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Unformatted text preview: Clickers with ID listed below need to be registered - #1774335 #1899119 #18FC749 #19025B4 K3 E3 H5 Q3 Physics 100 Fall 2007 Schedule Monday 9/3 Labor Day No Class Pretest 3 posted Monday 9/10 HW 2 due Tuesday Tuesday Wednesday 9/5 HW 3 posted HE 1 posted Wednesday 9/12 Pretest 3 due Pretest 4 posted Tuesday Wednesday 9/19 Pretest 5 posted Thursday Thursday Thursday 9/7 Friday Friday 9/13 HW 3 due HE 1 due HW 4 posted Friday 9/21 HW 4 due Monday 9/17 Pretest 4 due Physics 100 Fall 2007 Energy and Work Energy a conserved quantity can't be created or destroyed can be transformed or transferred between objects is the capacity to do work Work mechanical means of transferring energy work = force distance (where force and distance in same direction) Physics 100 Fall 2007 Energy and Work Energy: the capacity to make things happen F = mg Work: is the transfer of energy Work = Force Distance W W = = F d h F h = mg h Where did energy of lifters go? - into potential energy of piano Gravitational potential energy = mgh Energy is Conserved Physics 100 Fall 2007 Challenge: Get the piano up to the second floor Ramp or Straight Lift? RAMP Straight Lift 2000N Physics 100 Fall 2007 Force from ramp Net Force d Weight = mg Component into ramp h F = mg W = FdW = F d = W Physics 100 Fall 2007 F d W = mg h Work on piano = change in energy of piano = same! Why Ramps Work The work done to move a piano up a ramp does not depend on the ramp's slope The work done mgh is equal to the increase in the piano's gravitational potential energy The blue and red triangles are similar in shape Fnet d h mg Note: h Fnet = d mg So, Work Done = Fnet d = mg h Physics 100 Fall 2007 Transfer of Energy Mechanical energy has two principal forms Kinetic energy energy of motion Potential energy energy stored in forces Your work transfers energy from you to the piano You do work on the piano Your chemical potential energy decreases The piano's gravitational potential energy increases Energy is conserved Physics 100 Fall 2007 When the Piano Falls Gravitational Potential Energy = mgh Kinetic Energy = ? The piano falls from rest so equations for uniform accelerated motion apply After falling a distance h, the piano has h distance moved from rest 1 2 1 2 h v0 t at 0 gt (1) 2 2 v velocity after falling v v0 at 0 gt t (2) g So, substituting (2) into (1), we get mgh 1 v mg g 2 g 2 Kinetic Energy = mv2 These must be equal - 1 2 mv 2 Energy is conserved. Physics 100 Fall 2007 Hills are Ramps Find velocity at any point on the hill using the idea that energy is conserved: PE1 + KE1 = PE2 + KE2 mgh1 + mv12 = mgh2 + mv22 E = PE1 + KE1 E = PE2 + KE2 h1 h2 Answer A Is v2 greater than v1? A. Yes; B. No; C. Depends. Physics 100 Fall 2007 Seesaws Section 2.1 Torque and angular motion t Newton's laws Physics 100 Fall 2007 Introductory Question You and a child half your height lean out over the edge of a pool at the same angle. If you both let go simultaneously, who will tip over faster and hit the water first? A. You B. The small child Physics 100 Fall 2007 Observations about Seesaws A balanced seesaw rocks back and forth easily Equal-weight children balance a seesaw Unequal-weight children don't normally balance Moving heavier child inward restores balance Sitting closer to the pivot speeds up the motion Physics 100 Fall 2007 ...
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This note was uploaded on 03/26/2008 for the course PHYS 100 taught by Professor Tsui during the Fall '07 term at UNC.
- Fall '07