Physics 1 AcademicChapter 4

Newton’s First Law of Motion – InertiaSections 4.1 to 4.4

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Aristotle on Motion

In the 4th Century B.C., the Greeks were developing the basic ideas of motion.

The foremost Greek Scientist of the time was Aristotle.

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Aristotle on Motion

Aristotle divided motion into:› Natural Motion› Violent Motion

Natural motion was limited to motion that was:› Straight up› Straight down

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Aristotle on Motion

Objects would seek their natural resting place:› A boulder would end up on the ground.› Smoke would end up in the air.

The Greeks assumed that it was natural for:› Heavy things to fall.› Light things to rise.

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Aristotle on Motion

Aristotle also extended natural motion to include circular motion in the heavens.› He thought circular motion was without

beginning or end› He thought the heavens were without

beginning or end.

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Aristotle on Motion

Aristotle postulated that planets and stars moved in perfect circles.

He thought since all this motion was natural, that it was not caused by forces.

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Aristotle on Motion

Violent motion was imposed motion, caused by a push or a pull.› A cart moved because of the force applied

by the oxen.› A ship moved from the force of the wind› A boat move from the force applied by the

oars Violent motion was the result of some

external cause.

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Aristotle on Motion

For an object to move from its natural resting place it had to be pushed or pulled.

An object moving “against its nature” was the result of an external force.

The proper state of objects was at rest. This was believed for 2000 years, until

the 16th Century.

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Aristotle on Motion

It was believed that the Earth was in its natural resting place.

They also thought that there was no force strong enough to move the Earth.

Therefore, the Earth did not move.

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Copernicus and the Moving Earth Copernicus was the

first person to put forth the theory of the moving Earth.

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Copernicus and the Moving Earth He thought that the

easiest way to explain astronomical observations was that the Earth moved around the Sun.

He also thought the other planets moved around the Sun.

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Copernicus and the Moving Earth

This theory was very controversial. People believed the Earth was the

center of the Universe. Copernicus had to work in secret to

avoid persecution. Only at the end of his life did he

publish his ideas, the first copy reaching him the day he died.

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Copernicus and the Moving Earth

The Lunar Crater Copernicus

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Galileo on Motion

Galileo was the most important scientist in Italy in the later part of the Renaissance.

Strongly supported the ideas of Copernicus.

Was arrested as a result of these beliefs and put on trial.

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Galileo on Motion

Galileo is given credit for getting rid of the idea that a force is necessary to keep an object moving.

Force – a push or pull Friction

› Resists motion› Acts between materials that touch and are

in motion› Caused by irregularities of the surfaces of

objects.

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Galileo on Motion

Galileo said that only when friction is present do you need a force to keep an object moving.

In other words, in the absence of friction an object will keep moving forever.

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Galileo on Motion

To prove his ideas Galileo used a ball which he rolled down an inclined ramp and then allowed the ball to continue up another inclined ramp.

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Galileo on Motion

As the ball rolls down the ramp, it gains speed.

As the ball rolls up the second ramp, it loses speed

Without friction, it will end at the same height it started.

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Galileo on Motion

What if there was only a ramp to roll the ball down onto a level surface.

Where will the ball stop?

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Galileo on Motion

Where the ball stops depends on friction.

Galileo realized that the natural state was not at rest.

In the absence of friction, the ball will continue rolling forever.

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Galileo on Motion

Galileo stated that every object resists a change in its motion.

The resistance to change in motion is called inertia

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Galileo on Motion

Galileo addressed how things move rather than why.

He based his findings on experimentation rather than logic.

His ideas about motion and inertia discredited the work of Aristotle.

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Newton’s Law of Inertia

Newton restated Galileo’s ideas:

Every object continues in a state of rest, or of motion in a staight line at a constant speed unless compelled to change that state by forces exerted on it.

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Newton’s Law of Inertia

Simply stated:

Objects at rest tend to stay at rest. Objects in motion tend to stay in motion.

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Newton’s Law of Inertia

Take a table with a tablecloth and plates.

If done correctly, you can pull out the tablecloth and the plates will remain in place.

The plates are at rest, and want to stay at rest.

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Newton’s Law of Inertia

As the tablecloth is pulled quickly away, there is some friction and the plates will move slightly.

The friction is quickly overcome and the plates remain and the tablecloth is removed.

Go to the Videotape

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Newton’s Law of Inertia

Which slides further?› A hockey puck on a street?› A hockey puck on ice?› Why?

What would happen if the astronauts could throw an object out of the International Space Station?

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Newton’s Law of Inertia

Objects in a force-free environment will move in a straight line, forever.

Go to the Videotape