Both types of friction depend on the force
that is pushing the surfaces together.
 This is the NORMAL force {FN}
 FN always acts perpendicular to the
surfaces in contact.
 It also depends on the surfaces.
© 2002 Mike Maloney
Measuring Friction

Experimentally it has been found that







Friction = (coefficient of friction) x Normal Force
Ff = µ∙FN
µ (pronounced mu) is the coefficient of friction
It is a constant that depends on the surfaces that are
interacting.
It is the ratio of how much Friction Force you get from a
certain Normal Force.
There is a static coefficient µs
And a sliding coefficient µk
© 2002 Mike Maloney
Coefficient of Friction
The coefficient of
friction is always
given for a set of
surfaces, such as
rubber on concrete.
It does not make
sense to say the
coefficient of friction
of rubber, or metal.
The object needs a
second surface
material that it is
interacting with.
© 2002 Mike Maloney
Simple Friction Problem 1
 A 100
kg mass is being pulled at a constant
velocity by a 490 N force. What is the
coefficient of friction between the object and
the ground?
 How do we solve this one?
© 2002 Mike Maloney
One last force
the Spring Force
 The
SPRING FORCE is a little different
from other forces.
 The NORMAL force, GRAVITY, FRICTION
and APPLIED forces we have talked about
all have constant values.
 The SPRING force changes
© 2002 Mike Maloney
Spring Force
 What
happens when you (compress)
squish a spring? Does it get harder or
easier to the farther you push?
 RIGHT,
 What
it gets harder.
about when you pull a spring?
 Correct
again, it also gets harder to move.
© 2002 Mike Maloney
Spring Force
 The
spring force is dependent on the
displacement of the spring from its rest
position.
 The formula for the SPRING force looks
like this
 Fsp
= ksp∙x
Ksp = spring coefficient
 X = displacement from rest

© 2002 Mike Maloney
Spring Force
 Ksp
varies from spring to spring.
 The units of Ksp are N/m
 It is the measure of the amount of force
needed to stretch a spring a unit distance,
in our case the number of newtons needed
to stretch a spring 1 meter.
© 2002 Mike Maloney
Spring Force
 Even
though the SPRING force is a little
different from other forces it can still be
treated the same.
 It can be added and combined just like any
other force.
 We will see this guy again in a couple
chapters.
© 2002 Mike Maloney
Spring Example
 Suppose
a 100 N weight is hanging from a
spring. The spring is stretched 0.5 meters.
What is the Ksp of that particular spring.
= Ksp*x  Ksp = Fsp / x
 Ksp = 100 N / 0.5 m = 200 N/m
 Fsp
© 2002 Mike Maloney
Spring Example 2
 How
far does a spring stretch that has a Ksp
of 300 N/m when a Force of 100 N is
applied to it?
= Ksp*x  x = Fsp /Ksp
 X = 100 N / 300 N/m = 0.33 m
 Fsp
© 2002 Mike Maloney
Objectives
Can you …
 qualitiatively describe friction and what factors
contribute to it.
 differentiate between static and kinetic friciton.
 describe and use a simple model of friction.
 describe and use the Hookean model of a
spring.
 use models of friction and springs as part of
force problems.
© 2002 Mike Maloney