Page 1 Lecture 4: Momentum, Energy, Tides, and the Scientific Method Claire Max April 15 th, 2014...

Lecture 4: Lecture 4: Momentum, Energy, Momentum, Energy, Tides, and the Scientific MethodTides, and the Scientific Method

Claire Max

April 15th, 2014

Astro 18: Planets and Planetary Systems

UC Santa Cruz

Planetary newsPlanetary news

Outline of this lectureOutline of this lecture

• Newton’s Laws: Momentum and Angular Newton’s Laws: Momentum and Angular MomentumMomentum

• Types of energy, and conservation lawsTypes of energy, and conservation laws

• TidesTides

• The The ““scientific methodscientific method”” and what is science and what is science

Please remind Please remind me to take a me to take a

break at 12:45 break at 12:45 pm pm

NewtonNewton’’s second law of motions second law of motion

Force = mass X Force = mass X accelerationacceleration

• The arrow above the symbols means that they are The arrow above the symbols means that they are vectorsvectors: quantities that have both a magnitude and : quantities that have both a magnitude and a direction.a direction.

• Vector quantities are sometimes indicated by a Vector quantities are sometimes indicated by a boldbold font. font.

Consequence of NewtonConsequence of Newton’’s 2nd s 2nd LawLaw

• If thereIf there’’s no force, theres no force, there’’s no accelerations no acceleration

• Rate of change of velocity = 0Rate of change of velocity = 0

• Implies velocity = constantImplies velocity = constant

• ““A body in motion will stay in motionA body in motion will stay in motion”” = concept = concept of inertia. Newtonof inertia. Newton’’s first law follows from his 2s first law follows from his 2ndnd law!law!

NewtonNewton’’s second law re-s second law re-phrased in term of phrased in term of momentum conservationmomentum conservation

• Definition: momentum = mass x velocityDefinition: momentum = mass x velocity

– Symbol for momentum =Symbol for momentum = p p (a vector)(a vector)– pp = m = m vv– momentum has a direction because momentum has a direction because vv does does

• NewtonNewton’’s second law:s second law:

Force = mass x acceleration = rate of change of Force = mass x acceleration = rate of change of momentummomentum

FF = m = m aa = mass x rate of change of = mass x rate of change of velocityvelocityIf mass = constant, If mass = constant, FF = rate of change of (m = rate of change of (mvv))Define momentum as mass times velocity = mDefine momentum as mass times velocity = mvv

• If force = 0, momentum = mv = constantIf force = 0, momentum = mv = constant

Conservation of Angular Conservation of Angular MomentumMomentum

• Angular momentum conservation:Angular momentum conservation:

• The angular momentum of an object The angular momentum of an object cannot change unless an external twisting cannot change unless an external twisting force (torque) is acting on itforce (torque) is acting on it

• Earth experiences no twisting force as it Earth experiences no twisting force as it orbits the Sun, so its rotation and orbit orbits the Sun, so its rotation and orbit will continue indefinitelywill continue indefinitely

angular momentum = mass x velocity x radius

Angular momentum conservation Angular momentum conservation explains why objects rotate faster as explains why objects rotate faster as they shrink in radiusthey shrink in radius

Centrifugal forceCentrifugal force

Without the string, the ball Without the string, the ball would just keep moving in a would just keep moving in a straight linestraight line

Centrifugal forceCentrifugal forceFor a planet in orbit, gravity from For a planet in orbit, gravity from

the Sun takes the place of the the Sun takes the place of the stringstring

Concept QuestionConcept Question

• A cloud of interstellar gas is collapsing A cloud of interstellar gas is collapsing under the force of its own gravity. under the force of its own gravity.

• As it collapses, its rotational speedAs it collapses, its rotational speed

A.A. Depends on its massDepends on its massB.B. IncreasesIncreasesC.C. DecreasesDecreasesD.D. Is independent of its initial rotationIs independent of its initial rotation

• Why?Why?

What does this imply about What does this imply about the rotation rates of newly the rotation rates of newly born stars ?born stars ?

