Gravity

How come the astronauts float around when they are inside the space shuttle?A) The shuttle’s rockets compensate for gravityB) The shuttle is so far from earth that gravity is very, very weak.C) Earth’s atmosphere shields the shuttle from gravityD) The Moon’s gravity compensates for the Earth’sE) NASA didn’t pay the gravity billF) There is nothing holding the shuttle up

Orbits

What happens when something holds you up.

Gravity pulls you downFloor pushes you up

Forces balance - you don’t go anywhere.You know which way is up because that is the way the floor is pushing you

Gravity and Orbits - Not Free Fall

Help!

•Cut the cable•The elevator starts falling•The floor no longer holds you up•You fall too•You float around – no gravity•Because you and elevator are falling at the same rate, feels like weightlessness•Free fall can be up, down, or sideways

“The knack [to flying] lies in learning how to throw yourself at the ground and miss” - The Hitchhiker’s Guide to the Galaxy trilogy, in Life, the Universe, and Everything

Gravity and Orbits - Circular Orbits

2o

GMv

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Gravity and Orbits

How come the astronauts float around when they are inside the space shuttle?A) The shuttle’s rockets compensate for gravityB) The shuttle is so far from earth that gravity is very, very weak.C) The Earth’s atmosphere shields the shuttle from gravityD) The Moon’s gravity compensates for the Earth’sE) NASA didn’t pay the gravity billF) There is nothing holding the shuttle up

Non-circular orbits•If velocity is more or less than this, you get an ellipse

•If velocity is big enough, it leaves on a parabola – escape velocity

2 2e

GMv

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•If velocity is bigger, it leaves on a hyperbola

2o

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Two Body Orbits

Center of Mass•The Center of Mass is where the two masses would balance

•The objects go around each other

•How come the Sun doesn’t go around the Earth? It does!

(M + m)P2 = a3

Announcements

5/28

Date Read QuizToday Sec. 4.1 – 4.4, yes

Gravity LectureMonday Sec. 5.1 yesTuesday Sec. 5.2 – 5.3 yes

Test next Thursday!Information sheet

online soon?

Kepler vs. Newton

(M + m)P2 = a3

P2 = a3

Sun + Earth

M + m = 1 + .000003 1

Applies to planets around the Sun

Always applies

•Masses in Solar Masses

Gravity vs. Pressure = sphere

•Gravity wants to squish everything to a point•Pressure wants it to spread out•They balance to make a sphere

•If an object rotates, it makes a flattened (oblate) sphere•Small objects can be any shape

26,000 year cycle

Precession•Gravity pulls unevenly on the flattened Earth•Earth is like a top – its tilt changes over time

Gravity vs. Rotation = Disk Dust or gas or chunks - disk formsRotation keeps it from falling inGravity keeps it from escaping

•Rings around Planets•Spiral Galaxies•Accretion disks•Protoplanetary disks

Tidal Forces

Tidal Distortion (greatly exaggerated)

Big Tides Sun

Moon

Earth

Small Tides Moon

EarthSun

Spring (big) and Neap (small) Tides

Tides

Generally, high tide occurs twice every day. Why?A) There are two causes of tides, the Sun and the MoonB) The Moon causes two places on the Earth’s oceans to bulge and have high tideC) The second high tide is sort of an echo of the first tideD) The Moon goes around the Earth twice every day

Rotation causes bulge to be in wrong placeBulge has gravitational pull on MoonEarth rotation slowsMoon slung to higher, slower orbit

Bulges and the Earth/Moon system

Earth Moon

without rotation

Earth

Earth and Moon bulge want to alignStrong effect on the Moon

with rotation

Tidal locking

Why can we only see one side of the Moon?A) Because the Sun only shines on one side of the MoonB) Because the Moon doesn’t rotate, so we never see the back side of the MoonC) The Moon rotates at the same rate as it goes around the EarthD) It is like a rainbow; it doesn’t have another side

Moon

Moon

Moo

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Moon

Earth

planet moon

Tidal HeatingChanging shape as distance changesFlexing generates heat

Planet

Moon

Roche Limit

Roche Limit - Rings

Where is Titan [a very large moon of Saturn] compared to Saturn’s rings?A) It is closer than the ringsB) It is farther away than the ringsC) It is within one of the ringsD) It is between rings - there are rings both inside and outside it.

Roche Limit - Rings