Download Upcoming due dates

Upcoming due dates
Today:
• Planetarium show report from last week, if any
• Report on observing last Thursday, if you attended
Friday, Feb. 5: online assignment 2 (extra credit)
Monday, Feb. 8: test corrections, 1/2 pt extra credit for
each missed question corrected, up to 4 questions
I've already received several by email, but I won't be replying until
the 8th or 9th.
Quiz 1 papers still available for pickup here at front.
More dates
Wed., Feb. 10: Quiz 2
• Chapters 4 and 5 (motion, Newton’s laws, energy
light)
• Last 20 minutes of class, 20 multiple-choice questions
Mon., Feb. 15: optional tour of Ritter Observatory,
following the class session on telescopes
Mon. - Thurs., Feb. 22 - 26: next Brooks Observatory
telescope session
Horizontal motion combined with falling
Toss apple horizontally
No force acts on it after it leaves my hand except gravity
86
Therefore, constant speed in horizontal direction (ignore air
resistance)
Falling motion as if simply dropped
87
Fire a cannonball horizontally with greater and greater speed
...
88
• Time needed to reach ground same throughout (for short
distances)
• The faster it goes, the farther before hitting the ground
89
Curve of Earth’s surface: now indeed takes longer to hit
ground
90
Sufficient initial speed: never hits; circular orbit:
Circular speed
91
Initial speed slightly greater: orbit will be an ellipse
92
Initial speed great enough: coasts to rest at a great distance
from Earth. Orbit is a parabola.
Escape speed
Initial speed needed to reach parabolic (escape) orbit.
93
In parabolic orbit, the cannonball will coast to a stop at a
great distance (in the absence of any other gravitational
attraction) but will never return.
With an initial speed greater than escape speed, the orbit is a
hyperbola (similar shape); the cannonball keeps going
indefinitely; never comes to rest.
This is what it means to escape Earth’s gravity.
Actually, this phrase is misleading: Earth's gravity goes on indefinitely,
so it's not really possible to escape it.
All you can do is get so far away that some other object has a stronger
gravitational pull than the Earth - the Sun, for instance.
94
Summarize orbital motion
• Gravity is only force acting
• Two motions combined
– Free fall toward center of Earth
– Tangential motion (constant speed if orbit circular)
• Speed in orbit doesn’t depend on mass of satellite
• In absence of air resistance, object will stay in orbit
indefinitely
95
• To launch a satellite: get it above the atmosphere (60
miles or more); and give it a horizontal velocity equal to
circular speed (about 18,000 mph for Earth)
Energy
Energy is defined as the capacity to move an object from one
place to another, under the action of a force.
Some forms of energy:
Light Falling on matter, light can cause the matter to move.
Motion Energy of motion is also called kinetic energy. Its
amount depends on a moving object’s mass and speed.
Heat Properly called thermal energy. Really just the kinetic
energy of the moving particles that make up objects.
Raising the temperature of an object makes those particles
move faster.
97
Potential energy Also called stored energy.
Every process can be thought of as converting energy from
one form to another.
Stored chemical energy (or chemical potential energy)
• Stored in bonds between atoms in compounds, especially
in fuel
• Released when fuel burns
• Stored energy is converted into heat. In an engine, the
heat can be converted into directed energy of motion.
98
Stored gravitational energy (or gravitational potential energy)
When you drop an object, it
acquires kinetic energy as it falls.
Stored energy is being converted
to kinetic energy.
The higher above the ground it
starts, the more gravitational
potential energy it has.
99
Tossing an object upward, you give it some initial amount of
kinetic energy.
As it goes up, it slows down.
Kinetic energy is being converted
to stored gravitational energy.
As it starts to fall, the stored
energy is converted back to
energy of motion.
100
Conservation of energy
Energy cannot be created or destroyed, but only changed
from one form to another.
This is one of the most important physical laws.
If we think we find energy being created out of nothing, we
invent a new form of stored energy for it to be created from,
rather than give up this law.
With additional evidence, the new form of stored energy has
always turned out to make sense.
101
Orbits of the planets
Apply Newton’s Laws, with center of gravitation the Sun
• Planets fall toward Sun, while moving at a tangent
• Each moves with a velocity that is approximately equal to
circular, but not quite; hence elliptical orbits
• Slowing down when farther from Sun (Kepler’s 2nd law)
• Farther from Sun, Sun’s gravity is weaker
– Acceleration of gravity is smaller
– Circular velocity also; less speed needed to keep in
circular orbit
– Explains Kepler’s 3rd Law
If Sun’s gravity were turned o↵, planets would move in
straight line in direction in which going at that instant
103
Apply conservation of energy to planets’ orbits. Elliptical
orbit:
• Moves fastest when close to Sun
• Slows as it moves farther away or “rises” and kinetic
energy is converted to stored gravitational energy
• Converted back again as planet “falls” back toward Sun
Energy and the shape of the canonball’s orbit
Just the right amount of kinetic energy: circular orbit
104
More kinetic energy: elliptical orbit
105
Just enough energy to get entirely stored as gravitational
energy at a great distance: parabolic orbit or escape orbit
With even more energy, the orbit has the shape of a more
open curve called a hyperbola, and the cannonball never stops.
106
Why don’t we fall toward the center of the
Earth?
• The Earth’s surface pushes
up on us with a force that
is equal and opposite to
the force of gravity
• Result: zero acceleration
up or down
• This opposing force causes
the sensation of weight
107
The word “weight” thus has 2 legitimate meanings
1. The sensation of weight caused by the ground pressing up
on your feet. This force is what is measured by a scale
that uses a spring.
2. The force of gravity itself on an object, the usual meaning
of the word in physics classes.
Weightlessness = absence of the sensation of weight
(meaning 1)
108
“Weightlessness” experienced in orbit around the Earth is just
continual free fall, the same as experienced in an elevator (Fig. 4.3)
whose supporting cable has just been cut.
109
An astronaut in the Space Shuttle falls toward the Earth at
the same rate as the station.
• All falling objects experience the same acceleration under
gravity, regardless of mass.
• An astronaut outside the Shuttle will orbit right along
with it.
110
Interplanetary exploration missions
• Flyby
Can provide a gravity assist to boost a spacecraft toward
another planet.
111
By radio tracking the spacecraft, we learn exactly how
much the planet’s gravitational force accelerated the
spacecraft. This gives an accurate measurement of the
planet’s mass.
• Orbiter
112
• Atmospheric entry probe (may land on surface)
• Lander
• Rover
• Surface sample return
113
Space exploration of the planets so far:
Moon: all of above except atmospheric probe
Mercury: 3 flyby missions. Latest: Messenger in Jan. 2008
Venus: flyby missions, orbiters, entry probe, lander
Mars: flyby missions, orbiters, landers, rovers
Jupiter: flyby missions, orbiters, atmosphere probe
Saturn: flyby missions; Cassini/Huygens orbiter & Titan
entry probe
Uranus, Neptune: flyby missions only
Pluto: none so far — but New Horizons is on its way (2015)
Material on tides: Fig. 1 (from the special topic on tides in chapter 4) 114
to be continued