Chapter 3

How Things MoveGalileo asks the right Questions

Chapter 3

• Aristotle’s physics– Agrees with most people’s

commonsense feelings about motion

– It’s a wrong view to take and requires a closer look to see why and where Aristotle’s ideas of motion are wrong.

– His ideas will be discarded by Newton in his attempt to fundamentally explain motion.

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• Aristotle– He noticed that some motions

maintain themselves without assistance while others could be maintained only by some outside agent.

– Boiled motion into two categories:

• Natural Motion

• Violent Motion

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• Aristotle cont’d– Natural Motion

• Motion that could maintain itself– Examples: Solid objects falling,

liquids falling or running downhill, air rising, and flames leaping.

• The four natural motions occur on Earth because everything is made of four different substances--earth, water, air, and fire--and each of these substances are striving to reach their natural place.

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• Aristotle cont’d– He believed that all objects were

composed of these substances, and that an object’s motion was determined by how much of each element the object contained.

– For instance, • Wood floats because its mostly made

up of air• Hot air raise because its mostly made

up of fire• Page 67 in the book lists more

examples

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• Aristotle cont’d– One thing to keep in mind is that

all the motion discussed thus far is directly downward or upward. What about horizontal motion?

– Violent Motion• Motion that can only be maintained by

an outside agent.

– Examples: Pushing a boulder off a cliff, or pulling a wagon down the road

– Motion that mostly occurs horizontally.

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• Aristotle cont’d– Ultimately, Aristotle felt that all

motion on Earth was a combination of violent and natural motion.

– To explain the motion of the heavenly bodies, I.e. the planets and stars, Aristotle claimed that they were made of a 5th element called ether.

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• From Aristotle to Galileo– Although Aristotelian physics

explains a lot of different types of motion, the theory does have its weakness.

– Difficulties with Aristotelian physics:

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• From Aristotle to Galileo

• Galileo (1564-1642)– First recognizable person to

object with Aristotelian physics which had permeatted philosophy and culture until ~1600 A.D.

– Supported Copernican view of the universe

– Can be thought of as the “Grandfather” of experimental physics.

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• Galileo cont’d– 1st person we recognize as

practicing the scientific method– Performed experiments where he

rolled balls up and down inclines with varying smoothness

• This lead him to the conclusion that, in the absence of friction, a ball that started rolling on a horizontal surface would roll forever.

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• Galileo cont’d– His methods included the

following:• Experiments were designed to test specific

hypotheses.

• Idealizations of real-world conditions were made to eliminate any side effects that might obscure main effects.

• Limited the scope of inquiring by considering only one question at a time.

• Established quantitative methods to measure the motion of bodies, speed of falling objects, etc.

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• Inertia– Is the tendency of an object in motion to

stay in motion and an object at rest to stay at rest.

– Idea first purposed by French philosopher and scientist Rene Descarte.

• What happens to objects in the absence of gravity?

– All objects have inertia– Inertia doesn’t really explain anything,

rather is a word that stands for the unexplainable fact that unassisted objects do keep moving.

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• 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. In other words, all bodies have inertia.

– A body that is subject to no external forces must be unaccelerated. (most concise form)

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• More on Inertia– Space is a good place to test

motion without air resistance.• Planets keep moving around the sun

due to their inertia rather than some mystical force pushing.

• Inertia doesn’t provide the answer to why the planets travel around the sun in near circular orbits.

• Book lists other examples on page 72 through 74.

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• Motion– Rather than keep talking about

motion in a qualitative, its useful to talk about motion in a quantitative way.

– Scientists like to specify their measurements with the aid of a few basic quantities. The basic quantities useful in motion is distance, time, and direction.

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• Average Speed– Defined as the distance traveled divided

by the traveling time.

– A car’s speedometer gives you an idea of your speed. The one thing it doesn’t tell you is what direction you are heading.

– The concept speed is used to indicate the speed at every instance along an object’s path.

– Velocity is a term used to indicate speed and direction.

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• Speed– Equation: s = D / t– s = speed

• has units of meters per second or m/s

– D = distance• has units of meters or m

– t = time• has units of seconds

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• Acceleration– Defined as the change in speed

divided by the time it takes to make that change.

– Like Velocity, acceleration has a directional significance associated with it.

– The number part of acceleration is referred to as the acceleration.

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• 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.

– The problem with this law is that it seems to contradict our own common experience.

Chapter 3

• Galileo’s Law of Falling cont’d– Experience tells you that if you

drop a bowling ball and a feather that bowling ball will hit the ground.

– The reason is that the feather experiences much more air resistance than the bowling ball.

– This experiment was performed by the astronauts on their first voyage to the moon.

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• Galileo’s Law of Falling cont’d– The astronauts found that the

bowling ball and a feather fall to the ground at the same rate and hit the ground at the same time.

– The rate at which an objects falls to the ground is called the acceleration due to gravity.

– Free fall is motion that is influenced by gravity alone. Skydivers undergo free fall when they jump out of a perfectly good airplane.