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Lecture PowerPoint
Chapter 3
Chapter 3
How Things Move: Galileo Asks
the Right Questions
Physics:
Concepts & Connections
4th Edition
Art Hobson
© 2007 Pearson Prentice Hall
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Units of Chapter 3
Aristotelian Physics: A Commonsense
View
How Do We Know? Difficulties with
Aristotelian Physics
The Law of Inertia: The Foundation of
Newtonian Physics
Measuring Motion: Speed and Velocity
Measuring Motion: Acceleration
Falling
3.1 Aristotelian Physics: A
Commonsense View
Aristotle distinguished three different
kinds of motion.
Natural motion: falling objects and
liquids, rising air and flames
Violent motion: needing a constant
push or pull to continue
Celestial motion: motion of the moon,
planets, sun, and stars
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3.2 How Do We Know? Difficulties with
Aristotelian Physics
3.2 How Do We Know? Difficulties with
Aristotelian Physics
Take a flat piece of paper and drop it.
Then wad the paper up and drop it again.
Aristotle predicts both fall at the same
speed.
Galileo did detailed studies of falling and
rolling objects, and formulated his Law of
Falling:
Aristotle also predicts that heavier
objects will fall faster than lighter ones. Is
this true?
3.2 How Do We Know? Difficulties with
Aristotelian Physics
Now, throw a ball across the room. Once
it leaves your hand, what keeps it
moving? Aristotle says there must be a
constant force to keep it in motion.
Galileo’s thought experiment: Let a ball
roll down an incline; it will speed up. Let
it roll up the incline; it will slow down. In
between, on a perfectly flat surface with
no friction, the ball will keep rolling at a
constant speed forever.
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.
3.2 How Do We Know? Difficulties with
Aristotelian Physics
Galileo’s method:
Experimentation, to test a specific
hypothesis
Idealization, to eliminate side effects
Consider only one question at a time
Quantitative methods: precise
measurement
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3.3 The Law of Inertia: The Foundation of
Newtonian Physics
Imagine we could turn off air resistance, friction,
and gravity. How would things move?
Descartes had the answer (which Newton took as
his first principle of motion):
The Law of Inertia: A body that is subject to no external
influences (also called external forces) 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.
3.3 The Law of Inertia: The
Foundation of Newtonian Physics
3.3 The Law of Inertia: The Foundation of
Newtonian Physics
For example, if you are driving a car on an
icy road – a VERY icy road – and try to
stop or turn, you will find that your car
continues to go in a straight line, as there
is little friction.
You can also watch videos of astronauts –
things will float around until somebody
grabs them, or they run into the wall.
3.3 The Law of Inertia: The
Foundation of Newtonian Physics
Where does outer space begin?
Astronauts in “outer space” still move
under the influence of gravity; they are
basically falling all the time, and thus
experience apparent weightlessness.
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3.4 Measuring Motion: Speed and
Velocity
Speed is the distance an
object moves divided by
the time it takes to move.
This coaster is moving at
35.2 cm/s.
3.4 Measuring Motion: Speed and
Velocity
Suppose you drive 100 km in one hour.
Your speed would be 100 km/h. But this is
your average speed over the whole trip –
how likely is it that you were traveling at
exactly 100 km/h the whole time?
Your instantaneous speed is your speed at
one particular moment; this is what your
speedometer measures.
3.4 Measuring Motion: Speed and
Velocity
So, are speed and velocity the same thing?
Not in physics! Speed is what you would
read on a speedometer; velocity is the
combination of speed and direction. So if I
tell you I am driving 100 km/h, that is my
speed; if I tell you I am driving 100 km/h
north, that is my velocity.
3.5 Measuring Motion: Acceleration
The law of inertia tells us that an
undisturbed object will keep moving
with a constant velocity.
If an object’s velocity is changing, it
is accelerating. Acceleration is the
rate of change of velocity:
acceleration = (change in velocity)/time
Acceleration is measured in (m/s)/s, or
m/s2.
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3.6 Falling
Falling objects speed up as they fall.
How do we know this? Hold your book
above the floor and let it drop. What is
its speed right when you let it go? (Don’t
throw it!) What is its speed when it hits
the floor?
3.6 Falling
We see that the speed is proportional to
the time – 10 m/s at 1 s, 20 m/s at 2 s,
and so forth.
What about the distance? Clearly it is
not proportional to the time. However, if
we look at the pattern of how the
distance changes from second to
second, we can see that the distance is
proportional to the square of the time.
3.6 Falling
This diagram shows an
object falling. Note that
both its distance and its
velocity are increasing.
From one second to the
next, the distance
traveled increases, but
the change in velocity is
the same.
3.6 Falling
The object in the diagram is changing its
speed at a rate of 10 m/s per second.
Therefore, its acceleration is 10 m/s2.
This is (approximately) the acceleration
due to gravity anywhere on the Earth.
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