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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
theyre already doing.
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
theyre already doing.
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?
  • Answer
  • 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.
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