Transcript gravity
Gravity Orbits How come the astronauts float around when they are inside the space shuttle? A) The shuttle’s rockets compensate for gravity B) The shuttle is so far from earth that gravity is very, very weak. C) Earth’s atmosphere shields the shuttle from gravity D) The Moon’s gravity compensates for the Earth’s E) NASA didn’t pay the gravity bill F) There is nothing holding the shuttle up Gravity and Orbits - Not Free Fall •Cuthappens the cablewhen something holds you What up.•The elevator starts falling •The floor no longer holds you up Gravity pulls you down Help! •You too you up Floorfall pushes •You float around no gravity Forces balance - you– don’t go anywhere. •Because you and elevator are falling You know which way is up because thatatis same rate, feels like weightlessness thethe way the floor is pushing you •Free fall can be up, down, or sideways Gravity and Orbits - Circular Orbits “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 GM v R 2 o Gravity and Orbits How come the astronauts float around when they are inside the space shuttle? A) The shuttle’s rockets compensate for gravity B) The shuttle is so far from earth that gravity is very, very weak. C) The Earth’s atmosphere shields the shuttle from gravity D) The Moon’s gravity compensates for the Earth’s E) NASA didn’t pay the gravity bill F) 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 2GM v R 2 e •If velocity is bigger, it leaves on a hyperbola GM v R 2 o Two Body Orbits •How come the Sun doesn’t go around the Earth? •The Center of Mass is where the two masses would balance •The objects go around each other (M + m)P2 = a3 It does! Center of Mass Announcements Date Today Monday Tuesday Read Sec. 4.1 – 4.4, Gravity Lecture Sec. 5.1 Sec. 5.2 – 5.3 Test next Thursday! Information sheet online soon? 5/28 Quiz yes yes yes Kepler vs. Newton P2 = a3 (M + m)P2 = a3 Applies to planets around the Sun Always applies •Masses in Solar Masses Sun + Earth M + m = 1 + .000003 1 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 Precession •Gravity pulls unevenly on the flattened Earth •Earth is like a top – its tilt changes over time 26,000 year cycle Gravity vs. Rotation = Disk Dust or gas or chunks - disk forms Rotation keeps it from falling in Gravity keeps it from escaping •Rings around Planets •Spiral Galaxies •Accretion disks •Protoplanetary disks Tidal Forces Tidal Distortion (greatly exaggerated) Spring (big) and Neap (small) Tides Sun Big Tides Earth Moon Earth Sun Small Tides Moon Tides Generally, high tide occurs twice every day. Why? A) There are two causes of tides, the Sun and the Moon B) The Moon causes two places on the Earth’s oceans to bulge and have high tide C) The second high tide is sort of an echo of the first tide D) The Moon goes around the Earth twice every day Bulges and the Earth/Moon system without with rotation Earth Moon Earth and Moon bulge want to align Strong effect on the Moon Rotation causes bulge to be in wrong place Bulge has gravitational pull on Moon Earth rotation slows Moon slung to higher, slower orbit Earth Moon Moon Why can we only see one side of the Moon? A) Because the Sun only shines on one side of the Moon B) Because the Moon doesn’t rotate, so we never see the back side of the Moon C) The Moon rotates at the same rate as it goes around the Earth D) It is like a rainbow; it doesn’t have another side Moon Tidal locking Moon Tidal Heating Changing shape as distance changes Flexing generates heat planet moon Roche Limit Planet Moon Roche Limit - Rings Roche Limit - Rings Where is Titan [a very large moon of Saturn] compared to Saturn’s rings? A) It is closer than the rings B) It is farther away than the rings C) It is within one of the rings D) It is between rings - there are rings both inside and outside it.