chap 3 laws of motion.web

8/12/2019 Chap 3 Laws of Motion.web

1/64

Chap 3: Laws of Motion

http://neo.jpl.nasa.gov/1950da/animations.html

What is motion?what do scientists define as motion ?

Why do the planets move the way they do?


2/64

Motion

What is motion?what do scientists define as motion ?

Why do the planets move the way they do?

Keep in mind the way scientists like to work:

ObserveDefine

Hypothesize/PredictTest

repeat


3/64

Note:the study of motion was motivated bythe motion of celestial objects.

Galileo studied motion;

was the first to use atelescope for astronomicalobservations;

saw that there wereojects that did not movearound the Earth !


4/64

Galileo Galilei (1564- 1642) the astronomer

Believed in the Copernican model.

Demonstrated that Keplers and Copernicus ideaswere right by making observations with histelescope.

2. The moons of Jupiter obey Keplers laws 3. Phases of Venus, supporting heliocentric theory

4. Observed that Mars, Jupiter and Saturn had nophases.

5. Observed individual stars in the Milky Way,thereby showing stellar parallax measuring ispossible.

1. The heavens are not perfect: geological features on the moon sunspots on the surface of the Sun


5/64

Galileo Galilei (1564-1642)



1. The heavens are not perfect: geological features on the moon sunspots on the surface of the Sun


6/64


2. The moons of Jupiter obeyKeplers laws


7/64

3. Phases of Venus, supporting heliocentric theory 4. Observed that Mars, Jupiter and Saturn had no

phases.



8/64

3. Phases of Venus, supporting heliocentric theory 4. Observed that Mars, Jupiter and Saturn had no phases.

http://astro.unl.edu/classaction/animations/renaissance/ptolemaic.html

geocentric heliocentric
http://astro.unl.edu/classaction/animations/renaissance/ptolemaic.htmlhttp://astro.unl.edu/classaction/animations/renaissance/ptolemaic.htmlhttp://localhost/var/www/apps/conversion/tmp/scratch_4/phases_venus.htmhttp://localhost/var/www/apps/conversion/tmp/scratch_4/phases_venus.htm


9/64




2. The moons of Jupiter obey Keplers laws 3. Phases of Venus, supporting heliocentric theory

4. Observed that Mars, Jupiter and Saturn had nophases.

5. Observed individual stars in the Milky Way,thereby showing stellar parallax measuring ispossible.


10/64

Galileo Galilei (1564- 1642) the physicist

Galileo also experimented with fallingand moving objects and crafted atheory of motion.

Galileos workshopat the DeutchesMuseumin Munich,Germany


11/64

Galileo Galilei (1564- 1642) the physicist

Galileo also experimented with fallingand moving objects and crafted atheory of motion.

1. An object in motion will continue moving alonga straight line with a constant speed until anunbalanced force acts on it.


12/64

dropped objectsmove down at10 m/s /s

t=3 s

v=30 m/s

WHY?

2.


13/64

Gravity makes things accelerate at 10 m/s 2

Acceleration of gravity isindependent of the mass of the falling object!

Iron ball

Wood ball


14/64

QuickTi me and aVC Coding

Fourth manned lunar landing withDavid R. Scott, Alfred M. Worden, and James B. Irwin.Landed at Hadley rilleon July 30, 1971.

www.hq.nasa.gov/alsj/a15/video15.html#closeout3
http://www.hq.nasa.gov/alsj/a15/video15.htmlhttp://www.hq.nasa.gov/alsj/a15/video15.html


15/64

Observation

How do we describe motion?Define __________ ?


16/64

How do we describe motion?

10 m/s South-East & direction

100 m/sdisplacement per time

(define it)

60 km/hr --> 30 km/hr --> 0changing displacement per time


17/64

How do we describe motion? (define it)

Speed: Rate at whichobject moves

Velocity: Speed and direction

Acceleration: Any changein speed or direction

Thank you, Galileo


18/64

Motion: speed, velocity, & acceleration

What about the mass?


