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