Download Friday Feb 6

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Phys 1830: Lecture 13
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The Vela Cloud
J. English
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Coming up:
Tides
General Relativity
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Previous Classes:
Workshop on image-making
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Doppler Shift
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Velocity Fields
This Class
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Motion
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Angular Momentum
Newton’s Laws
Gravity
Quiz
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Radio Arrays can generate spectral line
cubes.
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Saw 3D cube with
x+y spatial position
and z axes 
velocity.
Velocity field of
NGC 3556 using HI
21 cm emission.
• The 21 cm line is particularly useful
since radio arrays make images per
channel. Each channel represents a
narrow velocity range.
• Professional astronomy tools like the
Karma suite help astronomers interpret
these 3D cubes.
Example of another velocity field: The Vela
Cloud
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• The galaxies rotation can be seen on
right.
• Note that the cloud doesn’t have
any distinct flows.
Constructing Colour Images:
The W4 Chimney Region in the Milky Way summary
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• Radio Continuum data is coloured to
distinguish thermal plasma and synchrotron
emission.
• HI radio data is coloured to show the motion
in the gas. Blue is assigned to blueshifted gas
 rotation of the Milky Way.
Another example of velocity field:
English & Taylorsummary
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• H I gas in our Milky Way which could effect our
understanding of the Cosmic Microwave
Background Radiation by absorbing
microwaves.
• The animation shows that this gas doesn’t
have signatures of flow or rotation. Its motion
is turbulent.
Seeing Gravity: Laws of Motion
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Motion
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Angular Momentum
• Defined as the tendency of a body to
keep spinning (i.e. rotating) or
moving in a circle (i.e. revolving).
Motion
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• Must be conserved within a system.
– person on spinning chair brings
in their arms
– Bicycle wheel  transfer of
angular momentum
– Flashlight shows how an object
can beam light when spinning.
Newton’s Laws
Newton’s First Law: The law of inertia.
An object at rest will remain at rest, and an object moving in a straight line at
constant speed will not change its motion, unless an external force acts on it.
Newton’s First Law: The law of inertia
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• Demo with cart – ignoring friction:
1) Cart at rest needs a force to make it
move.
2) Cart in motion needs a force to make
it stop.
Newton’s First Law: The law of inertia
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• Inertia is the tendency of an object
to keep moving at the same speed
and in the same direction.
• Definitions:
– Velocity has
• Amplitude (speed)
• Direction
– Acceleration
• Change in amplitude or direction.
Newton’s Second Law: Acceleration
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F == force
m == mass
a == acceleration
• When a force is exerted on an
object, its acceleration is inversely
proportional to its mass:
• a = F/m
Question:
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• Demo:
– Ball on a string. The Force is the
tension on the string.
– If there is no string what
happens?
a) The ball drops instantly straight to
the ground.
b) The ball continues in the circle for
a while.
c) The ball travels in a straight line in
the direction it was already
moving.
Newton’s Second Law: Acceleration
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F == force
m == mass
a == acceleration
• When a force is exerted on an
object, its acceleration is inversely
proportional to its mass:
• a = F/m
Newton’s Second Law: Acceleration
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• Demo:
• Cart: starting from rest , what do we do
to change its speed.
- Constant F  constant a.
- Greater F  greater a.
- increase mass  decrease a.
- decrease mass  increase a.
Newton’s Third Law: Equal and Opposite Reaction
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• When object A exerts a force on
object B, object B exerts an equal
and opposite force on object A.
Demo:
a)Container with water and coin to drop in it.
b)2 carts push equally on each other.
c)Balloon on straw on string. Balloon goes in opposite direction
to airflow.
a) Elastic collapse causes a force that pushes air out of
balloon nozzle.
b) Air pushes back with equal force on elastic causes
balloon to move.
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Review Questions:
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• With your neighbours, see if you can
list Newton’s 3 laws of motion. (Hint:
use a word or a few to describe each.)
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Motion:
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• Angular Momentum
• Newton’s Laws
1. Inertia
2. Acceleration
3. Equal and opposite reaction
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Review Question:
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Fill in the blank:
• According to Newton's second law,
when the same force acts on two
bodies, the body with the larger
mass will have the ________
acceleration.
a) Larger
b) Smaller
c) Same