Download 3 Newton`s First Law of Motion—Inertia

3 Newton’s First Law of Motion—Inertia
Forces cause changes
in motion.
3 Newton’s First Law of Motion—Inertia
• A ball at rest in the middle of a flat
field is in equilibrium. No net force
acts on it.
• If you saw it begin to move across
the ground, you’d look for forces
that don’t balance to zero.
• We don’t believe that changes in
motion occur without cause.
3 Newton’s First Law of Motion—Inertia
3.1 Aristotle on Motion
Aristotle, the foremost Greek scientist,
studied motion and divided it into two
types: natural motion and violent motion.
3 Newton’s First Law of Motion—Inertia
3.2 Copernicus and the Moving Earth
Copernicus reasoned that the simplest way
to interpret astronomical observations was
to assume that Earth and the other planets
move around the sun.
3 Newton’s First Law of Motion—Inertia
3.3 Galileo on Motion
Galileo argued that only when friction is
present—as it usually is—is a force needed
to keep an object moving.
3 Newton’s First Law of Motion—Inertia
3.3 Galileo on Motion
Friction is the name given to the force that acts between
materials that touch as they move past each other.
• Friction is caused by the irregularities in the surfaces of
objects that are touching.
• Even very smooth surfaces have microscopic
irregularities that obstruct motion.
• If friction were absent, a moving object would need no
force to remain in motion.
3 Newton’s First Law of Motion—Inertia
3.3 Galileo on Motion
Galileo stated that this tendency of a moving body to keep
moving is natural and that every material object resists
changes to its state of motion.
The property of a body to resist changes to its state of
motion is called inertia.
3 Newton’s First Law of Motion—Inertia
3.3 Galileo on Motion
think!
A ball is rolled across a counter top and rolls slowly to a stop.
How would Aristotle interpret this behavior? How would
Galileo interpret it? How would you interpret it?
Answer: Aristotle would probably say that the ball stops
because it seeks its natural state of rest. Galileo would
probably say that the friction between the ball and the table
overcomes the ball’s natural tendency to continue rolling—
overcomes the ball’s inertia—and brings it to a stop. Only you
can answer the last question!
3 Newton’s First Law of Motion—Inertia
3.3 Galileo on Motion
According to Galileo, when is a force
needed to keep an object moving?
3 Newton’s First Law of Motion—Inertia
3.4 Newton’s Law of Inertia
Newton’s first law states that every object
continues in a state of rest, or of uniform
speed in a straight line, unless acted on by
an unbalanced force.
3 Newton’s First Law of Motion—Inertia
3.4 Newton’s Law of Inertia
Newton’s first law, usually called the law of inertia, is a
restatement of Galileo’s idea that a force is not needed to
keep an object moving.
3 Newton’s First Law of Motion—Inertia
3.4 Newton’s Law of Inertia
Objects at rest tend to remain at rest.
3 Newton’s First Law of Motion—Inertia
3.4 Newton’s Law of Inertia
The law of inertia provides a completely
different way of viewing motion from the
ancients.
• Objects continue to move by
themselves.
• Forces are needed to overcome
any friction that may be present and
to set objects in motion initially.
• Once the object is moving in a
force-free environment, it will move
in a straight line indefinitely.
3 Newton’s First Law of Motion—Inertia
3.4 Newton’s Law of Inertia
3 Newton’s First Law of Motion—Inertia
3.4 Newton’s Law of Inertia
3 Newton’s First Law of Motion—Inertia
3.4 Newton’s Law of Inertia
think!
A force of gravity between the sun and its planets holds the
planets in orbit around the sun. If that force of gravity
suddenly disappeared, in what kind of path would the
planets move?
Answer: Each planet would move in a straight line at
constant speed.
3 Newton’s First Law of Motion—Inertia
3.4 Newton’s Law of Inertia
What is Newton’s first law of motion?
3 Newton’s First Law of Motion—Inertia
3.5 Mass—A Measure of Inertia
The more mass an object has, the greater
its inertia and the more force it takes to
change its state of motion.