• Initial big gas cloud rotates slowly – Initial big gas cloud rotates slowly – perhaps just taking part in the overall perhaps just taking part in the overall rotation of the Galaxyrotation of the Galaxy

• As it collapses to form a star, its As it collapses to form a star, its angular velocity increasesangular velocity increases

• Hence newly formed stars frequently Hence newly formed stars frequently have high rotation rates (they spin have high rotation rates (they spin rapidly)rapidly)

Next topic: EnergyNext topic: Energy

• Energy: Energy:

– The capacity to make matter move, or to The capacity to make matter move, or to ““do do workwork””

• Energy comes in different formsEnergy comes in different forms

– Kinetic energy, potential energy (gravity), Kinetic energy, potential energy (gravity), radiative energy, energy in atoms & radiative energy, energy in atoms & molecules, electrical energy, mass molecules, electrical energy, mass energy, ....energy, ....

– Energy can change from one form to anotherEnergy can change from one form to another

• But total energy is always conservedBut total energy is always conserved

““Follow the energyFollow the energy”” is a good is a good rule in astronomyrule in astronomy

• Why does something take place?Why does something take place?– Ask where its energy comes fromAsk where its energy comes from

• Examples:Examples:– Heat from the Sun (nuclear reactions at its Heat from the Sun (nuclear reactions at its

core)core)– Weather on Earth (heat from the Sun)Weather on Earth (heat from the Sun)– Orbits of planets (determined by kinetic Orbits of planets (determined by kinetic

energy and gravitational potential energy)energy and gravitational potential energy)

Astrophysical energies are Astrophysical energies are huge!huge!

Kinetic energyKinetic energy

• Energy of motionEnergy of motion

• Kinetic energy KE = (1/2) m vKinetic energy KE = (1/2) m v22

– m is mass, v is velocitym is mass, v is velocity

• Units: if mass is in kilograms, velocity Units: if mass is in kilograms, velocity is in meters/sec, then energy is in is in meters/sec, then energy is in joulesjoules– 1 joule = 1 kg (m / sec)1 joule = 1 kg (m / sec)22 = 1 kg m = 1 kg m22 / sec / sec22

Kinetic energy, continuedKinetic energy, continued

• Kinetic energy is proportional to massKinetic energy is proportional to mass– more massive objects have more KEmore massive objects have more KE

• Kinetic energy is proportional to the Kinetic energy is proportional to the squaresquare of the velocityof the velocity– If you double your speed (e.g. from 30 to 60 If you double your speed (e.g. from 30 to 60

mph), your kinetic energy goes up by factor of mph), your kinetic energy goes up by factor of fourfour

– Auto accidents: front part of cars are made to Auto accidents: front part of cars are made to absorb energy, crumple up (to avoid squishing absorb energy, crumple up (to avoid squishing the passenger compartment and hence you).the passenger compartment and hence you).

» Must absorb 4 X more energy at 60 mph than Must absorb 4 X more energy at 60 mph than at 30 mphat 30 mph

» If it isnIf it isn’’t able to do so, passengers get hurtt able to do so, passengers get hurt

Units of energy (in Metric Units of energy (in Metric system)system)

• ““cgs unitscgs units”” : grams and cm : grams and cm

• ““mks unitsmks units”” : kg and meters : kg and meters

• Completely equivalent (choose which one to Completely equivalent (choose which one to use)use)

Potential energyPotential energy

• Energy that is available by Energy that is available by virtue of an objectvirtue of an object’’s positions position

• Most common example is Most common example is gravitational potential gravitational potential energyenergy

• If you stand at top of diving If you stand at top of diving board, you have the board, you have the potential to turn your potential to turn your gravitational potential gravitational potential energy into kinetic energy of energy into kinetic energy of motionmotion

Size of gravitational potential Size of gravitational potential energyenergy

Potential energy on surface of a Potential energy on surface of a big planet or moon: big planet or moon:

• PE = - m g hPE = - m g h

mass x (gravitational acceleration) x mass x (gravitational acceleration) x heightheight

• Units are same as kinetic energyUnits are same as kinetic energy

– kg x (meters /seckg x (meters /sec22) x meters = kg m) x meters = kg m22 / /

secsec22

• Increases with mass, height, Increases with mass, height,

gravitational acceleration ggravitational acceleration g

Conservation of energyConservation of energy

• Kinetic Energy + Potential Energy = const = EKinetic Energy + Potential Energy = const = E

• At surface of a big planet or moon:At surface of a big planet or moon:

• Implications:Implications:

