10/6/2015A.PH 105 PH /4 ----Friday, Sept. 28, 2007 Homework: PS5 done; PS6 has hard-copy part (sketches, #7) Exam

10/6/2015PH 105 of test #1

Clicker question [ungraded, anonymous]: Which 1-hour test would you rather take? A.12 questions B.20 questions C.32 questions Responses

10/6/2015A.PH 105 PH /4 ----Monday, Sept. 30, 2007 Test: error on key, see WebAssign for score (most people went up) Homework due Wednesday.

Clicker question ► {1} A man kicks a box, which then slides across the floor several meters before coming to a stop. The floor is horizontal. While the box is moving, and no longer in contact with the man’s foot, what forces are acting on the box? Ignore air resistance. A.gravity, normal, friction, and inertia B.gravity, normal, friction and the force of the kick C.gravity, normal and friction only D.gravity, normal, friction, inertia and the force of the kick E.friction only On test: Later

Anonymous clicker question: A.Exposed a misconception (that inertia is a force, for example) and helped me overcome it. B.Planted a misconception and then punished me for accepting it (a ”trick question”) The previous question Responses 21 33

Anonymous clicker question: Irrespectively of whether misconceptions should be probed on exams, A.I thought inertia was a force and it would have helped me if the instructor had pointed out in class that it’s not B.I’ve never thought that inertia was a force (except perhaps after it’s suggested by an exam question) Responses 22 30

10/6/2015A.PH 105 Chapter 7: Work and Energy Main points W=Fd cos  Work is a transfer of energy Can be turned to KE, ½ m v 2 Energy is conserved: KE initial + PE initial = KE initial + PE initial Many forms: KE gravitational PE Elastic PE Chemical PE, electrical, nuclear,…

10/6/2015A.PH 105 PH / Wednesday, Oct. 3, 2007 New grades were put on WebAssign Monday afternoon Homework due tonight (Chapter 7) Review Chapter 7: W = F d cos  Kinetic energy K = ½ mv 2 Potential Energy: Gravitational (U=mgh), elastic (½kx 2 ) Conservation of energy: K i +U i = K f +U f (really Ch. 8!) Remaining things from Chapter 7: Non-conservative forces: can’t get energy back out (e.g., friction) Conservative force work independent of path (e.g. gravity) can define PE (formally, U = - work done by force) Equilibrium (stable, unstable, neutral)

Energy Diagrams [i.e. U(x)] and Stable Equilibrium The x = 0 position is one of stable equilibrium Configurations of stable equilibrium correspond to those for which U(x) is a minimum

10/6/2015A.PH 105 Chapter 8: conservation of energy Energy is conserved in isolated system (did example, mass hits spring) Another example: ball thrown off building (active fig. 8.3)

Clicker question: If a second ball is thrown upward at 15º above horizontal instead of below, the speed when it hits the ground will be A.slower than with the downward throw B.faster than with the downward throw C.exactly the same as with the downward throw Same: because PE is the same in both cases, and initial KE is the same (v is the same) so the final KE must be the same.

10/6/2015A.PH 105 Chapter 8: Power Power P = rate of doing work = W / time Force F acting on object moving with velocity v for time t does work W = F d = F vt so P = F vt /t = Fv: P = F v

10/6/2015A.PH 105 Chapter 9 (for lab) Momentum defined by p = m v (a vector) Significance: total momentum of a system is conserved in collision (it is “isolated”) Elastic collision: kinetic energy is conserved too Inelastic collision: energy not conserved (extreme case: objects stick together, as with velcro) Example: a cart of mass 1 kg moving at 4 m/s collides (completely) inelastically with another stationary 1 kg cart.