The Moon

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The Moon
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Phases of the Moon
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Discussion
Why does the Moon have phases? How is this
different from a lunar eclipse?
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Discussion
If the Moon is full, approximately what time does
the Moon rise, cross the meridian, and set? On
this day the Sun rises at 600 AM, crosses the
meridian at noon, and sets at 600 PM.
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Discussion
If the Moon is new, approximately what time does
the Moon rise, cross the meridian, and set? On
this day the Sun rises at 600 AM, crosses the
meridian at noon, and sets at 600 PM.
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Lunar eclipse
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Discussion
What phase must the Moon be in for there to be a
lunar eclipse?
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Total Lunar eclipse of July 16, 2000
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Discussion
Why do you think the eclipsed Moon appears red?
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The Physics of motion
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Galileo
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Galileos Physics

• Inertia A moving object will stay in motion
  unless acted upon by a force.
• Rate of falling Gravity accelerates all
  objects, regardless of weight, by the same amount

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Speed
Example a car moving at 60 miles/hour
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Velocity
Velocity refers to not only how fast an object is
moving but its direction as well.
Example a car moving 60 miles/hour due west.
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Acceleration
Acceleration refers to any rate of change in the
velocity of an object. An acceleration can
mean a speeding up, a slowing down, or simply a
change in the direction of motion with no change
in speed.
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Units of acceleration
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Example
A car accelerates from a stop light at 10 m/sec2
following a straight path. So, at time t 0 the
cars speed is 0 m/sec. After one second of
acceleration, the cars speed is 10 m/sec
(velocity 10 m/sec south). After two seconds,
the cars speed is 20 m/sec.
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Discussion
Starting from rest, after one minute of
accelerating at 10 m/sec2 at what speed is the
car moving?
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Discussion
If I had two identical inclined planes placed so
that they faced each other and I rolled a ball
down one of the planes. How high up the second
inclined plane will the ball get before it stops
rolling with no friction? Explain.
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Discussion
What if the second inclined plane is replaced by
one which is half as steep as the first inclined
plane. How high will the ball reach on this
plane?
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Law of falling objects
In the absence of air resistance, all objects
fall with the same constant acceleration
regardless of the objects mass.
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Newton
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Newtons first law of motion
The Law of inertia
A body remains at rest or moves in a straight
line at a constant speed unless acted on by a net
external force. Inertia is the tendency of an
object to resist a change in velocity.
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Discussion
Using Newtons first law of motion why is it a
good idea to be wearing a seatbelt in case of an
car accident?
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Newtons second law of motion

• Force mass ? acceleration

If the same force is applied to an object with
half the mass, the acceleration of that object
will be twice as much.
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Discussion
Using Newtons 2nd law of motion, explain why you
can throw a baseball further than a shot-put.
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Discussion
If I pull on either side of the a pen as hard as
I can, what is the net force I exert on the pen?
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Discussion
Which will do more damage to your car. Hitting
a brick wall at 60 miles per hour which does
little damage to the brick wall. A head on
collision with another car traveling at 60 miles
per hour in the opposite direction with the same
mass such that both cars immediately come to
rest.
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Newtons third law of motion

• For any force there is always an equal and
• opposite reaction force.

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Example Walking
In order to walk, you have to push with your
foot back on the ground. The Earth pushes your
foot with an equal and opposite force. While
the forces are equal and opposite the response to
that force is not, because the masses are very
different.
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Discussion
If I put my car in neutral and try to push it
with a force F, according to Newtons third law
my car pushes back with the same force.
Therefore, the car should never move. Is Newton
wrong? Why or why not?
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Discussion
Your on a space walk outside the space station
using Nasas jetpack to move around when it
breaks down leaving you stranded 500 feet from
safety. All you have is a bag of tools. How do
you get back to the space station.
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Discussion
Consider an object in uniform circular motion.
That is, an object traveling in a circle with a
constant speed. Is there a force acting on this
object? Why or why not?
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Discussion
Consider an object in uniform circular motion.
That is, an object traveling in a circle with a
constant speed. How is the velocity of the
object changing and how must the force on the
object be directed to change its velocity in this
way?
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Discussion
Is there a force acting on the Moon? How can
you tell?
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Discussion
In firing a cannon, the projectile leaves the
muzzle at a certain velocity, say v0. I wish to
control the distance that the projectile will
travel. How do I accomplish this?
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Discussion
Newtons third law tells us that the force of the
Sun on the Earth is the same as the force of the
Earth on the Sun. Why then does the Earth orbit
the Sun instead of the other way around?
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The Sun is more Massive
The force of the Earth on the Sun is the same as
the force of the Sun on the Earth. But, the Sun
is 333,000 times more massive than the Earth.
From Newtons second law the Earth will be
accelerated 333,000 time more than the Sun.
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Orbits
Because of Newtons third law, it is not exactly
correct to say that the Earth orbits the Sun or
the Moon orbits the Earth. Instead, both
objects, the Earth and the Sun or the Moon and
the Earth, orbit a common point called the center
of mass.
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Discussion
If two stars with equal mass are held together by
gravity, describe their orbit.
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Newtons Universal Law of Gravity

• Every mass attracts every other mass through a
  force called gravity
• The force is directly proportional to the product
  of their masses
• The force is inversely proportional to the square
  of the distance between them

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Newtons law of gravity
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Discussion
Consider the gravitational force between two
objects with mass M1 and M2 separated by a
distance d. How would the gravitational force
change if the distance between them increases to
3 ? d. How will it change in the distance in
decreased to 0.1 ? d?
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Discussion
Suppose a new planet is discovered out in the
Kuiper belt. This planet has twice the mass of
the Earth but is also twice the size. Is the
surface gravity of this new planet greater than,
less than or the same as the surface gravity of
the Earth?
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Gravitational forces between spherical masses
d
The distance to use is the distance between the
two spheres centers.