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© 2005 Pearson Education Inc., publishing as Addison- Wesley The Planets Prof. Geoff Marcy Tides Energy: Potential and Kinetic Momentum Angular Momentum Lecture 7 2012 Sept. 19

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Announcements Chapter 4 Assignment due tomorrow. Read Chapter 4: The Physics of Everything

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© 2005 Pearson Education Inc., publishing as Addison-Wesley

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Tides: Daily oscillations of the sea level. Two high tides and two low tides each day. Bay of Fundy

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Hiking along California’s coast

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Today’s tide chart Two low tides per day: They happen at moonrise and moonset. Low tide: 6:16 am Low tide: 6:50 pm High tide: 1:26 am High tide: 12:49 pm

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Tides During One Month Questions: Why two high tides per day? Why one every 12:50 hours? Why does the amplitude change over the course of a month? Time During One Month (Days)

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Gravity field causes two tidal bulges

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Tidal friction lead to synchronous rotation. Moon is facing the earth the same way.

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Tides vary with the phase of the Moon because the sun interacts also Spring tide: HIGH because sun and moon act together during new and full moon

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© 2005 Pearson Education Inc., publishing as Addison-Wesley Tides vary with the phase of the Moon because the sun interacts also Spring tide: HIGH because sun and moon act together during new and full moon Neap tide: LOW because sun and moon’s gravity counter-act

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Today is a full moon. What times will the high tide occur in the bay area? A.6 am B.Noon C.6 pm D. Midnight

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© 2005 Pearson Education Inc., publishing as Addison-Wesley Real tides are more complicated

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Tidal friction… Tidal friction gradually slows Earth rotation, 1 s every 50,000 years (and makes Moon get farther from Earth). Earth’s + moon’s combined angular momentum is conserved. Earth is losing angular momentum, the moon gains it. Moon once rotated faster (or slower); tidal friction caused it to “lock” in synchronous rotation.

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© 2005 Pearson Education Inc., publishing as Addison-Wesley Synchronous Rotation …is when the rotation period of a planet equals the orbital period of its moon. 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.

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Energy ! © 2005 Pearson Education Inc., publishing as Addison-Wesley

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Basic Types of Energy Kinetic (motion) Potential (gravitational) Thermal (heat) Chemical energy (bonds) Nuclear energy (bonds) Light Mass-Energy can change type, but cannot be destroyed. Light Energy Potential Energy Kinetic Energy

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Potential Energy Converted to Kinetic Energy

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Potential Energy Potential Energy = mass * 9.8 m/s 2 * height = mgh

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Converting: Potential Energy to Kinetic Energy

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Potential Energy Converted to Kinetic Energy

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Conservation of Energy: Potential + Kinetic = Constant with time © 2005 Pearson Education Inc., publishing as Addison-Wesley

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Kinetic Energy: Converted to Heat

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Thermal Energy of the motion of atoms and molecules

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Thermal Energy: Kinetic Energy of the molecules

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Potential Energy (in battery) Converted to Electrical Energy © 2005 Pearson Education Inc., publishing as Addison-Wesley

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Conservation of Energy Energy can make matter move. Energy is conserved, but it can: –Transfer from one object to another –Change in form Note: Energy is conserved within a closed, confined system. In an open system, energy can be exchanged with the environment.

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Conservation of Momentum Definition of momentum: p = mass × velocity (Unit: kg m/s) Conservation of momentum: If no external force is applied, the total momentum of system is conserved. Useful: For all sorts of collisions. It applies also to atoms and molecules.

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Collisions of Balls on a Pool Table

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Interactive Quiz: Collisions Balls on a Pool Table 8 8 A B C Which shot will get a black ball in the corner pocket? A) B) C) D) This is not possible.

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Interactive Quiz: Collisions Balls on a Pool Table A B C Which shot will get a black ball in the corner pocket? A) B) C) D) This is not possible. Corner

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Interactive Quiz: Collisions of two trucks Two trucks of equal mass on an icy road: Before collision: Truck 1 is at rest. Truck 2 approached with 40 km/h. After the collision: Both trucks are damaged and stick together What is their final velocity? A) 10 km/h B) 20 km/h C) 40 km/h D) This is not possible, conservation of momentum prevents them from sticking Truck #1 Truck #2 40 km/h

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Interactive Quiz: Collisions of two trucks Two trucks of equal mass on an icy road: Before collision: Truck 1 is at rest. Truck 2 approached with 40 km/h. Truck 1: p 1 =M* 0 km/h Truck 2: p 2 =M* 40 km/h After collision: p=(2M)*v final Momentum conservation: p final = p 1 +p 2 (2M)*v final = M*40 km/h v final = 20km/h Truck #1 Truck #2

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Conservation of Angular Momentum Definition of angular momentum unit: kg m / s Conservation of angular momentum: In a central field like the sun’s gravity, the angular momentum of orbiting objects is conserved. Useful: Planets and comets in the sun’s gravity field Applies to rotating objects.

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Rotating Chair Demo Before: Radius large Angular velocity small After: Radius small Angular velocity large

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© 2005 Pearson Education Inc., publishing as Addison-Wesley Four conservation laws mass (for chemists only) energy momentum, angular momentum,

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Conservation of Mass For all practical purposes in chemistry: Yes but … (A) YES (B) NO 2H 2 +O 2 2 H 2 O Energy released ΔE= 483.6 kJ/mol 1 mol = 6.02 10 23 particles

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Mass is a form of Energy: E = mc 2

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