– Initial state: v = 0, total energy = m g hInitial state: v = 0, total energy = m g h– Final state: h = 0, total energy = (1/2) m vFinal state: h = 0, total energy = (1/2) m vfinalfinal

22

– m g h = (1/2) m vm g h = (1/2) m vfinalfinal22

– Solve for vSolve for vfinal: final: v vfinalfinal = ( 2 g h ) = ( 2 g h )1/21/2

Implications, continuedImplications, continued

• vvfinalfinal = ( 2 g h ) = ( 2 g h )1/21/2

• If you fall from a higher place (h large), your If you fall from a higher place (h large), your final velocity will be higherfinal velocity will be higher

• If you fall on the Moon (g small), your final If you fall on the Moon (g small), your final velocity will be lower than if you fall on Earthvelocity will be lower than if you fall on Earth

• Note that final velocity is Note that final velocity is independent of independent of massmass– GalileoGalileo’’s famous experiment at leaning tower of s famous experiment at leaning tower of

PisaPisa– Dropped heavy object and light object; they hit Dropped heavy object and light object; they hit

ground at same timeground at same time


• Can you think of examples where Can you think of examples where gravitational potential energy is gravitational potential energy is converted to kinetic energy?converted to kinetic energy?

– In our daily lives here on Earth? In our daily lives here on Earth?

– In the Solar System?In the Solar System?

Examples where potential Examples where potential energy is converted to kinetic energy is converted to kinetic energyenergy

More examples of potential More examples of potential energy converting to kinetic energy converting to kinetic energyenergy


• PendulumPendulum


• SkiingSkiing

Waterfall: what are roles of Waterfall: what are roles of potential and kinetic energy potential and kinetic energy here?here?

Gravitational Potential EnergyGravitational Potential Energy

• On Earth, depends on:On Earth, depends on:– objectobject’’s mass (m)s mass (m)– strength of gravity (strength of gravity (gg))– distance object could distance object could

potentially fallpotentially fall

Gravitational Potential EnergyGravitational Potential Energy

• In space, an object or gas cloud has more gravitational In space, an object or gas cloud has more gravitational energy when it is spread out than when it contracts.energy when it is spread out than when it contracts.

• A contracting cloud converts gravitational potential A contracting cloud converts gravitational potential energy to thermal energy. energy to thermal energy.

Energy can do Energy can do ““workwork””

Work = Force x DistanceWork = Force x Distance

( a physicist( a physicist’’s definition of work)s definition of work)

General expression for General expression for gravitational potential energygravitational potential energy

• PE = m g h PE = m g h – only applies on the only applies on the

surface of a big surface of a big planet or moonplanet or moon

• General expression General expression (holds (holds everywhere):everywhere):

More implications: For a More implications: For a planet in orbit around a starplanet in orbit around a star

• rr = distance between = distance between mm11 and and mm22

• Gravitational PE is Gravitational PE is negativenegative

• Speed of planet is largest Speed of planet is largest when it is closest to star: when it is closest to star:

• r is small, so ( Gmr is small, so ( Gm11mm2 2 / r ) is / r ) is largelarge

© Nick Strobel


• Imagine a straight shaft bored from the Imagine a straight shaft bored from the EarthEarth’’s surface, thru the center of the Earth, s surface, thru the center of the Earth, and out the other sideand out the other side

• Drop a baseball down this shaftDrop a baseball down this shaft

• What is the baseballWhat is the baseball’’s motions motion

– At the startAt the start

– At the center of the EarthAt the center of the Earth

– Just as it comes out the other sideJust as it comes out the other side

• At each of these points, describe the At each of these points, describe the acceleration acceleration and the and the velocity velocity of the baseballof the baseball

• Will the ballWill the ball’’s motion be periodic?s motion be periodic?

Temperature is a measure of Temperature is a measure of energyenergy

• Temperature measures average kinetic Temperature measures average kinetic energy of all the particles in a regionenergy of all the particles in a region

Lower T Higher T

Temperature scales: Kelvin, Temperature scales: Kelvin, Celsius, FahrenheitCelsius, Fahrenheit

Thermal energyThermal energy

• Thermal energy = N k T, N = no. of Thermal energy = N k T, N = no. of particlesparticles

• k = Boltzmannk = Boltzmann’’s const. = 1.4 x 10s const. = 1.4 x 10-23-23 J / J / deg Kdeg K

Lower thermal energy Higher thermal energy

Thermal energy is a measure of the total Thermal energy is a measure of the total kinetic energy of all the particles in a kinetic energy of all the particles in a substance. substance.