3 Newton’s First Law of Motion—Inertia
3.5 Mass—A Measure of Inertia
The amount of inertia an object has depends on its mass—
which is roughly the amount of material present in the object.
Mass is a measure of the inertia of an object.
3 Newton’s First Law of Motion—Inertia
3.5 Mass—A Measure of Inertia
You can tell how much matter is in a can when you kick it.
Kick an empty can and it moves. Kick a can filled with sand
and it doesn’t move as much.
3 Newton’s First Law of Motion—Inertia
3.5 Mass—A Measure of Inertia
Which has more mass, a feather pillow or a common
automobile battery?
Clearly an automobile battery is more difficult to set into
motion. This is evidence of the battery’s greater inertia and
hence its greater mass.
3 Newton’s First Law of Motion—Inertia
3.5 Mass—A Measure of Inertia
Mass Is Not Weight
Mass is often confused with weight.
• We often determine the amount of
matter in an object by measuring
its gravitational attraction to Earth.
However, mass is more
fundamental than weight.
• Mass is a measure of the amount
of material in an object.
3 Newton’s First Law of Motion—Inertia
3.5 Mass—A Measure of Inertia
We can define mass and weight as follows:
• Mass is the quantity of matter in an object. More
specifically, mass is a measure of the inertia, or
“laziness,” that an object exhibits in response to any
effort made to start it, stop it, or otherwise change its
state of motion.
• Weight is the force of gravity on an object.
3 Newton’s First Law of Motion—Inertia
3.5 Mass—A Measure of Inertia
Mass Is Inertia
The amount of material in a particular stone is the
same whether the stone is located on Earth, on the
moon, or in outer space.
• The mass of the stone is the same in all of
these locations.
• The weight of the stone would be very
different.
3 Newton’s First Law of Motion—Inertia
3.5 Mass—A Measure of Inertia
It’s just as difficult to
shake a stone in its
weightless state in
space as it is in its
weighted state on
Earth.
3 Newton’s First Law of Motion—Inertia
3.5 Mass—A Measure of Inertia
One Kilogram Weighs 10 Newtons
• In the United States, the traditional unit of
weight is the pound.
• In most parts of the world, however, the
measure of matter is commonly expressed in
units of mass, the kilogram (kg).
• At Earth’s surface, 1 kilogram has a weight of
2.2 pounds.
3 Newton’s First Law of Motion—Inertia
3.5 Mass—A Measure of Inertia
One kilogram of nails weighs 10
newtons, which is equal to 2.2
pounds. Away from Earth’s
surface, where the force of
gravity is less, the bag of nails
weighs less.
3 Newton’s First Law of Motion—Inertia
3.5 Mass—A Measure of Inertia
think!
Does a 2-kilogram bunch of bananas have twice as much
inertia as a 1-kilogram loaf of bread? Twice as much mass?
Twice as much volume? Twice as much weight, when
weighed in the same location?
Answer: Except for volume, the answer to all the questions is
yes. Bananas are much more dense than bread, so two
kilograms of bananas must occupy less volume than one
kilogram of bread.
3 Newton’s First Law of Motion—Inertia
3.5 Mass—A Measure of Inertia
What is the relationship between mass
and inertia?
3 Newton’s First Law of Motion—Inertia
3.6 The Moving Earth Again
Flip a coin in an airplane, and
it behaves as if the plane
were at rest. The coin keeps
up with you—inertia in action!
3 Newton’s First Law of Motion—Inertia
3.6 The Moving Earth Again
3 Newton’s First Law of Motion—Inertia
3.6 The Moving Earth Again
How does the law of inertia apply to
objects in motion?
3 Newton’s First Law of Motion—Inertia
An object accelerates
when a net force acts
on it.
3 Newton’s First Law of Motion—Inertia
Recall the definition of
acceleration:
The cause of acceleration is
force.
