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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.