Mars moon Phobos, as seen by ESA's Mars Express. Phobos is about 25 kilometers in length and does not have enough gravity to compress it into a ball. It orbits so close to Mars that sometime in the next 20 million years, tidal forces will break up Phobos into a ring whose pieces will slowly spiral down and crash onto the red planet. The Russian mission Phobos-Grunt is scheduled to launch and land on Phobos next year.

Out-of-class Activity:

Every clear Wednesday evening

for the rest of the semester.

Check website for details.

Kirkwood Observatory Open House

A100 Movie Special

Tuesday, March 23

Swain West 119

7:00 pm (153 minutes)

Winner of several awards

20 activity points!

Homework #6 is

due Wednesday, March 24, 2:30 pm

Homework #7 will be posted shortly. It will be due next Monday

Exam #2

Wednesday, March 31

Review session will be held next Monday, time and location to be announced

Newton’s Three Laws of Motion

Newton’s Law of Gravity

The Acceleration of Gravity (a force)

As objects fall, they accelerate (a = g = Fgrav/m).

We use the special symbol g to represent the acceleration due to the force of gravity.

At sea level on the Earth, g = 9.8 m/s each second,

or g = 9.8 m/s2.

The higher you drop the ball, the greater its velocity will be at impact (force will be acting on it longer).

Weight is the force of gravity acting upon an

object :

W = Fg = mg

Galileo demonstrated that g is the same for all objects, regardless of their mass!

Is Mass the Same Thing as Weight?● mass – the amount of matter in an object● weight – a measurement of the force due to

gravity acting upon an object

When in free-fall, you still have weight!

“weightless” is a misnomer

W = mg (weight) F = ma

● Objects do have weight in space

● Free-fall often confused with weightlessness

Now, some questions…

Tidal Forces Because the gravitational force decreases with (distance)2, the attractive

force experienced by one object (e.g., the Earth) due to the gravitational field of a second object (e.g., the Moon) varies with position (closest parts attracted most strongly).

● Now look at what happens when we measure the forces

relative to the center of the Earth.

Tidal Friction

Tidal Friction

● This fight between Moon’s pull & Earth’s rotation causes friction.

● Earth’s rotation slows down (1 sec every 50,000 yrs.)

● Conservation of angular momentum causes the Moon to move farther away from Earth.

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.

● Tidal friction on the Moon has ceased since its tidal bulges are always aligned with Earth.

● Most of the large moons in the solar system are in synchronous rotation.

Now we are ready to examine the solar system!!

We want to understand how the solar system was formed and how it got to be in the state that it is today.

Look for patterns and physical characteristics of the solar system.

What does any theory of the formation and evolution of the solar System have to account for?

The Sun:

A fairly typical star

Predominately H and He

Most of the mass in the solar system.

Rotates in same sense that planets

orbit.

Need to understanding the similarities and differences between the planets, moons, asteroids, & comets

Nine planetseig

ht

People of earth.

Help!!!!

Planetary orbits:

1) Prograde

2) approximately coplanar

3) approximately circular

Rotation:

1) Mostly Prograde

2) Includes sun

3) Includes large moons

Terrestrial Jovian

Two “flavors” of planets

Size – “smaller” Location – closer to Sun Composition – rocky/metallic Temperature – hotter Rings – none Rotation rate – slow Surface – solid Atmosphere – “minimal”

Terrestrial Planets

Mer

cury

Mar

s

Ven

us

Ear

th

Mercury

No moons

Venus

No moons

Earth

One moonMars

Two moons

Jovian Planets

Size – “larger” Location – farther from Sun Composition – gaseous (mostly H,He) Temperature – cold Rings – ubiquitous Rotation rate – fast Surface – not solid Atmosphere – substantial

Jupi

ter

Nep

tune

Sat

urn

Ura

nus

Jupiter

>61 moonsUranus

> 27 moons

Neptune

> 13 moons

Saturn

> 31 moons

Surface features of solid objects in solar system

Craters are ubiquitous

There are lots of smaller objects in the Solar System,

some are rocky and some are icy

small

Rocky

Odd-shapes

nearly circular orbits

orbit planes are near Ecliptic Plane

orbits in inner part of solar system

Asteroids

The “asteroid belt”

Asteroids

Mars’ moons and the asteroid Gaspra

Deimos

Gaspra

Phobos

Comets “small” icy highly eccentric orbitsall orbit inclinations

Comet Wild

Halley’s Comet

Comet Tempel 1

(“Deep Impact”)

Comets are found mainly in two regions of the solar system

UB313(1500 miles)

Kuiper Belt Objects

So how do we account for what we see in the solar system?