Download Chapter 3: Newton`s First Law of Motion – Inertia 1 You see a ball in

Chapter 3: Newton’s First Law of Motion – Inertia
1
You see a ball in the middle
of a ball field. It suddenly
starts moving. Why?
Historical views on motion:
Aristotle: 4th century BC
Copernicus (1473-1543)
Divided motion into two types:
Simplest way to interpret astronomical
observations was to assume that Earth
and the other planets move around the
sun.
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Natural motion – straight up or straight down
o Boulder falling down, smoke rising up
o Objects seek their natural resting
places
 Circular motion was natural for
the heavens (no beginning, no
end)
 Planets and stars moved in
perfect circles around Earth
Violent motion – imposed motion
o Result of forces that pushed or pulled
o Carts pulled by horses, ships pushed
by wind
o External cause of violent motion –
motion was imparted to objects
Objects in their natural resting places could
not move by themselves – must be pushed or
pulled.
Believed that the proper state of objects was
one of rest, unless they were being pulled or
pushed, or were moving toward their natural
resting place.
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Earth is in its natural resting place
o No force is large enough to move it
o Earth did not move
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Galileo (1564-1642)
Supporter of Copernicus
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If friction were absent, a ball moving
horizontally would move forever.
o No push or pull required once it is
set in motion.
Was tried and sentenced to house
arrest
One of his greatest contributions to
physics was to demolish the idea that a
force is necessary to keep an object
moving.
Friction: name given to the force that
acts between materials in contact
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c. Galileo found that a ball rolling on a smooth
horizontal plane has almost constant velocity.
Very controversial idea
o People preferred to
believe that the Earth was
the center of the universe
Worked on his ideas in secret to
avoid persecution.
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Caused by irregularities in the
surfaces of objects
Microscopic irregularities exist in
very smooth objects that obstruct
motion
Without friction, a moving object
would need no force to remain in
motion
o Only when friction is
present is a force needed
to keep an object moving.
Tested idea by rolling balls along
plane surfaces inclined at
different angles.
Chapter 3: Newton’s First Law of Motion – Inertia
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Galileo (continued)
Galileo used 2 inclined planes facing each other.
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A ball released to roll down one plane would roll up the other to reach nearly
the same height
o Smoother planes had better results
o Final height would be nearly equal to initial height
If the second plane was longer and had a smaller angle, the ball reached the
same height.
If the angle of the incline were 0° (horizontal), the ball would roll forever if
friction were not present.
Disputed Aristotle. It was not the nature of a ball to come to rest.
Stated that it is the tendency of a moving body to keep moving; it is natural for an
object to resist changes in its motion
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Inertia – property of a body to resist changes to its state of motion
Interested in how things move, not why!
Experimentation, not logic, is the best test of knowledge.
Findings about motion and concept of inertia discredited Aristotle’s theory of
motion.
Chapter 3: Newton’s First Law of Motion – Inertia
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Newton’s Law of Inertia – First Law of Motion
Isaac Newton was born December 25, 1642 (the year Galileo died).
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By the age of 24, he had developed his laws of motion
Laws of motion replaced Aristotelian ideas that had dominated science for 2000
years!
Newton’s first law of motion, also known as the law of inertia, restates Galileo’s idea
that a force is not needed to keep an object moving.
“Every object continues in a state of rest, or of uniform speed in a straight line,
unless acted on by a nonzero net force.”
Objects tend to keep on doing what they’re already doing.
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Objects in a state of rest tend to remain at rest.
o Only a force will change that state
Objects in motion
Ex. A hockey puck sliding …
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On the road soon comes to a stop
On ice, slides much farther
o Friction is very small
On an air table, friction is virtually absent
o Puck slides with no apparent loss of speed.
In the absence of forces,
a moving object tends to
move in a straight line
indefinitely.
In space, it will travel
forever!
Chapter 3: Newton’s First Law of Motion – Inertia
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Inertia
Objects move (or don’t) by virtue of their own inertia.
Law of inertia provides a different way of viewing motion.
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Ancients thought continual forces were needed to maintain motion
Now, we know objects continue to move by themselves
o Forces are needed to overcome friction and to set the object in motion
o Once in motion in a force-free environment, the object will continue to
move in a straight line indefinitely.
Mass – A Measure of Inertia
Mass – the amount of material present in an
object
The more mass an object has, the greater its
inertia and the more force it takes to change
its state of motion.
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Mass is the measure of inertia of an object
Mass ≠ Volume
o Mass is the amount of material and
is measured in kilograms
o Volume is a measure of space and is
measured in cm3, m3, and liters.
o Large mass does not necessarily
mean a large volume
Mass is not weight.
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Mass is amount of material in
an object
Weight is the gravitational
force acting on the object
Mass is more fundamental than
weight
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weight can change because it
depends on location
mass depends only on the
number and kinds of atoms
that make up an object
o doesn’t change
Mass is inertia. The amount of material in a particular object is the same regardless of
its location – Earth, moon, space. Therefore, mass is the same.
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Same force is required to shake the object back-and-forth, in all 3 locations.
Inertia is a property of the object, not its location.
Chapter 3: Newton’s First Law of Motion – Inertia
Weight – Depends on location because it depends
on the force of gravity exerted on the object.
Definitions
Weight – force of gravity on an object
Mass – quantity of matter in an object; measure
of the inertia, or “laziness” that an object
exhibits in response to any effort to start, stop,
or otherwise change its state of motion.
Mass and weight are different, but are
proportional in any location.
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Large mass, large weight
Little mass, little weight
In the same location, twice the mass =
twice the weight
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Units of weight vs. mass.
In the U.S., we describe matter
by its gravitational pull to the
Earth, its weight. Weight is
measured in pounds.
In the rest of the world, matter is
described by its mass. The unit
of mass is the kilogram.
1 kg has a weight of 2.2 pounds
SI unit of force is the newton
1 kg weighs 10 newtons
You will use these facts when
converting.
The Moving Earth
When Copernicus announced that the Earth was moving around the Sun, opponents to the
idea argued that the Earth couldn’t be moving. One of their arguments went like this:
Bird at rest in a tree sees a worm on the ground below. The bird drops down and catches
the worm. This action would not be possible if the Earth were in motion.
If Copernicus was correct, the Earth would have to move at 107,000 km/h to circle the
sun in one year. That’s 30 km/s!
Birds do catch worms, so is Copernicus right? How would you defend his idea? Can you
give any examples to support his claim?
Objects move with the Earth
The Earth moves at 30 km/s.
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So do the tree, the bird, the worm,
even the air!
Everything on Earth moves at 30 km/s.
The law of inertia states that objects in
motion remain in motion if no unbalanced
forces act on them.
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Objects on Earth move with Earth as
the Earth orbits the sun.
The horizontal motion of the Earth has
no bearing on the bird’s motion as it
drops to the ground.
Try this! Stand next to a wall. Jump up
so your feet no longer touch the floor.
Does the wall (moving at 30 km/s) slam
into you? Why not?
The 30 km/s is the speed of the earth
relative to the sun, not the speed of the
wall relative to you.
Objects move with vehicles
Flip a coin while riding in a moving car or
while flying in an airplane. The coin
behaves as if the car and the plane were
at rest.