3 Newton’s First Law of Motion—Inertia
•6.1 Force Causes Acceleration
Unbalanced forces acting on an
object cause the object to accelerate.
3 Newton’s First Law of Motion—Inertia
•6.1 Force Causes Acceleration
When a hockey puck is at rest, the net force on it
(gravity and the support force) is balanced, so the puck
is in equilibrium.
Hit the puck (that is, apply an unbalanced force to it)
and the puck experiences a change in motion—it
accelerates.
Apply another force by striking the puck again, and the
puck’s motion changes again.
3 Newton’s First Law of Motion—Inertia
•6.1 Force Causes Acceleration
What causes an object to accelerate?
3 Newton’s First Law of Motion—Inertia
•6.2 Mass Resists Acceleration
For a constant force, an increase in
the mass will result in a decrease in
the acceleration.
3 Newton’s First Law of Motion—Inertia
•6.2 Mass Resists Acceleration
The acceleration produced depends on the mass
that is pushed.
3 Newton’s First Law of Motion—Inertia
•6.2 Mass Resists Acceleration
How does an increase in mass
affect acceleration?
3 Newton’s First Law of Motion—Inertia
•6.3 Newton’s Second Law
Newton’s second law states that the
acceleration produced by a net force
on an object is directly proportional to
the magnitude of the net force, is in
the same direction as the net force,
and is inversely proportional to the
mass of the object.
3 Newton’s First Law of Motion—Inertia
•6.3 Newton’s Second Law
Newton’s second law describes the relationship among an
object's mass, an object's acceleration, and the net force on
an object.
3 Newton’s First Law of Motion—Inertia
•6.3 Newton’s Second Law
By using consistent units, such as newtons (N) for
force, kilograms (kg) for mass, and meters per second
squared (m/s2) for acceleration, we get the exact
equation:
or
3 Newton’s First Law of Motion—Inertia
•6.3 Newton’s Second Law
The acceleration is equal to the net
force divided by the mass.
• If the net force acting on an
object doubles, its acceleration
is doubled.
• If the mass is doubled, then
acceleration will be halved.
• If both the net force and the
mass are doubled, the
acceleration will be unchanged.
3 Newton’s First Law of Motion—Inertia
•6.3 Newton’s Second Law
think!
If a car can accelerate at 2 m/s2, what acceleration can it
attain if it is towing another car of equal mass?
Answer: The same force on twice the mass produces half
the acceleration, or 1 m/s2.
3 Newton’s First Law of Motion—Inertia
•6.3 Newton’s Second Law
do the math!
A car has a mass of 1000 kg. What is the acceleration
produced by a force of 2000 N?
3 Newton’s First Law of Motion—Inertia
•6.3 Newton’s Second Law
do the math!
If the force is 4000 N, what is the acceleration?
Doubling the force on the same mass simply doubles the
acceleration.
3 Newton’s First Law of Motion—Inertia
•6.3 Newton’s Second Law
do the math!
How much force, or thrust, must a 30,000-kg jet plane
develop to achieve an acceleration of 1.5 m/s2?
Arrange Newton’s second law to read:
force = mass × acceleration
F = ma
= (30,000 kg)(1.5 m/s2)
= 45,000 kg•m/s2
= 45,000 N
3 Newton’s First Law of Motion—Inertia
•6.4 Friction
The force of friction between the
surfaces depends on the kinds of
material in contact and how much the
surfaces are pressed together.
3 Newton’s First Law of Motion—Inertia
•6.4 Friction
Rubber against concrete produces more friction than steel
against steel, so concrete road dividers have replaced steel
rails.
The friction produced by a tire rubbing against the concrete
is more effective in slowing the car than the friction
produced by a steel car body sliding against a steel rail.
3 Newton’s First Law of Motion—Inertia
•6.4 Friction
The direction of the force of friction always opposes the
direction of motion.
a. Push the crate to the right and friction acts toward the left.
b. The sack falls downward and air friction acts upward.
3 Newton’s First Law of Motion—Inertia
•6.4 Friction
think!
