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## Aristotle, Galileo and Newton and Newton

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### Title: Aristotle, Copernicus, Galileo and Newton Author: Fort Thomas Schools Last modified by: Auch, Timothy Created Date: 9/12/2008 6:35:56 PM Document presentation ... – PowerPoint PPT presentation

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Title: Aristotle, Galileo and Newton and Newton

1
Aristotle, Galileo and Newton and Newtons Laws
of Motion
• Chapter 3.1-3.6
• Chapter 6.1-6.3
• Chapter 7.1-7.4

2
ARISTOTLE
384 BC 322 BC Ancient Greece One of the first
to try to explain the natural world Geocentric
view of the universe Ideas based on observations
that seemed to be true Thought that objects in
motion must have a force keeping them moving
3
GALILEO
1564 1642 Italy Perhaps the first true
scientist. Rolled and dropped objects to discover
the true aspects of motionthat objects in motion
do not need a force to keep them moving
4
NEWTON
1642-1727 England Developed laws for motion and
gravity that explain why objects move, and worked
with optics
Simply put, things tend to keep on doing what
5
3.1 Aristotle on Motion
1. Objects do not move without a force.
2. Objects in motion always require a force to keep
them moving.
3. Objects seek their natural state, which is at
rest.
4. Mechanical equilibrium can only be static.

6
3.1 Galileo and Newton on Motion
1. Objects do not change motion without unbalanced
force.
2. Objects in motion do not always require a force
to keep them moving.
3. Objects have two natural states of motion, at
rest (static equilibrium) and moving at a
constant speed and direction (dynamic
equilibrium).

Simply put, things tend to keep on doing what
7
3.4 Newtons Law of Inertia
• Is a force required to keep an object moving?
• Newtons first law, usually called the law of
inertia, is a restatement of Galileos idea that
a force is not needed to keep an object moving.
• Galileo argued that only when friction is present
is a force needed to keep an object moving.
• Galileo stated that if friction were entirely
absent, a ball moving horizontally would move
forever at the same speed and in the same
direction (at a constant velocity).

8
3.4 Newtons 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.

9
The First Law of Motion
• Objects at rest stay at rest and objects in
motion at a constant velocity continue at a
constant velocity unless acted upon by an
unbalanced force. (also called the law of
inertia).
• Inertia the tendency of an object to resist
acceleration
• Inertia is not a force, its a property of
matter.
• Mass and inertia are proportional. More mass,
more inertia

10
3.5 MassA Measure of Inertia
Which has more mass, a feather pillow or a common
automobile battery? Which has more volume?
Which has a higher density? What has the most
inertia?
The pillow has a larger size (volume) but a
smaller mass than the battery. The battery is
more dense.
11
3.5 MassA Measure of Inertia
The stones inertia, or mass, is a property of
the stone and not its location. The same force
would be required to shake the stone with the
same rhythm whether the stone was on Earth, on
the moon, or in a force-free region of outer
space.
12
First Law of Motion
• The question is not why objects keep moving, but
why they dont keep moving.

13
3.4 Newtons Law of Inertia
Objects at rest tend to remain at rest.
14
3.4 Newtons Law of Inertia
15
3.4 Newtons Law of Inertia
16
REFLECT ON THE MEANING OF THIS CARTOON
17
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?

18
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 and Newton
interpret it?
• Aristotle would say that the ball stops because
it seeks its natural state of rest.
• Galileo and Newton would say that the friction
between the ball and the table overcomes the
balls natural tendency to continue
rollingovercomes the balls inertiaand brings
it to a stop.

19
3.6 The Moving Earth Again
• The law of inertia states that objects in motion
remain in motion and that objects at rest remain
at rest if no unbalanced forces act on them.

Constant speed and same direction!
20
• If the Earth is rotating at 800 mph at our
latitude, why cant we sense this?

