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# "Physics" by Walker--Chapter 6

This set is based on Chapter 6, Applications of Newton's Laws, of the textbook "Physics" by James S. Walker.
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Is friction a force?
Yes, friction is a force.
What is the origin of
the force we call friction?
The origin of friction is
the microscopic hills and valleys
of two surfaces bumping together.
What is the force that accelerates you forward
as you walk?
As you walk, you are accelerated forward
by the friction between your shoes and the ground.
What two types of friction
Kinetic friction and static friction.
What is kinetic friction?
"Kinetic friction is the friction encountered
when surfaces slide against one another
with a finite relative speed.
The force generated by this friction...
acts to oppose the sliding motion
at the point of contact between the surfaces."
(page 148)
What is the symbol for
kinetic friction?
The symbol for kinetic friction is
ƒ₍k₎
(f sub k)
How is the force of kinetic friction
related to the normal force?
The force of kinetic friction and the normal force
are proportional. In mathematical terms:
ƒ₍k₎ = µ₍k₎ × N
What is µ₍k₎
and how is it pronounced?
µ₍k₎ is the coefficient of kinetic friction.
It is pronounced "mew sub k".
It is a constant, and always positive.
It has no dimension. Typical values range 0-1.
The larger µ₍k₎ is, the larger the friction is.
Does the force of kinetic friction
vary according to the speed
of the surfaces?
No, the force of kinetic friction is
independent of the speed of the surfaces.
Does the force of kinetic friction
depend on the area of contact
between the surfaces?
No, the force of kinetic friction is
independent of the area of contact
between the surfaces.
What is static friction?
Static friction is the frictional force that
keeps two surfaces from moving
relative to one another.
(page 152)
What is the origin of
the force of static friction?
The origin of the force of static friction is
the microscopic hills and valleys of two surfaces.
When the surfaces are static relative to one another,
the hills and valleys become more enmeshed
than if the surfaces were moving.
Therefore, static friction tends to be stronger
than kinetic friction.
What is the symbol for
the force of static friction?
The symbol for static friction is ƒ₍s₎.
If a book is at rest on a table,
and there are no horizontal forces pulling on it,
what is the magnitude
of the force of static friction?
If a book is at rest on a table, and there are
no horizontal forces pulling on it,
the magnitude of the force of static friction is 0.
This is because no force is needed
to keep the book from sliding. (p. 152)
Suppose a book is at rest on a table,
and a small pulling force is applied,
but the book doesn't move.
What is the force of static friction?
If a book is at rest on a table,
and a small pulling force is applied
but the book doesn't move,
the force of static friction is
equal to the pulling force.
What do we call the upper limit
of the force that can be exerted
by static friction?
The upper limit to the force
that can be exerted by static friction
is called ƒ₍s, max₎.
When that is exceeded, kinetic friction takes over.
(page 152)
What values can static friction have?
Static friction can have any value
from zero to ƒ₍s, max₎. We can write this as:
0 ≤ ƒ₍s₎ ≤ ƒ₍s, max₎
What is the relationship of
the maximum force of static friction
to the normal force?
The maximum force of static friction
is proportional to the normal force.
ƒ₍s, max₎ = μ₍s₎N
What is μ₍s₎
and how is it pronounced?
μ₍s₎ is the coefficient of static friction.
It's pronounced "mew sub s". It is a constant of proportionality and has no dimension.
Usually it is greater than the coefficient
of kinetic friction.
It is sometimes greater than 1.
Does the force of static friction
depend on the area of contact
between the surfaces?
No. Like the force of kinetic friction,
the force of static friction is
independent of the area of contact
between the surfaces.
What is the direction of
the force of static friction?
The force of static friction is parallel
to the surface of contact, and opposite
to the direction the object would move
if there were no friction. (page 153)
Does an ideal pulley reduce
the tension in a rope?
No, an ideal pulley does not reduce
the tension in a rope.
It simply changes the direction of the tension.
What is the spring constant
of a spring?
The spring constant is a constant of proportionality
between the force a spring exerts and the amount
by which it is stretched or compressed.
(page 159)
What mathematical expression is used for
the force constant of a spring?
The spring constant is expressed by:
F = kx
...where F is force, k is the spring constant, and x is
the amount the spring is stretched or compressed.
What unit is used
for a spring constant?
The unit for a spring constant is
newtons per meter,
or N / m.
