Announcements

Announcements

• 50 students have still not registered and joined our class on Astronomy Place.

• The assignments for Wednesday is the tutorial “Orbits & Kepler’s Laws” on Astronomy Place. You need to register and join our class if you want to get credit for it.

• Remember - assignments, homework, quizzes and in-class activities count for 25% of your grade. They are mostly easy points.

More Announcements

• Quiz and Review - Monday Feb 9

• 1st Mid-Term Exam - Wednesday Feb 11Will cover chapters 1-6

Objectives: Energy & Motion(from syllabus)

• State the nature of Energy

• Describe the different types of energy

• Describe and apply the concept of conservation of energy

• Describe the structure of an atom

• Describe the relation between temperature and motion

• Identify concepts: velocity, acceleration, momentum, force, mass

Objectives (continued)

• Describe the cause-effect relation between force & motion (Newton’s law of motion)Illustrate it by concrete examples.

• Describe the relation between matter (mass), distance & gravity (Newton’s law of gravity)

• Apply the theories of motion & gravity to explain astronomical & everyday phenomena

• Use Newton’s version of Kepler’s 3rd law to determine the mass of astronomical bodies

What have we learned?

Matter and Energy:

Matter - material, composed of atoms, electrons, …

Energy – makes matter move, can do work!

Atom

nucleus electron

e-

proton

neutron

p+n

Three Basic Types of Energy

• kinetic– energy of motion

• potential– stored energy

• radiative– energy transported by light

Energy is CONSERVED!

Transferred, transformed, NOT created or destroyed

KE + PE + RE = total energy = constant

E=mc2

Mass and Energy are the same thing, measured in different units (e.g. kg & joules)hot pot has more mass than a cold one

you have more mass when running fast

Motion

• speed – rate at which an object moves, i.e. the distance traveled per unit time [m/s; mi/hr]

• velocity – an object’s speed in a certain direction, e.g. “10 m/s moving east”

• acceleration – a change in an object’s velocity, i.e. a change in either speed or direction is an acceleration [m/s2]

• momentum -- (mass x velocity), oomph

Newton’s Theory of Motion

Acceleration = Force / Mass

Force = push or pullMass = resistance to force (inertia),

depends on amount of matter, not size

Acceleration = change in velocity, speed up, slow down, turn

Memorize this equation

Orbital Motion

• Direction of planets motion continually changes (speed changes slightly for most and drastically for Mercury & Pluto)

Need a FORCE

• Planet’s motion is ACCELERATED(direction and speed change)Force is needed.Newton said FORCE is GRAVITY

Newton’s Theory of Gravity

• Source of Gravitational Force is MASSEvery object attracts every other object by the force of gravity.

• More Mass stronger gravity

• Larger Distance weaker gravity

Fgravity = G M m / D2

DistanceMass 1mass 2

NumberFor units

Force

Tests

• Know acceleration = F/m at Earth’s surface

• Know distance to Moon

• Calculate acceleration at Moon’s distance

• Compare to acceleration found from Moon’s period and distance

• Does this prove Newton’s theories of motion and gravity are correct?

Yes? / No?

Activity: orbital motion

Problems of Heliocentric Model

• Objects fall straight down

• Atmosphere and Moons stay with planets

Uranus & Neptune

• Laplace (1800) calculated Uranus orbit from effects on other planets

Its ONLY a THEORY

• Newton’s theories of motion and gravity were used to predict the locations of Uranus and Neptune which astronomers then looked and found with telescopes

• Newton’s theories of motion and gravity are used to plot the orbits of spacecraft, comets and asteroids.

• Newton’s theories of motion and gravity are used to determine the masses of astronomical bodies.

Philosophy

• Mechanical UniverseGiven knowledge of where everything

is and how it is moving now (and enough computer power) can calculate

everything that will happen in the future!

• Do you like this idea?

Appeal

• Universalall motion, terrestrial as well as

celestial

• Accurate predictions

• Aesthetically pleasingsimple, few assumptions, explains lots

Application: Masses

• From a=F/M2 and F=GM1M2/D2

get Newton’s version of Kepler’s 3rd law

€

M1 + M2 =4π 2

G

D3

P 2=V 3P

2πG

Don’t memorize this formula

If can measure period and either separation or speed of two astronomical bodies orbiting each other can determine the sum of their masses.

Is it correct?

• Discrepancies:– orbit of Mercury– high speeds (e.g. cyclotron)

• Improved Theory– Einstein’s theory of Motion & Gravity

(Relativity)Mass warps space-timeWarped space-time tells masses how to move

What is a FACT & what is a THEORY?

Think about it!

Assignment for Wednesday

• Wednesday - Chapter 6 Light (& matter)

• Assignment for Wednesday Feb. 4, tutorial“Orbits & Kepler’s Laws” on Astronomy Place To get credit you must both register & join our class

• Problems with Astronomy Place - clear browser cache & try again (instructions linked to course web page and on Angel)