VIEWS: 0 PAGES: 35 POSTED ON: 7/15/2013
Quiz information on the course website Include : Quiz answers (posted by ~5pm Tuesdays) Quiz problems Quiz rubrics (posted by 5pm following Tuesdays) Quiz score will also be posted by the end of the following week. Quizzes will be returned in your DL section that meet after the following quiz. (I.e. Quiz3 will be returned later next week) What about Quiz 1? Average 8.69 Those who has not gotten them back will get them in the first DLM this week. Answer, rubrics are on the course web site. Request regrade? => Submit your quiz along with Quiz Re- evaluation Request Form (available from the course website) to me AFTER the lecture by lecture 6 (Feb12) What about Quiz 2? Quiz 2 will be returned in DLM 7 this week. Quiz 3 8:30-8:50am TODAY Have your calculator ready Closed book Next lecture February 5 Quiz 4 will cover the material from today’s lecture, FNT’s from DLM 5, material from DLM6&7 this week, including FNTs for DLM7 but NOT FNT’s for DLM8. Energy systems so far Emass-spring KE Etherm Ebond Distance from Speed al Phase the equilibrium T position PEgra Eelectri v Enuclea c height r Energy is converted from one form to another, but NEVER created nor destroyed. If the energy of an object increases, something else must have given that object its energy. Conservation of Energy Etot = 10 Joule 5 3 2 Nature happens… Energy Interaction Model Etot = 10 Joule 7 2 1 Energy Interaction Model Etot = 10 Joule 7 2 1 (-3J) + (+5J) + (-2J) = 0 ∆Eorange + ∆ Emelon + ∆ Egrape = 0 Etotal = Eorange + Emelon + Egrape Conservation of Energy FNT 2.1.-1 Equal mass, identical initial speeds Which rock has the greatest speed just Before it hits the ground? Conservation of Energy FNT 2.1.-1 Equal mass, identical initial speeds Which rock has the greatest speed just Before it hits the ground? Increase in the KE system is the same as the decrease in the PEgrav system ∆PEgravX + ∆ KEX = 0 (PEgravX)final - (PEgravX)initial + (KEX)final - (KEX)initial = 0 0 - (PEgravX)initial + (KEX)final - (KEX)initial = 0 => (KEX)final = (KEX)initial + (PEgravX)initial Wait a minute! (KEX)initial = (KEY)initial = (KEZ)initial (PEgravX )initial= (PEgravY )initial= (PEgravZ )initial Conservation of Energy FNT 2.1.-1 Equal mass, identical initial speeds Which rock has the greatest speed just Before it hits the ground? Total energy of the system remains unchanged EtotX = PEgravX + KEX = Constant How do the total energies of the three rocks compare initially? Same How do the total energies of the three rocks compare finally (or at anytime) ? Same Bowling Ball What is the height of the bowling ball after one full swing? (a) Same (b) Higher (c) Lower Bowling Ball What is the height of the bowling ball after one full swing? (a) Same (Assume friction is negligable) Bowling Ball c a b When is the speed of the bowling ball maximum? (a) Starting point (b) When rope is vertical (c) At point c. Bowling Ball c a b When is the speed of the bowling ball maximum? (b) When rope is vertical Bowling Ball c a b When is the PEgravity of the bowling ball maximum? (a) Starting point (b) When rope is vertical (c) At point c Bowling Ball c a b When is the PEgravity of the bowling ball maximum? (a) Starting point (c) At point c. Conservation of Energy Consider a simple pendulum: At the height (peak) of the amplitude, the object is at rest. PEgravity = mgh (define h above the low point) At the bottom of the motion, the object is moving quickly, and h=0. KE = (1/2) m Dv2 Conservation of Energy dictates that: DPEgravity = - DKE mgDh = - (1/2) m Dv2 Etotal = PEgrav + KE = constant All of the PE goes into KE, and then back again! Bowling Ball Initial Final (Still in KE PEgrav motion) Speed Height Bowling Ball Final Initial KE PEgrav (In motion) Speed Height Bowling Ball Initial Final (Still in KE PEgrav motion) Speed Height Potential Energy: Springs • Springs contain energy when you stretch or compress them. We will use them a lot in Physics 7. • The indicator is how much the spring is stretched or compressed, Dx, from its equilibrium position. DPE spring = (1/2) kDx 2 x • k is a measure of the “stiffness” of the spring, with units [k] = kg/s2. • Dx: Much easier to stretch a spring a little bit than a lot! Mass-Spring Systems DPEmass- spring = (1/2) kDy2 +C • k is a property of the spring only • PEmass-spring does not depend on mass • PE = 0 arbitrary Mass-Spring Systems KE PEmass- spring Speed ∆y Mass-Spring Systems KE PEmass- Speed spring ∆y Conservation of Energy Just like a simple pendulum: • At the peak of the amplitude, the object is at rest. PEmass-spring = (1/2) m Dy2(define y from the equilibrium position) • At the equilibrium position, the object is moving quickly, and Dy =0. KE = (1/2) m Dv2 Conservation of Energy dictates that: DPEspring-mass = - DKE (1/2)k Dy2 = - (1/2) m Dv2 Etotal = PEspring-mass + KE = constant All of the PE goes into KE, and then back again! Graphing Energies What are the x-axis, y axis? Units? x axis (independent variable: height) y axis (dependent variable: PEgrav) Which quantity (energy) is the easiest to graph? Etot ? PEgrav? What about KE? Where should the origin (0) be placed? Where does it most make sense? Should the floor be 0m? Potential Energy and Forces: Springs, Gravitational The indicator is how much the spring is stretched or compressed, Dx, from its equilibrium position. x DPEspring = (1/2) kDx2 The indicator is the change in vertical distance that the object moved (I.e. change in the distance between the center of the Earth and the object) ∆PEgrav = h PE vs displacement: Force [-] Displacement from equilibrium y [+] PE vs displacement: Force direction of force [-] Displacement from equilibrium y [+] PE vs displacement: Force direction of force [-] Displacement from equilibrium y [+] PE vs displacement: Force On this side force pushes down Forces from potentials point in direction Equilibrium that (locally) lowers PE On this side [-] Displacement from equilibrium y [+] force pushes up Potential Energy vs r and Forces • Force is always in direction that decreases PE • Force is related to the slope -- NOT the value of PE • The steeper the PE vs r graph, the larger the force What does this to do with real world?? Why does it take more • Three-phase model of matter energy to vaporize than to melt? Whst is Ebond? • Energy-interaction model r • Mass-spring oscillator • Particle model of matter Particle model of bond energy Particle model of thermal energy We will model real atoms of liquids and solids as oscillating masses and springs •Thermodynamics Particle Model of Matter • Ideal gas model • Statistical model of thermodynamics Introduction to the Particle Model Potential Energy between two atoms P E Repulsive: Atoms push apart as they get too close Flattening: atoms have negligible forces at large separation. separation r Distance between the atoms Closed Book Make sure above boxes are filled!