What do we mean by
the equilibrium position of a spring?
The equilibrium position of a spring
is the position at which there is
no stretching or compression force on the spring.
What values (positive or negative)
should we use when we are describing
the stretching or compression of a spring?
Use negative values to give
magnitude of compression of a spring,
because you are describing
the equilibrium position minus a certain length.
Use positive values to give
the magnitude of stretching of a spring,
because you are describing
the equilibrium position plus a certain length.
What is Hooke's Law?
Hooke's Law is: A spring stretched or compressed
by the amount x from its equilibrium length
exerts a force whose x component is given by:
Fₓ = −kx
Can Hooke's Law be expressed by either
Fₓ = −kx or
F = kx ?
Yes. For Hooke's Law:
Fₓ = −kx gives magnitude and direction;
F = kx gives magnitude only. (page 160)
Is Hooke's Law always true
for a spring?
No, Hooke's Law is not always true for a spring.
It is possible to stretch a spring so much
that it will be permanently deformed.
In general, though, Hooke's Law
is a good rule of thumb.
What does it mean when an object
is in translational equilibrium?
When an object is in translational equilibrium,
the net force acting on it is zero.
When there are two dimensions, this implies that
the net force is zero for each component of the vector.
What is an ideal pulley?
An ideal pulley has no mass
and no friction.
If an object is in equilibrium,
what is its acceleration?
If an object is in equilibrium,
its acceleration is 0.
To make an object move in a circle
with constant speed, a force must act on it
that is directed toward
___ ______ __ ___ ______ .
To make an object move in a circle with constant speed,
a force must act on it that is directed
toward the center of the circle. (page 169)
When an object moves in a circle
of radius r with constant speed v,
its centripetal acceleration is....?
When an object moves in a circle of radius r
with constant speed v,
its centripetal acceleration is:
a₍cp₎ = v² / r
(page 171)
A force must be applied to an object
to give it circular motion. For an object
of mass m, the net force acting on it
must have a magnitude given by...?
A force must be applied to an object
to give it circular motion. For an object
of mass m, the net force acting on it
must have a magnitude given by:
ƒ₍cp₎ = ma₍cp₎ = m (v² / r)
(page 171)
A centrifuge, a common device
in biological and medical laboratories,
uses large ___________ accelerations
to perform such tasks as separating
red and white blood cells from serum.
(page 174)
A centrifuge, a common device
in biological and medical laboratories,
uses large centripetal accelerations
to perform such tasks as separating
red and white blood cells from serum.
(page 174)
What is tension?
Tension is the force transmitted through a string.
The tension is the same throughout the length
of an ideal string. (page 176)
Objects connected by strings
have the same _________ of acceleration.
Objects connected by strings have
the same magnitude of acceleration.
(page 176)
Objects that experience zero net force
obey Newton's ______ Law.
Objects that experience zero net force
obey Newton's First Law.
(page 188)
All objects experience forces--
the question is whether the object
experiences a _____ force.
All objects experience forces
--the question is whether the object
experiences a net force.
(page 188)
Moving at constant velocity
is equivalent to being at _____ .
Moving at constant velocity
is equivalent to being at rest .
(page 188)
Objects that experience a nonzero net force
obey Newton's ______ Law.
Objects that experience a nonzero net force
obey Newton's Second Law.
(page 189)
For linear accelerated motion:
--Net force is parallel to motion.
--Velocity changes in magnitude
but not in direction.
For linear accelerated motion:
--Net force is parallel to motion.
--Velocity changes in magnitude but not in direction.
(page 189)
For parabolic motion:
--Constant net force acts at an _____ to motion.
--Velocity changes in both magnitude
and direction.
For parabolic motion:
--Constant net force acts at an angle to motion.
--Velocity changes in both magnitude and direction.
(page 189)
For circular motion (constant speed):
--Net force is constant in magnitude
but always points toward the ______
of the circle.
--Velocity changes in direction
but not in magnitude.
For circular motion (constant speed):
--Net force is constant in magnitude but always points
toward the center of the circle. Thus, the net force
is always at a right angle to the object's velocity.
--Velocity changes in direction but not in magnitude.
(page 189)
The acceleration magnitude
is proportional to F and
_________ proportional to m.
The acceleration magnitude
is proportional to F and
inversely proportional to m.
(page 189)