It therefore depends both on temperature AND It therefore depends both on temperature AND densitydensity

Difference between Difference between temperature and heat flowtemperature and heat flow

• Heat flow: rate of spontaneous transfer of Heat flow: rate of spontaneous transfer of thermal energy from a higher T system to a lower thermal energy from a higher T system to a lower T systemT system

• Heat can flow via conduction, convection, or Heat can flow via conduction, convection, or radiationradiation

• Example from Bennett: much faster heat flow if Example from Bennett: much faster heat flow if you stick your hand in boiling water than if you you stick your hand in boiling water than if you stick your arm in a hot oven.stick your arm in a hot oven.

– Why?Why?

Other forms of energyOther forms of energy

• Energy in atoms and moleculesEnergy in atoms and molecules

• Radiative energyRadiative energy

• Mass energyMass energy

Atomic structure: energy in Atomic structure: energy in atoms and moleculesatoms and molecules

Energy in atoms (in this case, Energy in atoms (in this case, in the electrons surrounding in the electrons surrounding the nucleus)the nucleus)

Discrete energy levelsDiscrete energy levels

• Electrons Electrons inside atoms inside atoms can take on can take on only discrete only discrete energy levelsenergy levels

• Analogy to Analogy to ladder with ladder with specific stepsspecific steps

Energy levels in hydrogen Energy levels in hydrogen atomatom

1 eV = 1 electron volt = 1.6 x 10-19 joule

Radiative energyRadiative energy

• Energy carried by lightEnergy carried by light

• Atoms radiate light when their Atoms radiate light when their electrons make transitions from one electrons make transitions from one energy level to anotherenergy level to another

• Hot matter radiates light, transfers Hot matter radiates light, transfers heat to surrounding cooler matterheat to surrounding cooler matter

Mass energyMass energy

• Albert Einstein: E = m cAlbert Einstein: E = m c22

• Energy = mass x (speed of light)Energy = mass x (speed of light)22

• Examples where mass is actually converted Examples where mass is actually converted into other forms of energy:into other forms of energy:

– In core of Sun and hydrogen bombs (nuclear In core of Sun and hydrogen bombs (nuclear fusion)fusion)

– In nuclear reactors (nuclear fission)In nuclear reactors (nuclear fission)

• Example from Bennett: the mass-energy of a 1 Example from Bennett: the mass-energy of a 1 kg rock represents 7.5 x 10kg rock represents 7.5 x 1099 more energy than more energy than burning a barrel of oil!burning a barrel of oil!

Conservation of EnergyConservation of Energy

• Energy can be neither created nor Energy can be neither created nor destroyed.destroyed.

• It It cancan change form or be exchanged change form or be exchanged between objects.between objects.

• The total energy content of the The total energy content of the Universe was determined in the Big Universe was determined in the Big Bang and remains the same today.Bang and remains the same today.

What have we learned?What have we learned?

• Why do objects move at constant velocity if no force Why do objects move at constant velocity if no force acts on them?acts on them?

– Conservation of momentumConservation of momentum

• What keeps a planet rotating and orbiting the Sun?What keeps a planet rotating and orbiting the Sun?

– Conservation of angular momentumConservation of angular momentum

• Where do objects get their energy?Where do objects get their energy?

– Conservation of energy: energy cannot be created Conservation of energy: energy cannot be created or destroyed but only transformed from one type or destroyed but only transformed from one type to another. to another.

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

– Energy sources: heat flow, radiation flow (light), Energy sources: heat flow, radiation flow (light), nuclear fission and fusion, gravitational potential nuclear fission and fusion, gravitational potential energy, …energy, …

TidesTides

•TidesTides are due to the difference are due to the difference between the force of gravity on between the force of gravity on opposite sides of a planet or moonopposite sides of a planet or moon– Tides can have far-reaching effects Tides can have far-reaching effects on planets and their moonson planets and their moons

The physics behind tidesThe physics behind tides

• Gravitational force is strongest on side of the Gravitational force is strongest on side of the Earth closest to Moon, weakest on other sideEarth closest to Moon, weakest on other side

Strength of gravitational forces

How strong is tidal force?How strong is tidal force?