Two forces act on a book resting on a table: its weight and
the support force from the table. Does a force of friction act
as well?
Answer: No, not unless the book tends to slide or does slide
across the table. Friction forces occur only when an object
tends to slide or is sliding.
3 Newton’s First Law of Motion—Inertia
•6.4 Friction
What factors affect the force of
friction between surfaces?
3 Newton’s First Law of Motion—Inertia
•6.6 Free Fall Explained
F stands for the force (weight) acting on the cannonball, and
m stands for the correspondingly large mass of the
cannonball. The small F and m stand for the weight and
mass of the stone.
• The ratio of weight to mass is the same for these or
any objects.
• All freely falling objects undergo the same acceleration
at the same place on Earth.
3 Newton’s First Law of Motion—Inertia
•6.6 Free Fall Explained
The ratio of weight (F) to
mass (m) is the same for
the 10-kg cannonball and
the 1-kg stone.
3 Newton’s First Law of Motion—Inertia
•6.6 Free Fall Explained
Why do all freely falling objects fall with
the same acceleration?
3 Newton’s First Law of Motion—Inertia
For every force, there
is an equal and
opposite force.
3 Newton’s First Law of Motion—Inertia
•7.1 Forces and Interactions
A force is always part of a mutual
action that involves another force.
3 Newton’s First Law of Motion—Inertia
•7.1 Forces and Interactions
In the simplest sense, a force is a push or a pull.
A mutual action is an interaction between one thing
and another.
3 Newton’s First Law of Motion—Inertia
•7.1 Forces and Interactions
When you push on the wall, the wall pushes on you.
3 Newton’s First Law of Motion—Inertia
•7.1 Forces and Interactions
The interaction that drives the nail is the same as the
one that halts the hammer.
3 Newton’s First Law of Motion—Inertia
•7.1 Forces and Interactions
think!
Does a stick of dynamite contain force? Explain.
Answer: No. Force is not something an object has, like
mass. Force is an interaction between one object and
another. An object may possess the capability of exerting a
force on another object, but it cannot possess force as a
thing in itself. Later we will see that something like a stick of
dynamite possesses energy.
3 Newton’s First Law of Motion—Inertia
•7.1 Forces and Interactions
Why do forces always occur in pairs?
3 Newton’s First Law of Motion—Inertia
•7.2 Newton’s Third Law
Newton’s third law states that
whenever one object exerts a force on
a second object, the second object
exerts an equal and opposite force on
the first object.
3 Newton’s First Law of Motion—Inertia
•7.2 Newton’s Third Law
Newton’s third law describes the relationship between two
forces in an interaction.
• One force is called the action force.
• The other force is called the reaction force.
• Neither force exists without the other.
• They are equal in strength and opposite in direction.
• They occur at the same time (simultaneously).
•Newton’s third law is often stated: “To every action
there is always an equal opposing reaction.”
3 Newton’s First Law of Motion—Inertia
•7.2 Newton’s Third Law
The interactions depend on friction.
A person trying to walk on ice, where friction is minimal,
may not be able to exert an action force against the ice.
Without the action force there cannot be a reaction force,
and thus there is no resulting forward motion.
3 Newton’s First Law of Motion—Inertia
•7.2 Newton’s Third Law
When the girl jumps to shore, the boat moves backward.
3 Newton’s First Law of Motion—Inertia
•7.2 Newton’s Third Law
The dog wags the tail and the tail wags the dog.
3 Newton’s First Law of Motion—Inertia
•7.3 Identifying Action and Reaction
Sometimes the identity of the pair of action and reaction
forces in an interaction is not immediately obvious.
• For example, what are the action and reaction forces
in the case of a falling boulder?
• If we call the action Earth exerting a force on the
boulder, then the reaction is the boulder
simultaneously exerting a force on Earth.
3 Newton’s First Law of Motion—Inertia
•7.3 Identifying Action and Reaction
When action is A exerts force on B, the reaction is simply B
exerts force on A.
