1 / 13

Announcements

Announcements. Read Ch. 6 (Work & Energy): All of Chapter 6 CAPA Set #8 due on Friday at 10 pm (shorter assignment) This week in Section, Assignment #5 : Work & Energy Print and bring to your Lab Section. Midterm Exam Tuesday October 11 at 7:30 pm

garran
Download Presentation

Announcements

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Announcements • Read Ch. 6 (Work & Energy): All of Chapter 6 • CAPA Set #8 due on Friday at 10 pm (shorter assignment) • This week in Section, Assignment #5: Work & Energy Print and bring to your Lab Section. • Midterm Exam Tuesday October 11 at 7:30 pm • Practice exam and solutions versions have a couple differences. Apologies. Refer to solutions version. • Office Hours… Nagle – Monday 2-3 pm Gamow F221 Uzdensky – Tuesday 11 am – 12 Help Room Nagle/Kinney – Wed. 1:45-3:45 Help Room Kinney – Thursday 11 am – 12 Help Room *

  2. Exam #2 Tuesday, October 11 evening at 7:30-9:15 PM:

  3. Reminders: Work and Kinetic Energy Work – Force acting and resulting in displacement along direction of force Kinetic Energy – Energy of object in motion

  4. Room Frequency BA Clicker Question Two marbles, one twice as massive as the other, are dropped to the ground from the roof of a building. (Assume no air resistance.) Just before hitting the ground, the heavier marble has? As much kinetic energy as the lighter one. Twice as much kinetic energy as the lighter one. Half as much kinetic energy as the lighter one. Four times as much kinetic energy as the lighter one. Impossible to determine. KE = (1/2) mv2, so if you double the mass you double the KE.

  5. Room Frequency BA Clicker Question A 1 kg mass is moved part way around a square loop as shown. The square is 1 meter on a side and the final position of the mass is 0.5 m below its original position. Assume that g = 10 m/s2. What is the work done by the force of gravity during this journey? 0 J 10 J 5 J 0 J –10 J –5 J +10 J -5 J 0 J Total work done W = (-5) + (0) + (+10) + (0) = +5 J

  6. Work-Energy Principle The work done by the net force on a single object is equal to the change in kinetic energy of that object: Wnet = WFnet = ΔKE = KEf - KEi Since work is a transfer of energy via a force, this is a statement of conservation of energy. Only applies if energy is not transferred to another form (e.g. a change in potential energy, heat, chemical energy, etc.)

  7. Work-Energy Principle Wnet = WFnet = ΔKE = KEf - KEi Push a book across a frictionless table with constant force Fext #1: Fnet = Fext (since N and mg cancel) #2: Fnet = ma  a = Fext / m #3: W = +FextDx Fext = W / Dx #4: vf2 –vi2 = 2a Dx vf2 –vi2 = 2(Fext/m) Dx vf2 –vi2 = 2((W/Dx)/m) Dx ½ m(vf2 –vi2)= W ½ mvf2 –½mvi2 = DKE = W

  8. Room Frequency BA Clicker Question A car of mass m is moving with speed v. The driver applies the brakes and the car skids to a stop. What was the magnitude of the work done by the friction force on the tires? |Wfric| = FfricΔx = μK N Δx = |ΔKE|= ½ mv2 If the car’s speed were doubled, the distance it would skid before stopping would be: A) Doubled, B) Tripled, C) Quadrupled, D) Halved. μK N Δx = ½ mv2

  9. Potential Energy PE is a storedenergy associated with the position or geometry of a physical system Several varieties: gravitational, elastic, spring, …. Definition: PE is the amount of work done on a system by an external force when Kinetic Energy does not change and no heat energy flows in or out of the system. ΔPE = Wext when ΔKE = 0 Again, effectively a re-statement of conservation of energy.

  10. Demonstration For a moment, they are at rest near the top (KE = 0). Later, they are moving quite fast (large KE). Where did the energy come from since energy is conserved? Gravitational Potential Energy

  11. Gravitational Potential Energy Lift mass m at a constant speed by a height Δy As height increases, PEgrav increases. ΔPEgrav = Wext ΔPEgrav = +FextΔy ΔPEgrav= +mg Δy Often define PEgrav= 0 when y = 0. PE is always defined relative to a “reference level” where it is zero. PEgrav = mgy Fnet= ma = Fext – mg=0 Fext = mg

  12. Mechanical Energy Emechanical = KE + PE Conservation of Mechanical Energy: KE can change into PE and PE into KE, but the total (KE+PE) is constant for an isolatedsystem with no lost energy (dissipation). Emechanical = (KE + PE) = constant DEmechanical = D(KE + PE) = 0 Investigate swing: http://phet.colorado.edu/sims/pendulum-lab/pendulum-lab_en.html

  13. Conservation of Mechanical Energy Emechanical = KE + PE = constant (isolated system, no dissipation) Consider mass mswinging attached to a string of length L. The swing is released from rest at a height h. What is the speed v of the swing when it reaches height h/2? 0 KE = ½ mv2 PEgrav = mgy

More Related