21
3.6 The Moving Earth Again
• Objects Move With Earth

You can refute this argument using the idea of
inertia. Earth moves at 30 km/s, but so do the
tree, the worm below, and even the air in
between. Objects on Earth move with Earth as
Earth moves around the sun.
22
3.6 The Moving Earth Again
Earth does not need to be at rest for the bird to
catch the worm.
23
3.6 The Moving Earth Again
• Copernicus announced the idea of a moving Earth
in the sixteenth century. One of the arguments
against a moving Earth was
• Consider a bird sitting at rest in the top of a
tall tree.
• The bird sees a worm, drops down vertically, and
catches it.
• It was argued that this would not be possible if
Earth moved as Copernicus suggested.
• The fact that birds do catch worms from high tree
branches seemed to be clear evidence that Earth
must be at rest.

24
3.6 The Moving Earth Again
• A person flips a coin into the air while on a jet
that is traveling 500 mph. Where will the coin
land and why?

Flip a coin in an airplane, and it behaves as if
the plane were at rest. The coin keeps up with
youinertia in action!
25
3.6 The Moving Earth Again
• Objects Move With Vehicles

If we flip a coin in a high-speed car, bus, or
plane, we can catch the vertically moving coin as
we would if the vehicle were at rest. We see
evidence for the law of inertia when the
horizontal motion of the coin before, during, and
after the catch is the same. The vertical force
of gravity affects only the vertical motion of
the coin.
26
3.6 The Moving Earth Again
3 x 106 kg mass damper
• http//www.boreme.com/boreme/funny-2008/taipei-101
-damper-p1.php

27
3.6 The Moving Earth Again
How does the law of inertia apply to objects in
motion?
• The law of inertia states that objects in motion
remain in motion and that objects at rest remain
at rest if no unbalanced forces act on them.

28
3.3 Galileo on Motion
According to Galileo and Newton, when is a force
needed to keep an object moving?
Only when friction (or some other oppositional
force) is present is a force needed to keep an
object moving.
29
The Second Law of Motion
• The net force equals mass times acceleration.

Fnet ma or a Fnet/m Explains the
relationship between Net force, mass and
acceleration.
30
Newtons Second Law
• ?F represents the vector sum of all forces acting
on an object.
• ?F Fnet
• Units for force mass units (kg) ? acceleration
units (m/s2)
• The units kgm/s2 are also called newtons (N).

31
Classroom Practice Problem
• Space-shuttle astronauts experience accelerations
of about 35 m/s2 during takeoff. What force does
a 75 kg astronaut experience during an
acceleration of this magnitude?
• Answer 2600 kgm/s2 or 2600 N

32
Newtons 2nd law of motion
• Increasing the force will increase the
acceleration.
• Which produces a greater acceleration on a 3-kg
model airplane, a force of 5 N or a force of 7 N?
• Answer the 7 N force
• Increasing the mass will decrease the
acceleration.
• A force of 5 N is exerted on two model airplanes,
one with a mass of 3 kg and one with a mass of 4
kg. Which has a greater acceleration?
• Answer the 3 kg airplane

33
Newtons 2nd law of motion shows two general
relationships in science
• The rate of acceleration is directly related to
net force
• Also called directly proportional

34
Newtons 2nd law of motion shows two general
relationships in science
• The rate of acceleration is inversely related to
the objects mass
• Also called inversely proportional

35
The Third Law of Motion
• For every action force, there is an equal and
opposite reaction force.

Forces act in pairs!
36
Newtons Third Law
• Forces always exist in pairs.
• You push down on the chair, the chair pushes up
on you
• Called the action force and reaction force
• Occur simultaneously so either force is the
action force

37
Newtons Third Law
• For every action force there is an equal and
opposite reaction force.
• The forces act on different objects.
• Therefore, they do not balance or cancel each
other.
• The motion of each object depends on the net
force on that object.

38
What do you think?
• Two football players, Alex and Jason, collide
head-on. They have the same mass and the same
speed before the collision. How does the force on
Alex compare to the force on Jason? Why do you
think so?

