Download 2008oct23

Physics 1A, Section 6
October 23, 2008
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
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
Thursday, October 23:
• Quiz Problem 48
• Quiz Problem 11
• Optional, but helpful, to look at these in
advance.
Quiz
Problem
48:
kinetic
friction
Quiz
Problem
48
• Answer:
a) m1a = T + m1g(sinq – m1cosq),
m2a = -T + m2g(sinq – m2cosq)
b) a = g[sinq –
cosq(m1m1+m2m2)/(m1+m2)],
T = gcosq(m1-m2)m1m2/(m1+m2)
c) m1 > m2
Quiz
Problem
11:
static &
kinetic
friction
Quiz
Problem
11
• Answer:
a) a = g(m1scosq - sinq)
b) F = (m1+m2)g(m2kcosq + m1scosq)
Monday, October 27:
• Quiz Problem 29 (non-inertial frames)
• Quiz Problem 49 (springs)
• Optional, but helpful, to look at these in
advance.
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 ≈ R[1 + (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
Advanced Composition Explorer
– examines solar wind
Wilkinson Microwave Anisotropy
Probe – examines relic radiation
from Big Bang