Physics 1A, Section 6

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Physics 1A, Section 6

• October 23, 2008

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Second Quiz

• covers
• T.M.U. chapters 5-8 (and 1-4)
• lectures/sections through today (Oct. 23)
• homework 2-4 (and 1)
• open book (T.M.U., Feynman Vol. I), open class
  notes, open homework
• Pick it up tomorrow before/after the lecture

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Problem Solving HintsNormal 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

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Thursday, October 23

• Quiz Problem 48
• Quiz Problem 11
• Optional, but helpful, to look at these in
  advance.

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Quiz Problem 48kinetic friction
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Quiz Problem 48

• Answer
• m1a T m1g(sinq m1cosq), m2a -T m2g(sinq
  m2cosq)
• a gsinq cosq(m1m1m2m2)/(m1m2), T
  gcosq(m1-m2)m1m2/(m1m2)
• m1 gt m2

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Quiz Problem 11 static kinetic friction
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Quiz Problem 11

• Answer
• a g(m1scosq - sinq)
• F (m1m2)g(m2kcosq m1scosq)

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Monday, October 27

• Quiz Problem 29 (non-inertial frames)
• Quiz Problem 49 (springs)
• Optional, but helpful, to look at these in
  advance.

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Location of Lagrange Points

• L1,L2 DR R(ME/3MS)1/3
• about 0.01 A.U. from Earth toward or away from
  Sun
• unstable orbits
• L3 orbital radius R1 (5ME/12Ms)
• very unstable orbit
• L4,L5 three bodies form an equilateral triangle
• stable orbits

1 A.U. astronomical unit distance from Earth
to Sun
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Advanced Composition Explorer examines solar
wind
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Wilkinson Microwave Anisotropy Probe examines
relic radiation from Big Bang