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Physics 1a, Section 2

Physics 1a, Section 2. October 25, 2010. Second Quiz. Was due 3 hours ago. Problem Solving Hints: Normal Force and Friction. suggested order: 1) draw gravity and other fixed external forces (direction and magnitude are unambiguous)

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Physics 1a, Section 2

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  1. Physics 1a, Section 2 October 25, 2010

  2. Second Quiz • Was due 3 hours ago.

  3. Problem Solving Hints:Normal Force and Friction • suggested order: 1) draw gravity and other fixed external forces (direction and magnitude are unambiguous) 2) draw normal force (prevents interpenetration of objects) 3) draw friction force (perpendicular to normal force) • kinetic friction: F = mkN direction opposite to relative velocity • static friction: F ≤ msN direction and magnitude to prevent relative acceleration

  4. Quiz Problem 11: static & kinetic friction

  5. Quiz Problem 11 • Answer: • a = g(m1scosq - sinq) • F = (m1+m2)g(m2k + m1s)cosq

  6. Tension in a Rope Draped over a Table relevant for HW QP20 mass per unit length r L

  7. Tension in a Rope Draped over a Table mass per unit length r T’ = rgL T=T’ T=T’ T=T’ L T=0 T=0

  8. Fictitious (Inertial) Forces • Accelerating (non-inertial) frames: • Linear acceleration: • Fictitous force ma opposite direction of acceleration • Rotating frame: • Fictitious force mw2r outward • (If an object is moving with respect to the rotating frame, there is also a fictitious Coriolis force on it.) • Why introduce fictitious forces and work in non-inertial frame? • Often simplifies solution of the problem.

  9. Quiz Problem 29

  10. Quiz Problem 29 • Answer: • mA to the left • ma = mAcosq – mgsinq – mN, N = m(Asinq + gcosq) • a = A(cosq – msinq) – g(mcosq + sinq); q < 37°

  11. Homework due Wednesday • I think the intention is for you to work these problems in non-inertial frames with fictitious forces: • QP21 • QP28 • but they can be solved without that. • Good practice to try in non-inertial frame.

  12. Thursday, October 28: • Quiz Problem 49 (springs) • Quiz Problem 50 (energy) • Optional, but helpful, to try these in advance.

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