Prof.
Clements Notes/Hints for Physics Courses
Chapter
5 Only Document Open Stax College Physics or Introductory Physics
Courses
Friction, Springs
Terms
you should know at the end of this unit: friction, kinetic, static,
drag force, terminal velocity, spring, stretch, compress, force
constant
Friction
The
force of friction is a result of the microscopic interaction
of surfaces. I will not discuss the details of this interaction.
Instead I (and most physics books) will use the a model to calculate
the force of friction.
The
force of friction is always in the direction opposite (180 degrees)
to the velocity of the object. If the object is not moving the
direction of the force of friction is opposite the direction of the
net applied force on the object.
Static
Friction fs
≤ μsN,
μs is the coefficient of friction. N
is the magnitude of the normal force. On a horizontal surface the
magnitude of N is equal to the weight of the object. Static
means velocity = 0. Notice that the force of static friction is a
variable force. Try pushing on something heavy with just one of your
fingers. You should feel that the force increases until the object
starts to move. If the force of static friction was always equal to
μsN, the force of static
friction could make things move. You would set something on a table
and the object would start to move. The universe does not work this
way. Instead, when a force is applied by some agent to an object,
the static friction will oppose the tendency to move and will have a
value equal to the applied force (if possible). As the force from
the agent increases, the force of static fiction will increase. This
continues up to the maximum value of the static friction force. With
any additional applied force the object starts to move and we switch
the calculation of friction to the kinetic friction equation.
Kinetic
Friction fk = μkN,
The direction of the force of kinetic friction is opposite to the
velocity for kinetic friction.
You
should look at the table of friction coefficients in your textbook.
The
maximum value of the force of static friction
is usually larger than the force of kinetic friction.
What
force makes you move forward when you walk?
The
force of static friction on your foot makes you move forward. This
is a Newton's Third Law force pair with the force the foot applies to
the ground.
A
string is used to pull on a 3 kg object across a horizontal surface
which has a coefficient of kinetic friction of 0.5. The coefficient
of static friction is 0.6. The acceleration of the object is 0.3
m/s2.
a)
What is the value of the tension in the string just before the object
starts to move?
b)
What is the tension in the string while the object is moving?
Friction
on Inclines
Make
a drawing that shows a 5 kg object on a frictionless plane which is
inclined at 25 degrees to the horizontal. Show all of the forces
that act on the object. a) Find the magnitude of the normal
force and the magnitude of the force acting on the object in a
direction of “down the plane”. b) Now suppose that a 3 kg object
is connected with a string at the upward side of the 5 kg object.
The string passes over a pulley at the top of the ramp and the 3 kg
object hangs vertically from the string. The 3 kg object does not
touch the ground. The system starts at rest. Calculate the
acceleration of the 3 kg object. Let the up-the-plane direction be
the positive direction. c) Calculate the tension in the string.
d)
repeat part b) for the case of the coefficient of static friction =
0.12 and the coefficient of kinetic friction = 0.09
Drag
Force Concepts
I
am not discussing calculations for drag forces.
The
concepts of this section are:
1)
the drag force increases as the velocity increases
2)
the terminal velocity occurs when the drag force magnitude equals the
magnitude of the applied force
Elasticity:
Stress and Strain
For
small stretch and compression Hooke's Law is valid
F
= k
ΔL
k is the "force constant" in units of Newtons/meter.
ΔL
is the stretch or compression in meters
from the relaxed position of the spring.
If
a spring has a force constant of 60 N/m and it is stretched 4 cm the
force is
60
N/m x 0.04 meters = 2.4 Newtons
It
is important to note that the force is a "restoring" force.
If a spring is extended it applies a force in the opposite direction
to try to move the spring back to the original, unstretched,
position.
I
am only applying
these concepts to springs.
I
am not discussing shear.
Friction
on Inclines ... look for examples in my list of YouTube videos
Copyright©
2017 by Greg Clements Permission is granted to reproduce this
document as long as 1) this copyright notice is included, 2) no
charge of any kind is made, and, 3) the use is for an educational
purpose. Editing of the document to suit your own class style and
purposes is allowed.
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