7. Newton's Laws_student

7/25/2019 7. Newton's Laws_student

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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_dOEyAfk


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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

7. Newton's Laws_student

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