19/64

Yes, motion depends on mass, too.

Linear Momentum = mass velocity

Motion with Mass Momentum

Angular momentum is

rotational momentum of a spinning ororbiting object


20/64

Describing Motion:(basic ingredients)

1. change in position (displacement)2. time3. mass4. direction

So far..

speedvelocityaccelerationmomentum


21/64

Isaac Newton (1642-1727)

Born the year Galileodied Contemporary of Bach Derived laws of gravityand other laws of physics

If I have seen further, it is by standing on theshoulders of giants.

--Isaac Newton


22/64

Isaac Newton (1642 - 1727) Building on the results of Galileo and Kepler

Major achievements:1. Invented Calculus as a necessary tool to solve

mathematical problems related to motion

Adding physics interpretations to the mathematicaldescriptions of astronomy by Copernicus, Galileoand Kepler

2. Formulated the three laws of motion

3. Formulated the universal law of mutual gravitation
http://science.discovery.com/interactives/literacy/newton/newton.html


23/64

Newtons 3 Laws of Motion

1 2 3Inertia F = ma action = reaction

science.discovery.com/interactives/literacy/newton/newton.html
http://science.discovery.com/interactives/literacy/newton/newton.html


24/64

1. Newtons Laws of Motion

A body continues at rest orin uniform motion in a straight lineunless acted upon by some net force.

Also known as : The Law of Inertia


25/64

Newtons 1st:

object will stayat rest (or inuniform motion) until acted on by

a FORCE


26/64


An astronaut floating inspace will continue tofloat forever in a straight

line unless someexternal force isaccelerating him/her.


27/64

2. Newtons Laws of MotionThe accelerat ion, a, of a

body isdirectly proportional to

the net fo rce F,

in the same directionas the net force F ,

inversely proportionalto its mass, m .

a = F F = m a

m


28/64

Newtons 2nd:

F = ma(unbalanced forcescause changes inmotion.)


29/64


To every action,there is an equal andopposite reaction.

The same force that isaccelerating the boy

forward, is acceleratingthe skateboardbackward.

M = 70 kg

m = 1 kg

v = 7 m/s

V = ?


30/64

Newtons 3rd: action - reaction


31/64

Newtons 3rd:

action - reaction


32/64

The Universal Law of Gravity

Any two bodies are attracting eachother through gravitation, with aforce proportional to the product oftheir masses and inverselyproportional to the square of theirdistance:

F = - G Mm

d2

(G is the Universal constant of gravity.)

d


33/64

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 thesquare of the distance between their centers.


34/64

weight F m g

F = m a

All objects on Earth fall with the sameacceleration, g .g = 9.8 m/s 2 (about 10 m/s 2 --- Galileo)

The acceleration: a = g

Your weight is the force of Earth on YOU


35/64

How do we describe motion? Speed = distance / time Speed & direction => velocity

Change in velocity => acceleration Mass effect motion Momentum = mass x velocity

Force causes change in momentum,producing acceleration

Summary so far:


36/64

Motion described by:

speed, velocity, and acceleration determined by:

Newtons 3 Laws has: energy

Energy makes change


37/64


38/64

There are 2 forms of ENERGY

Kinetic energyis motion of waves,electrons,atoms,molecules,substances,

and objects.

Potential energyis stored energyand the

energy of positionof objects,nucleus,chemical


39/64


Kinetic energyis motion Electrical --- chargesRadiant --- EM energyThermal --- heatMotion --- Newtons Laws Sound --- waves through substances


40/64


Potential energyis stored energyand theenergy of positionChemical --- stored in bonds (atom/molecule)Stored mechanical --- springs, rubber bandNuclear --- stored in nucleus (fusion/fission)

Gravitational --- stored in position


41/64


42/64

Gravitational Potential Energy

On Earth, GPE depends on: objects mass (m) strength of gravity ( g ) distance object could

potentially fall


43/64

Gravitational Potential EnergyIn space, an object or gas cloud has more

gravitational energy when it is spread out thanwhen it contracts.

