for wednesday, february 25 no new reading assignment quiz #2 friday (bring a calculator)...
TRANSCRIPT
For Wednesday, February 25
No new reading assignment
QUIZ #2 FRIDAY (BRING A CALCULATOR)
Assignments: Homework #3 (due today)
Measuring Planet Orbits
Can measure size of planet’s orbit (a) if:• measure P from star’s wobble• determine M for star from its light (and as long as
Mplanet << Mstar)
PLANET’S FORCE ON STAR
STAR’S FORCE ON PLANET
Example: planet orbiting star HD189733
Thought Question:A new planet is discovered orbiting a star that is 4
times as massive as the Sun. Astronomers find that it takes 0.5 yr to make one orbit. What does this say about the planet’s orbit?
A. The planet orbits 1 AU from its star.
B. The planet orbits more than 1 AU from its star.
C. The planet orbits less than 1 AU from its star.
D. It isn’t possible to tell how big the planet’s orbit is.
Thought Question:A planet is discovered orbiting another star.
Astronomers find that its average distance from its star is 1 AU, but it takes 3 years to make one orbit. What does this say about the star?
A. The host star is more massive than the Sun.
B. The host star is less massive than the Sun.
C. The host star is the same mass as the Sun.
D. It isn’t possible to tell what the star’s mass is.
Impacts
JUPITER - 1994
EnergyThe ability to do work: move a distance against an
opposing force…
• Metric unit: Joule (J)
• English unit: Calorie
FORCE (FRICTION)
FORCE (GRAVITY)DISTANCE
DISTANCE
Energy
The ability to do work …• Metric unit: Joule (J)
• English unit: Calorie
energy to pick a burger off floor (move it up by 1 m): 1 J energy from eating candy bar: 106 J energy released by H bomb: 5 1015 J energy released by Sun each second: 4 1026 J
Types of Energy: Kinetic Energy
• kinetic energy: energy of motion
o gain kinetic energy by moving with a force
• thermal energy: kinetic energy involving random
motions of atoms and other particles higher temperature
more thermal energy
EXAMPLE: GAS IN A BOX
FORCE
(GRAVITY)
Tunguska Event
SIBERIA - 1908
7:14 am (local)
Chelyabinsk Meteor
RUSSIA – 2013
9:20 am (local)
Thought Question:How fast does a 1010 kg asteroid have to be traveling to
have a kinetic energy equal to an H bomb (5 1015 J)?
A. 1 m / s
B. 1 km / s
C. 103 km / s
D. 106 km / s
Meteors
• meteors tend to hit on the leading side of Earth
• impact speed is usually at least Earth’s orbit speed (~30 km/s)
For Friday, Feb. 27
QUIZ #2
Types of Energy: Potential Energy
• potential energy: energy of position – a kind of “stored” energy
• increases as distance between objects increases• defined to be 0 at infinite separation (d=∞)
POTENTIAL ENERGY DECREASES AS IT ROLLS DOWN…
BECOMING KINETIC ENERGY
Thought Question:A spacecraft is moving away from the Sun
without firing its engines. What will happen to the spacecraft’s…
kinetic energy?
potential energy?
USAUSA
Answer: A. increasesB: decreasesC: stays the same
(Enter the two letters and hit send.)
CONSERVATION OF ENERGY:CONSERVATION OF ENERGY: energy can be transferred from object to object, or converted from one form to another, but never destroyed
Jupiter’s Moon Io
Example: Io volcanic eruption
If you shoot cannonballs into space at greater and greater speeds, what will happen?
low speed: crash into surface
medium speed: circular orbit
high speed: ellipse orbit (cannonball gets farther from Earth)
Newton’s Thought ExperimentFire cannonballs from tall mountain at different speeds:
Escape Velocity
If ship escapes, it must get far enough away that gravity can’t pull it back:
ON PLANET’S SURFACE
USAUSA
IN SPACE
Escape Velocity
Bound and Unbound Orbits
• BOUND ORBIT – object does not have enough kinetic energy to escape:
• UNBOUND ORBIT – object can eventually reach d = ∞
miniature golf analogy: imagine the Sun at bottom of a valley, and object rolling along the sides
Bound and Unbound Orbits
• BOUND ORBIT (ellipse or circle) — gravity able to hold object
• UNBOUND ORBIT (parabola or hyperbola) — object does not return after closest approach
Review Question:Two of the orbits in the diagram below have
the same orbit period. Which one doesn’t?
Enter the letters of the two answers and hit send.
Review Question:In the diagram below, the larger of the two
orbiting objects has 4 times as much mass as the other. Where is the center of mass?
Enter the number of the position.
0 1 2 3 4 5 6 7 8 9 10
Review Question:If you need to calculate the speed of Earth
orbiting the Sun, what mass and distance do you need to use?
A. Earth’s mass Earth-Sun distance
B. Sun’s mass Earth’s radius
C. Earth’s mass Earth’s radius
D. Sun’s mass Earth-Sun distance
Review Question:For the Dawn spacecraft going into orbit around the dwarf planet Ceres, which of the following do you
need to know to calculate the spacecraft’s orbit period? (Enter all letters you need.)
A.Ceres’ radius
B.Ceres’ semi-major axis
C.Dawn’s semi-major axis
D.Sun’s mass
E.Ceres’ mass
F.Dawn’s mass