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Test 1 Result: Section 1
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Test 1 Result: Section 2
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2
AST 103
Ch.4 Making Sense of the Universe:
Motion & Gravity
Prof. Ken Nagamine
UNLV
3
Sec 4.1: Describing Motion
• Our goals for learning:
• How do we describe motion?
• How is mass different from weight?
4
How do we describe motion?
Precise definitions to describe motion:
Speed: Rate at which object moves
example: speed of 10 m/s
Velocity: Speed and direction
example: 10 m/s, due east
Acceleration: Any change in velocity
units of speed/time (m/s2)
5
Momentum and Force
• momentum = mass × velocity
• “applying a force” ==>
acceleration (change in velocity)
==> change in momentum
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Angular Momentum
• Rotational momentum of a spinning or
orbiting object is known as angular
momentum
angular momentum = mass x velocity x radius
R
V
M
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Why do objects fall?
8
Quiz
Which object will hit
the ground first?
A. the piece of paper
B. the eraser
C. Both will hit at the same time.
D. We can’t predict which will hit first.
9
The Acceleration of Gravity (g)
• Galileo showed that
g is the same for all
falling objects,
regardless of their
mass.
Apollo 15 demonstration
http://www.youtube.com/watch?v=4mTsrRZEMwA
(search by “Galileo experiment on moon)
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Acceleration due to Gravity
• All falling objects
accelerate at the same
rate on Earth (not
counting friction of air
resistance).
• On Earth, acceleration
g~10 m/s2 : speed
increases 10 m/s with
each second of falling.
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Quiz
Is there a difference
between mass and weight?
A. Yes, there is a difference.
B. No, they are the same thing.
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How is mass different
from weight?
• Mass – the amount of matter in an
object
• Weight – the force that acts upon an
object
13
`Gedanken Experiment’
of Einstein
?
?
?
?
?
?
14
How is mass different
from weight?
?
?
?
?
15
How is mass different
from weight?
?
?
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How is mass different
from weight?
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Einstein realized that acceleration is
equivalent to gravity.
Weight can change according to the
environment, but mass cannot.
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Another good example:
Why are astronauts weightless
in space?
There is gravity
in space
Weightlessness is
due to a constant
state of free-fall
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Quiz
On the Moon,
A. My weight is the same, my mass is less.
B. My weight is less, my mass is the same.
C. My weight is more, my mass is the same.
D. My weight is more, my mass is less.
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What have we learned?
• How do we describe motion?
• Speed = distance / time
• Speed & direction => velocity
• Change in velocity => acceleration
• Momentum = mass x velocity
• Force causes change in momentum,
producing acceleration
21
What have we learned?
How is mass different from weight?
Mass = quantity of matter
Weight = force acting on mass
Objects are weightless in free-fall
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S. 4.2: Newton’s Laws of Motion
• How did Newton change our view of
the universe?
• What are Newton’s three laws of
motion?
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How did Newton change our
view of the universe?
• Realized the same physical laws
that operate on Earth also
operate in the heavens
•
one universe
• Discovered laws of motion and
gravity
• Much more: experiments with
Sir Isaac Newton
(1642-1727)
light; first reflecting telescope,
calculus, ....
24
Newton’s first law of motion
• An object moves at
constant velocity unless
a net force acts to change
its speed or direction.
In layman’s terms....
“a scooter will coast with a same
speed until you brake or kick!”
25
Newton’s second law
of motion
• Force = mass × acceleration
• The bigger the force, the
greater the resulting
acceleration on a given mass.
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Newton’s third law of
motion:
• For every force, there is always an equal
and opposite reaction force.
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Newton’s law of
universal gravitation
• The force of gravitational attraction exerted
by one object on another depends on
• the mass of the first object (M)
• the mass of the second object (m)
• the distance between the objects (r)
or, in math terms, F = GMm
r2
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What determines the strength of gravity?
The Universal Law of Gravitation:
1. Every mass attracts every other mass.
2. Attraction is directly proportional to the product of
their masses.
3. Attraction is inversely proportional to the square of
the distance between their centers.
© 2008 Pearson Education Inc, publishing as Pearson Addison-Wesley
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Quiz
Is the force Earth exerts on you larger, smaller,
or the same as the force you exert on Earth?
A. Earth exerts a larger force on me.
B. I exert a larger force on Earth.
C. Earth and I exert the same amount of force
on each other.
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m
M
31
moon
m
m
M
M
Earth
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Lecture-Tutorial (LT):
Newton’s Laws and Gravity (p.29-31)
• Work with a partner!
• Read the instructions and questions carefully.
• Discuss the concepts and your answers with one
•
•
•
another.
Come to a consensus answer you both agree on.
If you get stuck or are not sure of your answer, ask
another group.
If you get really stuck or don’t understand what the
LT is asking, ask for a help.
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