1. Physics 1A, Section 2
October 18, 2010

2. warning
Homework problem 7.16 is difficult! Leave some time in your schedule to work on it.

3. material to hit today
statics (equilibrium) – chapters 6.5 & 6.6 in text
S F = 0
S t = 0
t = r × F
force due to gravity
circular motion

4. combined gravity & torque problem
pivot m1 m2 q1 q2 M
What ratio of masses m1 and m2 is required to balance that system? Consider gravitational attraction of m1 and m2 to M, but neglect Earth’s gravity.

5. combined gravity & torque problem
pivot m1 m2 q1 q2
Answer:
m1sinq1cos2q1 = m2sinq2cos2q2
There is a normal force at the pivot to create force balance. M
What ratio of masses m1 and m2 is required to balance that system? Consider gravitational attraction of m1 and m2 to M, but neglect Earth’s gravity.

6. circular orbit m m d
Useful relations for circular motion: v = omega * r; omega = 2*pi/T
What is the orbital period of a binary star in circular orbit with separation d, if each star has mass m?

7. circular orbit m m d
Extra factor of 2 compared to formula 7.30 in book, which worked out case of m << M.
Answer:
T2 = [4p2/(Gm)] (d3/2)
What is the orbital period of a binary star in circular orbit with separation d, if each star has mass m?

8. Thursday, October 21: Quiz Problem 37 ???
Optional, but helpful, to try these problems in advance.