3 Newton’s First Law of Motion—Inertia
•7.3 Identifying Action and Reaction
When action is A exerts force on B, the reaction is simply B
exerts force on A.
3 Newton’s First Law of Motion—Inertia
•7.3 Identifying Action and Reaction
think!
We know that Earth pulls on the moon. Does the moon also
pull on Earth? If so, which pull is stronger?
Answer: Asking which pull is stronger is like asking which
distance is greater—between New York and San Francisco,
or between San Francisco and New York. The distances
either way are the same. It is the same with force pairs. Both
Earth and moon pull on each other with equal and opposite
forces.
3 Newton’s First Law of Motion—Inertia
•7.4 Action and Reaction on Different Masses
A given force exerted on a small mass
produces a greater acceleration than
the same force exerted on a large mass.
3 Newton’s First Law of Motion—Inertia
•7.4 Action and Reaction on Different Masses
The cannonball undergoes more acceleration than
the cannon because its mass is much smaller.
3 Newton’s First Law of Motion—Inertia
•7.4 Action and Reaction on Different Masses
F represents both the action and reaction forces; m (large),
the mass of the cannon; and m (small), the mass of the
cannonball.
Do you see why the change in the velocity of the cannonball is
greater compared with the change in velocity of the cannon?
3 Newton’s First Law of Motion—Inertia
•7.4 Action and Reaction on Different Masses
The balloon recoils from the escaping
air and climbs upward.
3 Newton’s First Law of Motion—Inertia
•7.4 Action and Reaction on Different Masses
The rocket recoils from the
“molecular cannonballs” it
fires and climbs upward.
3 Newton’s First Law of Motion—Inertia
•7.4 Action and Reaction on Different Masses
Why do objects that experience the same
amount of force accelerate at different rates?
3 Newton’s First Law of Motion—Inertia
•7.7 Action Equals Reaction
For every interaction between things,
there is always a pair of oppositely
directed forces that are equal in
strength.
3 Newton’s First Law of Motion—Inertia
•7.7 Action Equals Reaction
If you hit the wall, it will hit you equally hard.
3 Newton’s First Law of Motion—Inertia
•7.7 Action Equals Reaction
If a sheet of paper is held in midair, the heavyweight
champion of the world could not strike the paper with a
force of 200 N (45 pounds).
The paper is not capable of exerting a reaction force of
200 N, and you cannot have an action force without a
reaction force.
If the paper is against the wall, then the wall will easily
assist the paper in providing 200 N of reaction force, and
more if needed!
3 Newton’s First Law of Motion—Inertia
•7.7 Action Equals Reaction
What must occur in every interaction
between things?
3 Newton’s First Law of Motion—Inertia
Assessment Questions
1.
Two thousand years ago, people thought that Earth did not
move. One major reason for thinking this was that
a. no force was large enough to move the Earth.
b. Earth’s motion would be unnatural.
c. Earth was near the center of the universe.
d. Earth moved in a perfect circle.
Answer: A
3 Newton’s First Law of Motion—Inertia
Assessment Questions
2.
According to Aristotle and his followers over centuries, Earth was at
the center of the universe. The first European to effectively challenge
that notion was
a. Copernicus.
b. Galileo.
c. Newton.
d. Einstein.
Answer: A
3 Newton’s First Law of Motion—Inertia
Assessment Questions
3.
Galileo’s conclusions about motion helped advance science because
they were based on
a. experiments rather than philosophical discussions.
b. philosophical discussions rather than experiments.
c. nonmathematical thinking.
d. Aristotle’s theories of motion.
Answer: A
3 Newton’s First Law of Motion—Inertia
Assessment Questions
4.
If gravity between the sun and Earth suddenly vanished, Earth would
continue moving in a(n)
a. curved path.
b. straight-line path.
c. outward spiral path.
d. inward spiral path.
Answer: B
3 Newton’s First Law of Motion—Inertia
Assessment Questions
5.