• Tidal forces fall off like 1 / rTidal forces fall off like 1 / r33

• ““RegularRegular”” gravitational force falls off like 1 / r gravitational force falls off like 1 / r22

x1 x2 Moon

distance r

F1 F2

Derivation of tidal force using Derivation of tidal force using calculuscalculus

x1 x2 Moon

distance r

F1 F2

What is the effect on the What is the effect on the Moon?Moon?On the Earth?On the Earth?

• Moon pulls backward on EarthMoon pulls backward on Earth’’s tidal bulge, slows s tidal bulge, slows rotation rate of Earth. Day gets longer (very slowly).rotation rate of Earth. Day gets longer (very slowly).

• Tidal bulge pulls Moon ahead in its orbit, makes it Tidal bulge pulls Moon ahead in its orbit, makes it spiral outwards away from Earth (very slowly)spiral outwards away from Earth (very slowly)

Force on Moon depends strongly Force on Moon depends strongly on distance between Earth and on distance between Earth and MoonMoon

• Tidal recession of Moon was very fast when Earth Tidal recession of Moon was very fast when Earth and Moon were close together; is slower nowand Moon were close together; is slower now

Lengthening of EarthLengthening of Earth’’s s dayday

• EarthEarth’’s day: Evidence from growth s day: Evidence from growth bands in fossil bivalve shells and coralsbands in fossil bivalve shells and corals– There were ~400 days per solar year about There were ~400 days per solar year about

350 million years ago. So an Earth-day was 350 million years ago. So an Earth-day was shorter.shorter.

• Historical records of eclipses imply day Historical records of eclipses imply day is slightly longer now than it was is slightly longer now than it was ~2000 years ago~2000 years ago

Tidal origin of MoonTidal origin of Moon’’s s synchronous rotationsynchronous rotation

• Just as tides on Earth slow EarthJust as tides on Earth slow Earth’’s day, s day, tides on Moon slow Moontides on Moon slow Moon’’s rotation rates rotation rate– Yes, rock bulges a bit, forming Yes, rock bulges a bit, forming ““tidestides”” on on

MoonMoon

• MoonMoon’’s s ““dayday”” slowed down so much that slowed down so much that now it only rotates once a month now it only rotates once a month – Called Synchronous RotationCalled Synchronous Rotation

• Once that happened, MoonOnce that happened, Moon’’s tidal bulge s tidal bulge always pointed toward Earth, so Moonalways pointed toward Earth, so Moon’’s s day wonday won’’t slow down still moret slow down still more

Synchronous rotation Synchronous rotation elsewhere in Solar System: elsewhere in Solar System: Pluto and CharonPluto and Charon

• Pluto-Charon: Pluto-Charon: Each spins on its Each spins on its axis in same axis in same length of time length of time they orbit around they orbit around each othereach other– Same hemisphere Same hemisphere

of Pluto always of Pluto always faces Charonfaces Charon

– Same hemisphere Same hemisphere of Charon always of Charon always faces Plutofaces Pluto

Tidal forces elsewhere in Tidal forces elsewhere in Solar SystemSolar System

• Most inner moons of giant planets Most inner moons of giant planets rotate synchronouslyrotate synchronously

• MercuryMercury’’s rotation was slowed by tides s rotation was slowed by tides from Sunfrom Sun– Now after two Mercury orbits around Sun, Now after two Mercury orbits around Sun,

planet has rotated on its axis three timesplanet has rotated on its axis three times– Called an Called an ““orbital resonanceorbital resonance””

Tidal heating: Io is best Tidal heating: Io is best exampleexample

• Io is JupiterIo is Jupiter’’s closest moons closest moon

• IoIo’’s orbit is kept non-circular by s orbit is kept non-circular by Europa, another of JupiterEuropa, another of Jupiter’’s s moonsmoons

• Continued flexing and bulging Continued flexing and bulging produces internal motions of Ioproduces internal motions of Io’’s s rocks, friction, internal heatingrocks, friction, internal heating

• Hot enough inside Io to melt Hot enough inside Io to melt rock, form molten lavarock, form molten lava

• Erupts to surface in > 200 Erupts to surface in > 200 volcanoesvolcanoes

• Total heat flow several trillion Total heat flow several trillion watts (!)watts (!)


