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.
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
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.
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