To say that 1 kg of matter weighs 10 N is to say that 1 kg of matter
a. will weigh 10 N everywhere.
b. has ten times less volume than 10 kg of matter.
c. has ten times more inertia than 10 kg of matter.
d. is attracted to Earth with 10 N of force.
Answer: D
3 Newton’s First Law of Motion—Inertia
Assessment Questions
6.
The Earth moves about 30 km/s relative to the sun. But when you
jump upward in front of a wall, the wall doesn’t slam into you at 30
km/s. A good explanation for why it doesn’t is that
a. the sun’s influence on you is negligible.
b. the air in the room is also moving.
c. both you and the wall are moving at the same speed, before,
during, and after your jump.
d. the inertia of you and the wall is negligible compared with that of
the sun.
Answer: C
3 Newton’s First Law of Motion—Inertia
•Assessment Questions
1.
An object will accelerate when
a. SF = 0.
b. it is unbalanced.
c. it is pushed or pulled with a net force.
d. its mass increases.
Answer: C
3 Newton’s First Law of Motion—Inertia
•Assessment Questions
2.
When a net force acts on an object, its acceleration depends on the
object’s
a. initial speed.
b. mass.
c. volume.
d. weight.
Answer: B
3 Newton’s First Law of Motion—Inertia
•Assessment Questions
3.
A cart is pushed and undergoes a certain acceleration. Consider how
the acceleration would compare if it were pushed with twice the net
force while its mass increased by four. Then its acceleration would be
a. one quarter.
b. half.
c. twice.
d. the same.
Answer: B
3 Newton’s First Law of Motion—Inertia
•Assessment Questions
4.
Friction is a force like any other force and affects motion. Friction
occurs in
a. solids sliding over one another.
b. fluids.
c. air.
d. all of these
Answer: D
3 Newton’s First Law of Motion—Inertia
•Assessment Questions
6.
The reason a 20-kg rock falls no faster than a 10-kg rock in free fall is
that
a. air resistance is negligible.
b. the force of gravity on both is the same.
c. their speeds are the same.
d. the force/mass ratio is the same.
Answer: D
3 Newton’s First Law of Motion—Inertia
•Assessment Questions
1.
A force interaction requires at least a(n)
a. single force.
b. pair of forces.
c. action force.
d. reaction force.
Answer: B
3 Newton’s First Law of Motion—Inertia
•Assessment Questions
2.
Whenever one object exerts a force on a second object, the second
object exerts a force on the first that is
a. opposite in direction and equal in magnitude at the same time.
b. in the same direction and equal in magnitude a moment later.
c. opposite in direction and greater in magnitude at the same time.
d. in the same direction and weaker in magnitude a moment later.
Answer: A
3 Newton’s First Law of Motion—Inertia
•Assessment Questions
3.
The force that directly propels a motor scooter along a highway is that
provided by the
a. engine.
b. fuel.
c. tires.
d. road.
Answer: D
3 Newton’s First Law of Motion—Inertia
•Assessment Questions
4.
When you jump vertically upward, strictly speaking, you cause Earth
to
a. move downward.
b. also move upward with you.
c. remain stationary.
d. move sideways a bit.
Answer: A
3 Newton’s First Law of Motion—Inertia
•Assessment Questions
5.
A system undergoes acceleration only when acted on by a(n)
a. net force.
b. pair of forces.
c. action and reaction forces.
d. internal interactions.
Answer: A
3 Newton’s First Law of Motion—Inertia
•Assessment Questions
6.
If a net force acts on a horse while it is pulling a wagon, the
horse
a. accelerates.
b. is restrained.
c. is pulled backward by an equal and opposite net force.
d. cannot move.
Answer: A
3 Newton’s First Law of Motion—Inertia
•Assessment Questions
7.
At a pizza shop, the cook throws the pizza dough in the air. The
amount of force the cook exerts on the dough depends on the
a. mass of the dough.
b. strength of the cook.
c. weight of the dough.
d. height of the cook.
Answer: A