• Science fiction stories like to describe what would Science fiction stories like to describe what would happen to you if your space ship accidentally happen to you if your space ship accidentally came too close to a black holecame too close to a black hole– For the purposes of this question, consider a black hole a For the purposes of this question, consider a black hole a

very small region of space where gravity is extremely very small region of space where gravity is extremely intense. intense.

• If your space ship flies too close to the black hole, If your space ship flies too close to the black hole, it is stated that it is stated that ““youyou’’ll be torn apart by tidal forcesll be torn apart by tidal forces””

• Draw a stick-figure of yourself, and show in a Draw a stick-figure of yourself, and show in a diagram how these diagram how these ““tidal forcestidal forces”” might might ““tear you tear you apart.apart.”” Show the differences in strength of the Show the differences in strength of the gravitational force on the different parts of your gravitational force on the different parts of your body. body.

The Scientific MethodThe Scientific Method

• What is a scientific theory?What is a scientific theory?

• How can we distinguish science from How can we distinguish science from non-science?non-science?

What is a scientific theory?What is a scientific theory?

• The word The word ““theorytheory”” has a somewhat different has a somewhat different meaning in science than in everyday life.meaning in science than in everyday life.

• A A scientific theoryscientific theory must: must:— Explain a wide variety of observations with a few simple Explain a wide variety of observations with a few simple

principles, principles, — Be supported by a large, compelling body of evidence,Be supported by a large, compelling body of evidence,— Must not have failed crucial tests of its validity,Must not have failed crucial tests of its validity,— Must be amenable to modification if new data require this.Must be amenable to modification if new data require this.

• NewtonNewton’’s laws of gravitation are a good examples laws of gravitation are a good example– They explain a wide body of observations, have lots of evidence, They explain a wide body of observations, have lots of evidence,

but under some (very unusual) circumstances they require but under some (very unusual) circumstances they require modification.modification.

– Near black holes and neutron stars, gravity is so strong that Near black holes and neutron stars, gravity is so strong that EinsteinEinstein’’s theory of General Relativity applies, instead of s theory of General Relativity applies, instead of NewtonNewton’’s laws.s laws.

How can we distinguish How can we distinguish science from non-science from non-science?science?

• Defining science can be surprisingly Defining science can be surprisingly difficult.difficult.

• ScienceScience from the Latin from the Latin scientiascientia, , meaning meaning ““knowledge.knowledge.””

• But not all knowledge comes from But not all knowledge comes from science… science…

The idealized The idealized scientific methodscientific method

• Based on Based on proposing and proposing and testing testing hypotheseshypotheses

• Hypothesis = Hypothesis = educated guesseducated guess

But science doesnBut science doesn’’t always t always proceed in this idealized wayproceed in this idealized way

• Sometimes we start by Sometimes we start by ““just lookingjust looking”” and and then coming up with possible then coming up with possible explanations.explanations.

• Sometimes we follow our intuition rather Sometimes we follow our intuition rather than a particular line of hard evidence.than a particular line of hard evidence.

• There are frequently several blind alleys There are frequently several blind alleys that donthat don’’t work out, before a successful t work out, before a successful theory is developed and tested.theory is developed and tested.

Hallmarks of scienceHallmarks of science

• Useful criteria Useful criteria to decide to decide whether an whether an argument is argument is scientific or notscientific or not

Hallmarks of Science: #1Hallmarks of Science: #1

• In ancient times, actions of the gods In ancient times, actions of the gods were invoked as explanations for were invoked as explanations for things that were hard to understandthings that were hard to understand

• But modern science seeks But modern science seeks explanations for observed phenomena explanations for observed phenomena that rely solely on natural causesthat rely solely on natural causes

• There are other kinds of explanations, There are other kinds of explanations, but they donbut they don’’t come under the t come under the heading heading ““sciencescience””, but rather are , but rather are different kinds of discussionsdifferent kinds of discussions


• Science progresses through the Science progresses through the creation and testing of models of creation and testing of models of nature that explain the observations as nature that explain the observations as simply as possible.simply as possible.

• Example: By early 1600s, there were Example: By early 1600s, there were several competing models of planetary several competing models of planetary motion (Ptolemy, Copernicus, Kepler, motion (Ptolemy, Copernicus, Kepler, …) Kepler…) Kepler’’s gained acceptance because s gained acceptance because it worked the best.it worked the best.


