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ASTRONOMY 161 Introduction to Solar System Astronomy

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Title: ASTRONOMY 161 Introduction to Solar System Astronomy


1
ASTRONOMY 161Introduction to Solar System
Astronomy
Class 7
2
A161 1st Midterm Results (last year)
E D C B A
3
Few midterm sample questions
  • On the first day of winter the sun rises
  • A) Due South
  • B) South of East
  • C) Due East
  • D) North of East
  • E) Due West

4
Few midterm sample questions cont
  • If you could observe Earth through a telescope
    from Mars, Earth would
  • A) always be in gibbous or full phase
  • B) always be in crescent phase
  • C) go through all phases from crescent to full
  • D) have largest apparent diameter when in full
    phase
  • E) appear at full phase when seen at opposition

5
Few midterm sample questions cont
  • Where is the full moon just after sunset?
  • (A) Near the Western horizon
  • (B) Nearly overhead
  • (C) Close to the North star
  • (D) Close to the South star
  • (E) Near the Eastern horizon

6
Newtons LawsWednesday, October 7
7
Few opening questions
  • 1) While driving a car, you throw a baseball in a
    direction perpendicular to your motion. What
    happens?
  • 2) Now you throw a baseball in the direction of
    your motion. What happens?
  • 3) While driving a car, you press the gas pedal.
    What happens?
  • 4) While driving a car, you press the brake. What
    happens?

8
Newtons Laws Key Concepts
  • Three Laws of Motion
  • (1) An object remains at rest, or moves in a
    straight line at constant speed, unless acted on
    by an outside force.
  • (2) The acceleration of an object is directly
    proportional to force, and inversely proportional
    to mass.
  • (3) For every action, there is an equal and
    opposite reaction.
  • Law of Gravity
  • (4) The gravitational force between masses M and
    m, separated by distance r, is

9
Isaac Newton (1643-1727) English
  • Discovered
  • three laws of motion,
  • one law of universal gravitation.

10
(No Transcript)
11
Newtons great book
  • Newtons laws are universal in scope, and
    mathematical in form.

12
(1) Newtons First Law of Motion
  • An object remains at rest, or moves in a straight
    line at constant speed, unless acted on by an
    outside force.
  • Precise mathematical laws require precise
    definitions of terms
  • SPEED rate at which an object changes its
    position. Example 65 miles/hour.
  • VELOCITY speed plus direction of travel.
    Example 65 miles/hour to the north.

13
Acceleration
  • rate at which an object changes its velocity.
  • Acceleration can involve
  • increase in speed
  • (2) decrease in speed
  • OR
  • (3) change in direction.

14
Force
  • a push or pull acting on an object.
  • Examples
  • gravity pull
  • electrostatic attraction pull
  • electrostatic repulsion push

15
(2) Newtons Second Law of Motion
  • The acceleration of an object is directly
    proportional to the force acting on it, and
    inversely proportional to its mass.
  • In mathematical form
  • Or alternatively

16
Example of Newtons Second Law
  • A package of cookies has mass
    m 0.454 kilograms,
  • And experiences gravitational acceleration
    g 9.8 meters/second2
  • How large is the force acting on the cookies?

17
(3) Newtons Third Law of Motion
  • For every action, there is
    an equal and opposite reaction.
  • Whenever A exerts a force on B, B exerts a force
    on A thats equal in size and opposite in
    direction.
  • All forces come in pairs.

18
Example of Newtons Third Law
  • Cookies push on hand F 1 pound, downward.
  • Hand pushes on cookies F 1 pound, upward.
  • Remove hand!
  • Earth pulls on cookies F 1 pound, downward.
  • Cookies pull on earth F 1 pound, upward.

19
  • THIRD Law states
    force on Earth force on cookies
  • SECOND Law states
    acceleration force divided by mass
  • Mass of Earth 1025 x mass of cookies
  • Therefore, acceleration of cookies
    1025 x acceleration of Earth.
  • (Cookies reach a high speed while the Earth
    hardly budges.)

20
  • Butwhy do the cookies and the Earth exert a
    force on each other?
  • Newtons Law of Gravity states that gravity is an
    attractive force acting between ALL pairs of
    massive objects.
  • Gravity depends on
  • (1) MASSES of the two objects,
  • (2) DISTANCES between the objects.

21
(4) Newtons Law of Gravity
  • The gravitational force between two objects
  • F gravitational force
  • M mass of one object
  • m mass of the second object
  • r distance between centers of objects
  • G universal constant of gravitation

22
  • Gravitational force varies
    directly with mass and
    inversely with square of distance.
  • Double the distance between objects
    Force 1/4 as large.
  • Triple the distance between objects
    Force 1/9 as large.

23
Example What is gravitational force between
Earth and cookies?
24
Example Encore What is acceleration of
cookies?
25
  • Newtons question can GRAVITY be the force
    keeping the Moon in its orbit?
  • Newtons approximation Moon is on a circular
    orbit.
  • Even if its orbit were perfectly circular, the
    Moon would still be accelerated.

26
  • The Moons orbital speed
  • radius of orbit r 3.8 x 108 m
  • circumference of orbit 2pr 2.4 x 109 m
  • orbital period P 27.3 days 2.4 x 106 sec
  • orbital speed
  • v (2pr)/P 103 m/sec 1 km/sec!

27
Acceleration required to
keep Moon on a circular orbit
28
Acceleration provided by gravity
29
Bottom Line
  • If gravity goes as one over the square of the
    distance,
  • Then it provides the right acceleration to keep
    the Moon on its orbit (to keep it falling).
  • Triumph for Newton!!

30
Few closing questions
  • 1) Imagine we move Earth twice as far away from
    the Sun. How long would be the year on Earth?
  • 2) How many times faster or slower would Earth
    move in its new orbit?
  • 3) How long would be the day on such an Earth?
  • 4) Imagine you decrease the mass of Earth by a
    factor of two. What would be a mass of a kilogram
    of cookies there?
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