39
What do you think?
• Suppose Alex has twice the mass of Jason. How
would the forces compare?
• Why do you think so?
• Sketch as before.
• Suppose Alex has twice the mass and Jason is at
rest. How would the forces compare?
• Why do you think so?
• Sketch as before.

40
7.3 Identifying Action and Reaction
When action is A exerts force on B, the reaction
is simply B exerts force on A.
41
7.3 Identifying Action and Reaction
When action is A exerts force on B, the reaction
is simply B exerts force on A.
42
7.2 Newtons Third Law
The dog wags the tail and the tail wags the dog.
http//www.nasa.gov/audience/forstudents/brainbite
s/nonflash/bb_home_bolt.html
43
7.4 Action and Reaction on Different Masses
The balloon recoils from the escaping air and
climbs upward.
A common misconception is that a balloon is
propelled by the impact of exhaust gases against
the atmosphere.
Each molecule of exhaust gas acts like a tiny
molecular cannonball shot downward from the
balloon.
44
7.4 Action and Reaction on Different Masses
The rocket recoils from the molecular
cannonballs it fires and climbs upward.
Gas pushes on rocket
Rocket pushes on gas
45
7.4 Action and Reaction on Different Masses
What can you say about the action and reaction
forces experienced by the cannon and the
cannonball?
• The force the cannon exerts on the cannonball is
exactly equal and opposite to the force the
cannonball exerts on the cannon.

cannon
cannonball
46
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.
Which has the greater change in motion and why?
Same force
Different masses
Different accelerations
The cannonball! Because it has less mass.
47
Hammer Striking a Nail
• What are the action/reaction pairs for a hammer
striking a nail into wood?
• Force of hammer on nail force of nail on hammer
• Force of wood on nail force of nail on wood
• Which of the action/reaction forces above act on
the nail?
• Force of hammer on nail (downward)
• Force of wood on nail (upward)
• Does the nail move? If so, how?
• Fhammer-on-nail gt Fwood-on-nail so the nail
• accelerates downward

48
Hammer Striking a Nail
• What forces act on the hammer?
• Force of nail on hammer (upward)
• Force of hand on hammer (downward)
• Does the hammer move? If so, how?
• Fnail-on-hammer gt Fhand-on-hammer so the hammer
accelerates upward or slows down
• The hammer and nail accelerate in opposite
directions.

49
Action-Reaction A Book on a Desk
• Action Force
• The desk pushes up on the book.
• Reaction Force
• The book pushes down on the desk.
• The book pulls up on Earth.
• Earth pulls down on the book (force of gravity).

50
Action-Reaction A Falling Book
• Reaction
• The book pulls up on Earth.
• What is the result of the reaction force?
• Unbalanced force produces a very small upward
acceleration (because the mass of Earth is so
large).
• Action
• Earth pulls down on the book (force of gravity).
• What is the result of the action force (if this
is the only force on the book)?
• Unbalanced force produces an acceleration of
-9.81 m/s2.

51
Chapter 7, Page 118
• 9. The cannon and cannonball have very different
accelerations because equal forces on unequal
masses produce unequal accelerations. The more
massive cannon has more inertia and is harder to
accelerate than the much less massive cannonball.

52
Chapter 7, Page 118
• 10. The force that propels a rocket is the force
of the exhaust gases pushing the rocket in
reaction to the rocket pushing the exhaust gases.

Rocket pushes on exhaust gas
Exhaust gas pushes on rocket
RECOIL
53
6.1 Force Causes Acceleration
What causes an object to accelerate?
Unbalanced forces acting on an object cause the
object to accelerate.
Net Force gt 0
54
3.4 Newtons Law of Inertia
• Objects at Rest
• Objects in a state of rest tend to remain at
rest.
• Only a force will change that state.

55
3.4 Newtons Law of Inertia
• Objects in Motion
• In the absence of forces, a moving object tends
to move in a straight line indefinitely.
• Toss an object from a space station located in
the vacuum of outer space, and the object will
move forever due to inertia.

56
3.4 Newtons 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?

57
3.4 Newtons 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.