Chapter 3
How Things Move
Sections
• 3.1- Aristotelian Physics: A commonsense View
• 3.2- How Do We Know? Difficulties with
Aristotelian Physics
• 3.3- The Law of Inertia: The Foundation of
Newtonian Physics
• 3.4- Measuring Motion: Speed and Velocity
• 3.5- Measuring Motion: Acceleration
• 3.6- Falling
3.1- Aristotelian Physics: A
Commonsense View
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- Aristotle’s physics- theorized that there were three
kinds of motion: natural, violent and celestial motion.
- Natural motion- Unassisted motion like falling is one
form of natural motion
- Violent motion- horizontal motion
- Celestial motion- Aristotle believed that the moon,
sun, planets, and stars were made of a substance called
ether. Ether moved in perfect circles around Earth.
-Ether- in theory ether had no weight and was
incorruptible.
3.2- How Do We kow? Difficulties
with Aristotelian Physics
• Aristotelian physics has certain weaknesses. For
Example: If a crumpled piece of paper about the same
size as a baseball are dropped they fall the same speed,
except when dropped for a long distance there is a small
difference do to air resistance. This outcome could not
be explained my Aristotle’s theory.
• Air resistance- The resistance to the motion object
through the air due to the object’s collisions with
numerous air molecules.
• When air is not present during to ball and paper
experiment, the objects fall at exactly the same speed.
3.2
• Galileo’s Law of Falling- If Air resistance is negligible, then any two
objects that are dropped together will fall together, regardless of
their weights and their shapes, and regardless of the substances of
which they are made.
• Problem with violent motion: When arrow is shot from a bow, it can
travel great distances without constant external assistance. So what
keeps it moving? Aristotle had great difficulty explaining this
question with his theory.
• Friction- The force that one surface exerts on another due to the
roughness of the surfaces.
• Galileo theorized that in absence of friction, a ball that once started
rolling on a horizontal surface would roll forever. This statement was
completely different from those made by Aristotle.
Galileo’s Methods
• Experiments- Designed to test specific hypotheses.
• Idealizations- Of real-world conditions, to eliminate any
side effects that might obscure the main effects.
• Limiting the scope of the inquiry- by considering only one
question at a time. For example, Galileo separated
horizontal from vertical motion, studying only one of
them at a time.
• Quantitative Methods- Galileo went to great lengths to
measure the motion of bodies. He understood that a
theory capable of making quantitative predictions was
more powerful than one that could make only descriptive
predictions, because quantitative predictions were more
specific and could be experimentally tested in greater
detail.
3.3- The Law of Inertia: The
Foundation of Newtonian Physics
• Rene’ Descartes a French philosopher, was the first to
imagine the absence of gravity. He asked the question,
“What would happen if gravity was turned off?”
• His theory was that if gravity absent, an object released
in mid-air would not fall to the ground. Therefore if you
were to throw the object if would travel at the same
speed in a straight line forever.
• The Law of Inertia- A body that is subject to no external
influences will stay at rest if it was at rest to begin with
and will keep moving if it was moving to begin with; in
the latter case, its motion will be in a straight line at an
unchanging speed. All objects have inertia.
Space, The Final Frontier
• Outer Space- refers to those
regions of the universe outside
Earth and outside other
astronomical objects.
• In outer space there is zero
gravity. A good example of this
is when astronauts travel to the
moon, they fire their rocket
boosters to achieve the
desired speed, and then coast
all the way to the moon.
• Earth coasts in outer space.
This explains what Copernicus
could not, when asked how
earth was to continue moving.
3.4- Measuring Motion: Speed and
Velocity
• In order to obtain quantitative predictions about our
physical existence; further more to discover more
accurate predictions, numerical methods must be used.
• Average speed- An object’s average speed is its
distance traveled during a time interval divided by the
duration of that time interval. Measured in meters per
second.
• Instantaneous Speed (speed)- The average speed
during a time interval that is so short that the speed
hardly changes. Speedometers measure this. Measured
in meters per second.
• Velocity- refers to speed and direction. Speed and
velocity are not interchangeable.
3.5 Measuring Motion: Acceleration
• Concise Law of Inertia- A body that is subject to
no external forces must maintain an unchanging
velocity.
• Acceleration- An accelerated object is one
whose velocity is changing. Quntitatively, the
acceleration is the change in velocity during a
time interval divided by the duration of that time
interval. It can be measured in (km/hr)/s, or in
(m/s)/s=m/s^2.
3.6- Falling
• Free Fall- An object like this whose falling is influenced
by gravity alone.
• Acceleration due to gravity- The acceleration of any
freely falling object. On Earth this is about 10 m/s^2 or,
more precisely, 9.8 m/s^2.
• Proportional- One quantity is proportional to a second
quantity if, whenever the second is multiplied by some
number, the first is multiplied by the same number. One
quantity is proportional to the square of a second
quantity if, whenever the second is multiplied by some
number, the first is multiplied by the same number
squared. (HAHAHA!)