A contracting cloud converts gravitationalpotential energy to thermal energy.


44/64

Mass-Energy Mass itself is a form of potential

energy

E = mc 2

A small amount of masscan release a great deal ofenergy

Concentrated energy canspontaneously turn into

particles (for example, in particle accelerators)


45/64

Energy

Energy can be neither created nordestroyed.

It can change form or be exchanged

between objects. The total energy content of the Universe

was determined in the Big Bang and

remains the same today.

Energy is Conserved


46/64

Conservation of energy(energy before = energy after )

Conservation of momentum

Anything else conserved?


47/64

Conservation of Momentum

The total momentumof interacting objectscannot change

unless an externalforce acts on them

Interacting objects

exchange momentumthrough equal andopposite forces

BEFORE

AFTER


48/64

Conservation of angular momentum

Angular momentumconservation alsoexplains why objectsrotate faster as theyshrink in radius


49/64

MASS, WEIGHT does it matter?

YES!MASS is the a m o u n t of matter of the object.Weight is the amoun t of fo rce on the object.


50/64

Lets apply EVERYTHING we know about motion to orbital motion


51/64


52/64

and so on and on .


53/64

The Sunexerts aforce on theplanets(and viceversa!)

Orbital Motion

How do gravity and energy together allow us


54/64

How do gravity and energy together allow usto understand orbits?

Total orbital energy (gravi ta t ional+kinet ic)

stays constant if thereis no external force

Less kinetic energy;

More gravitational energy.

More kinetic energy;Less gravitational

energy

Orbits cannot changespontaneously .

How Can an Orbit Change ?


55/64

Friction: atmosphericdrag, or tidal flexingof a fluid object

A gravitationalencounter.

How Can an Orbit Change ?

An object gains or lose orbital energy. HOW does that happen?


56/64

Conservation of Angular Momentum

The angular momentum of an object cannotchange unless an external force (torque) isacting on it

Earths rotation and orbit will continueforever because it cant get rid of angular momentum

angular momentum = (mass x velocity) x radius of orbit

S f


57/64

So far Why do objects move at constant velocity

if no force acts on them? Conservation of momentum

Where do objects get their energy?

Conservation of energy: energy cannotbe created or destroyed but onlytransformed from one type to another.

Energy comes in three basic types:kinetic, potential, radiative.


58/64

In order to stay on a closed orbit, an objecthas to be within a certain range of

velocities:

Too slow =>Object falls

back downto Earth


59/64

In order to stay on a closed orbit, anobject has to be within a certain range ofvelocities:

Too fast =>ObjectescapesEarthsgravity


60/64

If an object gainsenough orbital energy, itmay escape (changefrom a bound tounbound orbit)

Escape velocity from

Earth 11 km/s fromsea level (about 40,000km/hr)

Escape Velocity


61/64

AstroTour Velocity, Acceleration, Inertia

Click here to launch this AstroTour (Requires an active Internet connection.)
http://media.wwnorton.com/college/astronomy/animations/astrotour/Anim11/html/Velocity_Acceleration_Inertia.htmlhttp://media.wwnorton.com/college/astronomy/animations/astrotour/Anim11/html/Velocity_Acceleration_Inertia.html


62/64

AstroTour Newtons Laws and Universal Gravitation

http://media.wwnorton.com/college/astronomy/animations/astrotour/Anim03/Projectile_Motion.htmlhttp://media.wwnorton.com/college/astronomy/animations/astrotour/Anim03/Projectile_Motion.html


63/64

AstroTour Elliptical Orbits


End of Chapter 3
http://media.wwnorton.com/college/astronomy/animations/astrotour/Anim02/Elliptical%20orbit.htmlhttp://media.wwnorton.com/college/astronomy/animations/astrotour/Anim02/Elliptical%20orbit.html


64/64

End of Chapter 3

Now you know !

chap 3 laws of motion.web

Documents