Transcript B) NO

Lecture 7
2012 Sept. 13
• Tides
• Energy: Potential and Kinetic
• Momentum
• Angular Momentum
The Planets
Prof. Geoff Marcy
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Education Inc.,
Announcements
• Chapter 4 Assignment due tomorrow.
• Read Chapter 5: Light!
• Homework: MasteringAstronomy
Assignment Chapt. 5
Last time:
1. Newton’s three laws
2. Elliptical orbits of planets
Today:
1. Tides (Why is do we always see the
same side of the moon?)
2. Conservation laws (Why useful?)
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Tides
Daily oscillations of
the ocean surface level
Two high tides and two low tides
per day, one every 12hr 50 min.
Hiking along California’s coast
Today’s tide chart
Note: two low tides per day; Happen with moonrise and moonset.
A monthly tides gauge reading
Questions: Why two high tides per day? Why one every 12:50 hours?
Why does the amplitude change over the course of a month?
Gravity field causes two tidal bulges
Gravity field causes two tidal bulges
Tidal friction lead to synchronous
rotation.
•Moon is facing the earth the same way.
•Pluto and moon Charon rotate
synchronously around each other.
Tides vary with the phase of the Moon
because the sun interacts also
Spring tide: HIGH because sun and moon act together during new and full moon
Tides vary with the phase of the Moon
because the sun interacts also
Spring tide: HIGH because sun and moon act together during new and full moon
Neap tide: LOW because
sun and moon’s gravity
counter-act
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The next new moon will be on
th
Sept. 15 . What time will the low
tide occur in the bay area?
(pick closest time)
A. 6 am
B. Noon
C. 6 pm
D. Midnight
Real tides are more complicated
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publishing as Addison-Wesley
Real tides are more complicated
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publishing as Addison-Wesley
Tidal friction…
• Tidal friction gradually slows Earth rotation, 1 s every 50,000
years (and makes Moon get farther from Earth).
• Earth’s + moon’s combined angular momentum is conserved.
Earth is losing angular momentum, the moon gains it.
• Moon once rotated faster (or slower); tidal friction caused it to
“lock” in synchronous rotation.
Synchronous Rotation
• …is when the rotation period of a moon,
planet, or star equals its orbital period about
another object.
• Tidal friction on the Moon (caused by Earth)
has slowed its rotation down to a period of
one month.
• The Moon now rotates synchronously.
– We always see the same side of the Moon.
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Last time: How is mass different from weight?
• mass – the amount of matter in an object
• weight – the force that acts upon an object
You are weightless in
free-fall!
You can’t tell the
difference between:
- Free fall in gravity
- No gravity at all
Tennis on the moon
Would you hear Maria Sharapova grunting on the moon?
A) YES
B) NO
Tennis on the moon
Would you hear the players’ noises on the moon?
A) YES
B) NO
The gravity field on the moon is 6 times weaker than on Earth.
Could one still play tennis there?
A) YES
B) NO
Tennis on the moon
Would you hear the players’ noises on the moon?
A) YES
B) NO
The gravity field on the moon is 6 times weaker than on Earth.
Could one still play tennis there?
A) YES
B) NO
but balls would bounce much higher.
Could I ‘fix’ that by making the balls 6 times as heavy?
A) YES
B) NO
Tennis on the moon
Would you hear the players’ noises on the moon?
A) YES
B) NO
The gravity field on the moon is 6 times weaker than on Earth.
Could one still play tennis there?
A) YES
B) NO
but balls would bound much higher.
Could I ‘fix’ that by making the balls 6 times as heavy?
A) YES
B) NO
No that would only make it harder to accelerate them.
All objects fall at the same rate!
Why are astronauts weightless in space?
• There IS gravity in space…
• weightlessness is due to a constant state of free-fall:
Four conservation laws
• mass (for chemists only)
• energy
• momentum,
• angular momentum,
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Conservation of Mass
2H2+O2  2 H2O
Energy released ΔE= 483.6 kJ/mol
1 mol = 6.021023 particles
(A) YES
(B) NO
For all practical
purposes in chemistry:
Yes but …
Mass is a form of Energy: E = mc2
Conservation of Mass
2H2+O2  2 H2O
Energy released ΔE= 483.6 kJ/mol
1 mol = 6.021023 particles
(A) YES
(B) NO
No, because there is a tiny
difference because the
atom are now is state of
lower energy.
