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

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

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

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

Hiking along California’s coast

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

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)

Gravity field causes two tidal bulges

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

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

© 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

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

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

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.

© 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.

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

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

Potential Energy Converted to Kinetic Energy

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

Converting: Potential Energy to Kinetic Energy

Potential Energy Converted to Kinetic Energy

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

Kinetic Energy: Converted to Heat

Thermal Energy of the motion of atoms and molecules

Thermal Energy: Kinetic Energy of the molecules

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

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.

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.

Collisions of Balls on a Pool Table

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.

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

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

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

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.

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

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

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= kJ/mol 1 mol = 6.02  particles

Mass is a form of Energy: E = mc 2