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 11
Will 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
electron
nucleus
p+n
proton
neutron
e-
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
2
E=mc
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 pull
Mass = 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 MASS
Every object attracts every other object by
the force of gravity.
• More Mass
stronger gravity
• Larger Distance
weaker gravity
Fgravity = G M m / D2
Force
Number
For units
Mass 1
mass 2
Distance
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 Universe
Given 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
• Universal
all motion, terrestrial as well as celestial
• Accurate predictions
• Aesthetically pleasing
simple, few assumptions, explains lots
Application: Masses
• From a=F/M2 and F=GM1M2/D2
get Newton’s version of Kepler’s 3rd law
4 D
V P
M1  M 2 

2
G P
2G
2
3
3
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-time
Warped 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)