Δm=ΔE / c2
Negligible for practical
purposes but measurable.
How do we produce energy?
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Basic Types of Energy
•
•
•
•
•
•
Kinetic (motion)
Potential (gravitational)
Thermal (heat)
Chemical energy (bonds)
Nuclear energy (bonds)
Light
Mass-Energy can change
type, but cannot be
destroyed.
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Kinetic Energy
Potential Energy
Converted to
Kinetic Energy
Potential Energy
Potential Energy = mass * 9.8 m/s2 * height = mgh
Converting:
Potential Energy to Kinetic Energy
Kinetic Energy
Converted to Heat
Potential Energy
Converted to Kinetic Energy
Thermal Energy
of the motion of atoms and molecules
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Thermal Energy:
Kinetic Energy of the molecules
Potential Energy (in battery)
Converted to Electrical Energy
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Conservation of Energy
• Energy can make matter move.
• Energy is conserved, but it can:
– Transfer from one object to another
– Change in form
• Note: Energy is conserved within a definite
system. In an open system, it can be exchanged
with the environment
Conservation of Momentum
Definition of momentum: p = mass × velocity
unit: kg m / s
Conservation of momentum: If no external force is applied, the total
momentum of system is conserved.
Useful:
For all sorts of collisions
Applies also to atoms and molecules
Not useful: If friction is involved.
Conservation of Energy:
Potential + Kinetic = Constant with time
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Collisions of Balls on a Pool Table
Interactive Quiz: Collisions Balls on a Pool Table
A
8
B
C
Which shot will get a black ball
in the corner pocket?
A)
B)
C)
D) This is not possible.
Interactive Quiz: Collisions Balls on a Pool Table
A
B
C
Which shot will get a black ball
in the corner pocket?
A)
B)
C)
D) This is not possible.
Interactive Quiz: Collisions Balls on a Pool Table
A
B
C
Which shot will get a black ball
in the corner pocket?
A)
B)
C)
D) This is not possible.
Interactive Quiz: Collisions Balls on a Pool Table
A
B
C
Which shot will get a black ball
in the corner pocket?
A)
B)
C)
D) This is not possible.
Interactive Quiz: Collisions of two trucks
Two trucks of equal mass on an icy road:
Before collision: Truck 1 is at rest.
Truck 2 approached with 40 km/h.
After the collision: Both trucks are damaged and stick together
What is their final velocity?
A) 10 km/h
B) 20 km/h
C) 40 km/h
D) This is not possible, conservation of
momentum prevents them from sticking
Interactive Quiz: Collisions of two trucks
Two trucks of equal mass on an icy road:
Before collision: Truck 1 is at rest.
Truck 2 approached with 40 km/h.
Truck 1:
p1=M* 0 km/h
Truck 2:
p2=M* 40 km/h
After collision: p=(2M)*vfinal
Momentum conservation:
p
= p1+p2
(2M)*vfinal = M*40 km/h
vfinal
= 20km/h
Is energy conserved for this collision process?
Interactive Quiz: Collisions of two trucks
Two trucks of equal mass on an icy road:
Before collision: Truck 1 is at rest.
Truck 2 approached with 40 km/h.
Truck 1: p1=M* 0 km/h
E1=½ M v2=0
Truck 2: p2=M* 40 km/h
E2=½ M (40km/h)2 =800M (km/h)2
After collision: p=(2M)*vfinal
Momentum conservation:
p
= p1+p2
(2M)*vfinal = M*40 km/h
vfinal
= 20km/h
E = ½ (2M) (20km/h)2=400M (km/h)2
Conservation of Angular Momentum
Definition of angular momentum
L = m v ´r = p´r
unit: kg m / s
Conservation of angular momentum: In a central field like the sun’s
gravity, the angular momentum of orbiting objects is conserved.
Planets and comets in the sun’s gravity field
Applies to rotating objects.
Not useful: If friction is involved.
Useful:
Changes in length of day
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Rotating Chair Demo
L=m v ´r
Before: Radius large
Angular velocity small
After: Radius small
Angular velocity large
Changes in length of day
Winter-summer atmosphere flow, storms
Motion in liquid iron core
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Postglacial rebound
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A Planet in a Elliptical Orbit
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What have we learned and seen today
1.Conservation of momentum, angular
momentum, and energy
2.What causes tides?
3.Why are the two tidal bulges?
4.Why do we always see the same side
of the moon?
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publishing as Addison-Wesley