# PHYSICS 151

##### University of Hawaii, Manoa Dept. of Physics & Astronomy

Instructor: Michael Nassir
Office: Watanabe Hall, Rm. 426, (808) 956-2922
E-mail: nassir @ hawaii.edu

## ANSWERS to MIDTERM #3 from SPRING 2004

Part A

1a. A (True)
1b. A (True)
1c. A (True)
1d. B (False)
2. E
3. A (the closer that the overall mass distribution is located to the spin axis, the smaller the moment of inertia of the object)
4. F
5. B and D (if moment of inertia is constant, then in order for L to be decreasing, angular speed must also be decreasing)
6a. C
6b. F (if R of a sphere doubles, its mass increases by 8x, so its entire moment of inertia increases by 32x)
7a. B
7b. E
7c. F
7d. G
8. B (want largest possible lever arm, and force as perpendicular as possible to lever arm direction)

Part B

1a. 8.6 m/s (conservation of energy: KE of pair of cars is converted to work done by friction force)
1b. 15 m/s = 34 mi/h (using conservation of momentum through the collision)

2. 0.76 away from the fulcrum, on Bryce's side (find position of Crystal so that total center-of-mass lies exactly at fulcrum position; OR sum torques of all three children about fulcrum/pivot and set equal to zero)

3. F_spring = 2.8 x 10^3 N