How Things Move3t.pdf Impo
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Transcript of How Things Move3t.pdf Impo
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Aristotelian Physics
Aristotle's physics agrees
with most people's
common sense, but modern scientists
discard it.
So what went wrong?
Here's what Aristotle
said:
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Aristotelian Physics
Aristotle’s classification of motion● Natural motion
● every object in the universe has a proper
place determined by a combination of four elements: earth, water, air, and fire
● any object not in its proper place will strive toget there
examples: – stones fall
– puffs of smoke rise
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Aristotelian Physics
● Natural motion (continued)
● straight up or straight down for all things on
earth
● beyond Earth, motion is circular
example: Sun and Moon continually circle
Earth
● Violent motion● produced by external pushes or pulls on objects
example: wind imposes motion on ships
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Aristotelian Physics
Aristotle said that a heavier object shouldnaturally fall much faster than a lighter object.
With modern technology, it is easy to
demonstrate that a feather and a rock fallat exactly the same rate in a vacuum tube.
There was a man who lived 400 years ago
who didn't need our modern technologyto figure out that Aristotle was wrong.
Who was he?
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Galileo's Physics
Aristotle had said that heavyobjects should fall noticeably faster
than lighter objects.
Galileo was the first scientist
to challenge him on this!
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.
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Galileo's Physics
Galileo correctly identified the influence of the forceof friction on objects.
When objects move they usually rub against a surface.
They can even rub against the air.
This rubbing force, which opposes their motion is called
friction.
To truly observe an object in its natural state of motion,
as Aristotle envisioned, you would have to get rid of the
interfering influence of friction.
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Galileo's Physics
In principle, it is next to impossible to get rid of allfriction, but, as technology has advanced, we can
now get pretty close.
When you drop a feather and a stone in theair, the stone obviously falls faster. Galileo
would say that friction with the air is holding
back the feather. What would Aristotle say?
When you drop a feather and a stone in avacuum, they fall together at the same rate.
Unfortunately, Galileo could not do this kind of
demonstration in a vacuum.
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Galileo's Physics
However, it is alleged that Galileo did do another kind of demonstration where the effects of
friction with the air could be minimized.
It is said that Galileo dropped a
10 kilogram cannonball and a1 kilogram stone from the
leaning tower of Pisa. Despite
the fact that the cannonball was
10 times heavier, it hit theground at nearly the same time
as the stone.
The acceleration of gravity should be the same for both.
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Galileo's Physics
Galileo observed that a ball
rolling down a steep ramp picks
up speed quickly.
A ball rolling down a
ramp that is less steepstill picks up speed but
at a slower rate.
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Galileo's Physics
Galileo reasoned that if you were to start a ball moving
on a horizontal surface, ideally it should never speed up
or slow down. It would keep on moving forever.
Of course, there would have to be no friction.
Today, we can come close to this by observing
objects in outer space or on low friction
surfaces such as ice.
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Galileo's Physics
In the absence of friction, a ball rolling down the incline on
the left tends to roll up to its initial height on the right.
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Galileo's Physics
Galileo also sought to know what an object'snatural state (or tendency) of motion should be.
Unlike Aristotle, Galileo came to a different
conclusion.
Galileo said that an object in motion will tend to
keep on moving in a straight line at constant
speed unless there was some unbalanced force(like friction) acting upon it.
This is the Law of Inertia
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Galileo's Physics
Galileo was one of the first to practice what we now call the scientific process.
His methods include:
1. Experiments designed to test ideas.
2. Idealizations to eliminate side effects that could
hide the truth.
3. Limiting the scope of the inquiry.
(One question at a time.)
4. Quantitative methods. He was not afraid to use
math.