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47 terms

Is friction a force?

Yes, friction is a force.

What is the origin of

the force we call friction?

the force we call friction?

The origin of friction is

the microscopic hills and valleys

of two surfaces bumping together.

the microscopic hills and valleys

of two surfaces bumping together.

What is the force that accelerates you forward

as you walk?

as you walk?

As you walk, you are accelerated forward

by the friction between your shoes and the ground.

by the friction between your shoes and the ground.

What two types of friction

are addressed in the textbook?

are addressed in the textbook?

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)

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?

kinetic friction?

The symbol for kinetic friction is

ƒ₍k₎

(f sub k)

ƒ₍k₎

(f sub k)

How is the force of kinetic friction

related to the normal force?

related to the normal force?

The force of kinetic friction and the normal force

are proportional. In mathematical terms:

ƒ₍k₎ = µ₍k₎ × N

are proportional. In mathematical terms:

ƒ₍k₎ = µ₍k₎ × N

What is µ₍k₎

and how is it pronounced?

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.

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?

vary according to the speed

of the surfaces?

No, the force of kinetic friction is

independent of the speed of the surfaces.

independent of the speed of the surfaces.

Does the force of kinetic friction

depend on the area of contact

between the surfaces?

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.

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)

keeps two surfaces from moving

relative to one another.

(page 152)

What is the origin of

the force of static friction?

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.

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 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?

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)

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?

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.

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?

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)

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₎

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

to the normal force?

The maximum force of static friction

is proportional to the normal force.

ƒ₍s, max₎ = μ₍s₎N

is proportional to the normal force.

ƒ₍s, max₎ = μ₍s₎N

What is μ₍s₎

and how is it pronounced?

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.

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?

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.

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?

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)

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?

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.

the tension in a rope.

It simply changes the direction of the tension.

What is the spring constant

of a spring?

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)

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 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.

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?

for a spring constant?

The unit for a spring constant is

newtons per meter,

or N / m.

newtons per meter,

or N / m.

What do we mean by

the equilibrium position of a spring?

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.

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?

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.

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

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 ?

Fₓ = −kx or

F = kx ?

Yes. For Hooke's Law:

Fₓ = −kx gives magnitude and direction;

F = kx gives magnitude only. (page 160)

Fₓ = −kx gives magnitude and direction;

F = kx gives magnitude only. (page 160)

Is Hooke's Law always true

for a spring?

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.

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?

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.

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.

and no friction.

If an object is in equilibrium,

what is its acceleration?

what is its acceleration?

If an object is in equilibrium,

its acceleration is 0.

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

___ ______ __ ___ ______ .

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)

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....?

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)

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...?

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)

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)

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)

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)

The tension is the same throughout the length

of an ideal string. (page 176)

Objects connected by strings

have the same _________ of acceleration.

have the same _________ of acceleration.

Objects connected by strings have

the same magnitude of acceleration.

(page 176)

the same magnitude of acceleration.

(page 176)

Objects that experience zero net force

obey Newton's ______ Law.

obey Newton's ______ Law.

Objects that experience zero net force

obey Newton's First Law.

(page 188)

obey Newton's First Law.

(page 188)

All objects experience forces--

the question is whether the object

experiences a _____ force.

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)

--the question is whether the object

experiences a net force.

(page 188)

Moving at constant velocity

is equivalent to being at _____ .

is equivalent to being at _____ .

Moving at constant velocity

is equivalent to being at rest .

(page 188)

is equivalent to being at rest .

(page 188)

Objects that experience a nonzero net force

obey Newton's ______ Law.

obey Newton's ______ Law.

Objects that experience a nonzero net force

obey Newton's Second Law.

(page 189)

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.

--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)

--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.

--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)

--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.

--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)

--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.

is proportional to F and

_________ proportional to m.

The acceleration magnitude

is proportional to F and

inversely proportional to m.

(page 189)

is proportional to F and

inversely proportional to m.

(page 189)