• A scientific model should make testable A scientific model should make testable predictions about natural phenomena.predictions about natural phenomena.

• If subsequent tests donIf subsequent tests don’’t agree with the t agree with the predictions, a scientist would be willing predictions, a scientist would be willing (even eager) to revise or even abandon (even eager) to revise or even abandon his/her model.his/her model.

• If someone, in the face of data that If someone, in the face of data that contradict his/her model, isncontradict his/her model, isn’’t willing to t willing to revise or abandon it, they are not using the revise or abandon it, they are not using the scientific method.scientific method.

Issues for Planetary ScienceIssues for Planetary Science

• Planets and their moons are hugely Planets and their moons are hugely variedvaried

• For example: We arenFor example: We aren’’t advanced t advanced enough to have an enough to have an a prioria priori theory that theory that would predict what a newly discovered would predict what a newly discovered moon of Jupiter or Saturn should be likemoon of Jupiter or Saturn should be like

• ““RetrodictionRetrodiction”” or or ““postdictionpostdiction”” rather rather than than ““predictionprediction””– Try to understand new observations using Try to understand new observations using

overarching principles based on previous overarching principles based on previous body of databody of data

Scientific Method: Main PointsScientific Method: Main Points

• How can we distinguish science from How can we distinguish science from non-science?non-science?– Science: seeks explanations that rely Science: seeks explanations that rely

solely on natural causes; progresses solely on natural causes; progresses through the creation and testing of through the creation and testing of models of nature; models must make models of nature; models must make testable predictionstestable predictions

• What is a scientific theory?What is a scientific theory?– A model that explains a wide variety of A model that explains a wide variety of

observations in terms of a few general observations in terms of a few general principles and that has survived repeated principles and that has survived repeated and varied testingand varied testing

What about astrology?What about astrology?

• How is astrology different from How is astrology different from astronomy?astronomy?

• Is astrology a scientific theory?Is astrology a scientific theory?

• Does astrology have scientific validity?Does astrology have scientific validity?

Astrology asks a different type of Astrology asks a different type of question than astronomyquestion than astronomy

• AstronomyAstronomy is a science focused on is a science focused on learning about how stars, planets, and learning about how stars, planets, and other celestial objects work.other celestial objects work.

• AstrologyAstrology is a search for ( hidden) is a search for ( hidden) influences influences on human liveson human lives based on the based on the positions of planets and stars in the positions of planets and stars in the sky.sky.

Does astrology have scientific Does astrology have scientific validity?validity?

• In principle the stars might influence human In principle the stars might influence human affairs.affairs.

• How do we know whether they do or not?How do we know whether they do or not?

• Scientific tests consistently show that Scientific tests consistently show that astrological predictions are no more accurate astrological predictions are no more accurate than we should expect from pure chance.than we should expect from pure chance.

• Proponents of astrology say that the act of Proponents of astrology say that the act of doing controlled experiments ruins the doing controlled experiments ruins the ““auraaura”” and thatand that’’s why predictions arens why predictions aren’’t accurate when t accurate when tested in a lab environment.tested in a lab environment.

• In my opinion this means that astrology doesnIn my opinion this means that astrology doesn’’t t come under the heading come under the heading ““sciencescience””, since it can, since it can’’t t make testable predictions.make testable predictions.

What have we learned?What have we learned?

• A scientific theory should:A scientific theory should:

— Explain wide variety of observations with a few simple Explain wide variety of observations with a few simple principles, principles,

— Be supported by a large, compelling body of evidence,Be supported by a large, compelling body of evidence,

— Must not have failed crucial tests of its validity,Must not have failed crucial tests of its validity,

— Be amenable to modification if new data require this.Be amenable to modification if new data require this.

• Astrology Astrology

– Search for influences on human lives based on the Search for influences on human lives based on the positions of planets and starspositions of planets and stars

– Thus far scientific tests show that astrological Thus far scientific tests show that astrological predictions are no more accurate than we should expect predictions are no more accurate than we should expect from pure chancefrom pure chance

Page 1 Lecture 4: Momentum, Energy, Tides, and the Scientific Method Claire Max April 15 th, 2014...

Documents

Transcript of Page 1 Lecture 4: Momentum, Energy, Tides, and the Scientific Method Claire Max April 15 th, 2014...