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Making Sense of the Universe:
Understanding Motion, Energy, and Gravity
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How do we describe motion?
Precise definitions to describe motion:Speed: Rate at which object moves
s p e e d =d i s t ance
tim e !un i ts o fm
s"
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)
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The Acceleration of Gravity
All falling objectsaccelerate at thesame rate (notcounting friction ofair resistance).
On Earth, g!10m/s2: speedincreases 10 m/swith each second offalling.
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The Acceleration of Gravity (g)
Galileo showed
that gis the same
for all fallingobjects, regardless
of their mass.
Demonstrated by
Apollo 15 Hammerand Feather expt.
http://www.youtube.com/watch?v=5C5_dOEyAfk
http://www.youtube.com/watch?v=5C5_dOEyAfkhttp://www.youtube.com/watch?v=5C5_dOEyAfk7/25/2019 7. Newton's Laws_student
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Momentum and Force
'Momentum' -- your total amount of motion Momentum = mass #velocity
Momentum is 'conserved'
A net forcechanges momentum, whichgenerally means an acceleration (change in
velocity) Rotational momentum of a spinning or orbiting
object is known as angular momentum
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Conservation of Angular Momentum
A net external twisting force (torque) must be
acting on an object in to change the angularmomentum of the object.
Earth experiences no twisting force as it orbitsthe Sun, so its rotation and orbit will continueindefinitely
angular momentum = mass xvelocity xradius
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Angular momentum conservation explains why objects
rotate faster as they shrink in radius:
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What is mass?
How is mass different from weight?
Mass the amount of matter in an object Weight the forcethat acts upon an object
You are weightless
in free-fall!
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Thought Question
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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There isgravity in
spaceWeightlessness is
due to a constant
state of free-fall
Why are astronauts
weightless in space?
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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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Newtons Laws of Motion
Our goals for learning:
How did Newton change our view of the
universe?
What are Newtons three laws of motion?
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Realized the same physicallaws that operate on Earthalso operate in the heavens$ one universe
Discovered laws of motionand gravity (PhilosophiaeNaturalis PrinipiaMathematica 1687)
Much more: Experiments withlight; first reflecting telescope(1672), calculus"Sir Isaac Newton
(1642-1727)
How did Newton change our
view of the universe?
Wh t N t th l
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What are Newtons three laws
of motion?
Newtons first law of
motion:An object moves at
constant velocity unless a netforce acts to change its
speed or direction.
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Examples of First Law
Which of these has a force acting on it andwhen?
Ball on a string
Inside planes and cars
Drying
Orbiting Objects: Satellites, planets
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Thought Question:
Is there a net force? Y/N
1. A car coming to a stop.
2. A bus speeding up.
3. An elevator moving up at constant speed.
4. A bicycle going around a curve.
5. A moon orbiting Jupiter.
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Newtons second law of motion
Force = mass #acceleration
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Shotput vs. baseball
If I throw each one, I put the same amount
of force into it.
F=mshot
#a F=mbsball
#a
If the shotput is twice as massive how far
does it go?
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Thought Question:
Is the object accelerating? Y/N
1. A car coming to a stop.
2. A bus speeding up.
3. An elevator moving up at constant speed.
4. A bicycle going around a curve.
5. A moon orbiting Jupiter.
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Newtons third law of motion:
For every force, there is always an equal andoppositereaction force.
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Understanding Our Universe, 1stEdition
Copyright 2012 W. W. Norton & Company
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Thought Question:
Is the force the Earth exerts on you larger, smaller,
or the same as the force you exert on it?
A. Earth exerts a larger force on you.
B. I exert a larger force on Earth.
C. Earth and I exert equal and opposite forces oneach other.
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What have we learned?
How did Newton change our view of the universe?
He discovered laws of motion & gravitation
He realized these same laws of physics were
identical in the universe and on Earth
What are Newtons Three Laws of Motion?
1. Object moves at constant velocity if no net force
is acting.
2. Force = mass
#
acceleration 3. For every force there is an equal and opposite
reaction force