due at the beginning of class in person, or at the end of...

Announcements:

•  Homeworks •  Due at the BEGINNING of class in person, or at

the end of class in Kate’s box, for 100% credit •  Kate’s Box: EMS room A234 •  Labeled “Dallas” “Astro 3” •  M-F 9-12, 1-5

•  After this time til end of next day: 50% credit •  Exams

•  You are allowed to bring in one 8 ½ x 11” sheet of paper with whatever you want written on it

© 2005 Pearson Education Inc., publishing as Addison-Wesley

4.4 The Force of Gravity

Our goals for learning:•What determines the strength of gravity?

•How does Newton’s law of gravity extend Kepler’s laws?

•How do gravity and energy together allow us to understand orbits?

•How does gravity cause tides?


What determines the strength of gravity?

The Universal Law of Gravitation1. 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..


How does Newton’s law of gravity extend Kepler’s laws?

• Ellipses are not the only orbital paths. Orbits can be:– bound (ellipses)– unbound

• Parabola• hyperbola

• Kepler’s first two laws apply to all orbiting objects, not just planets


• Newton generalized Kepler’s Third Law:Newton’s version of Kepler’s Third Law:If a small object orbits a larger one and you measure the orbiting object’s

orbital period AND average orbital distanceTHEN you can calculate the mass of the larger object.

Examples:• Calculate mass of Sun from Earth’s orbital period (1 year) and average distance (1 AU).• Calculate mass of Earth from orbital period and distance of a satellite.• Calculate mass of Jupiter from orbital period and distance of one of its moons.


Newton’s version of Kepler’s Third Law

p = orbital perioda=average orbital distance (between centers)(M1 + M2) = sum of object masses

p2= 4π2G(M1+M2)a3


How do gravity and energy together explain orbits?

• Orbits cannot change spontaneously.• An object’s orbit can only change if it somehow

gains or loses orbital energy =kinetic energy + gravitational potential energy

(due to orbit).


⇒ So what can make an object gain or lose orbital energy?

• Friction or atmospheric drag• A gravitational encounter.


• If an object gains enough orbital energy, it may escape (change from a bound to unbound orbit)

•escape velocity from Earth ≈ 11 km/s from sea level (about 40,000 km/hr)


How does gravity cause tides?


Tides vary with the phase of the Moon:


Special Topic: Why does the Moon always show the same face to Earth?

Moon rotates in the same amount of time that it orbits… But why?


Tidal friction…

• Tidal friction gradually slows Earth rotation (and makes Moon get farther from Earth).

• Moon once orbited faster (or slower); tidal friction caused it to “lock” in synchronous rotation.


Scientific Thinking

• It is a natural part of human behavior.• We draw conclusions based on our experiences.• Progress is made through “trial and error.”

3.4 The Nature of Science

• How can we distinguish science from nonscience?

• What is a scientific theory?

Our goals for learning:

How can we distinguish science from non-science?

• Defining science can be surprisingly difficult.• Science from the Latin scientia, meaning “knowledge.”• But not all knowledge comes from science…

The idealized scientific method

• Based on proposing and testing hypotheses

• hypothesis = educated guess


The Scientific Method1 Question2 Hypothesis

– a tentative explanation3 Prediction4 Test5 Result

– confirm, reject, or modify

should be the same no matter who conducts the test

But science rarely proceeds in this idealized way… For example:

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

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

Hallmarks of Science: #1

Modern science seeks explanations for observed phenomena that rely solely on natural causes.

(A scientific model cannot include divine intervention)


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

(Simplicity = “Occam’s razor”)


A scientific model must make testable predictions about natural phenomena that would force us to revise or abandon the model if the predictions do not agree with observations.

(a good theory is “falsifiable”)

What is a scientific theory?

• The word theory has a different meaning in science than in everyday life.

• In science, a theory is NOT the same as a hypothesis, rather:

• A scientific theory must:Explain a wide variety of observations with a few simple principles, ANDMust be supported by a large, compelling body of evidence.Must NOT have failed any crucial test of its validity.


Astrology• claims to study how the positions of the Sun,

Moon, & planets among the stars influence human behavior

• was the driving force which advanced ancient astronomy

• Kepler & Galileo were the last astronomers to cast horoscopes… since then astronomy grew apart from astrology into a modern science

• modern scientific tests of astrology fail …it is a pseudoscience

Theories that have stood the test of time and evidence

• Evolution• The big bang• Special and general relativity• Quantum mechanics• Plate tectonics

Dozens of theories you have never heard about were once respectable among scientists, but eventually discarded because data contradicted their predictions

5.1LightinEverydayLife•Whatisthedifferencebetweenenergyandpower?•Whatarethefourwaysinwhichlightandma@ercaninteractwitheachother?

Whatisthisaspectrumof?

Power•power:therateatwhichenergyisused/emi5ed•Itismeasuredinunitscalledwa5s.1wa@=1joulepersecond•A100wa@lightbulbradiates100joulesofenergyeverysecond.

[email protected]–[email protected]–[email protected]–[email protected]–ma@errepelslightinanotherdirecNon

5.2ProperNesofLight•Inwhatwayislightawave?•InwhatwayislightmadeofparNcles?•Howarewavelength,frequency,andthespeedoflightrelated?•Howarewavelengthandenergyrelatedforphotonsoflight?

Light:AvibraNoninanelectromagneNcfieldthroughwhichenergyistransported.Lightasawavefλ=c

f=frequency,in#/sλ=wavelength,inm/sc=speedoflight,=3x108m/s

SingleSlitDiffracNon

LightasaParNcle•Lightcanalsobetreatedasphotons–packetsofenergy.•Theenergycarriedbyeachphotondependsonitsfrequency(color)E=hf=hc/λ[“h”iscalledPlanck’sConstant]•Shorterwavelengthlightcarriesmoreenergyperphoton–therefore,bluerlightcarriesmoreenergyperphoton.

5.4LightandMa@erOurgoalsforlearning:•HowcanweuseemissionorabsorpNonlinestodeterminethecomposiNonofadistantobject?•Arethereanymaterialobjectsthatdon’tgiveoffanylight?•WhatarethetworulesofthermalradiaNon?

Phasesofma@erstronglydependontemperature

due at the beginning of class in person, or at the end of...

Documents