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Hawaii PHYS 151 exam151m3-05sprExam

Course Code
PHYS 151
Study Guide

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Physics 151 Roster No.:
April 15, 2005
Midterm Exam #3, Part A
Exam time limit: 50 minutes. You may use calculators and both sides of 2 pages of notes, handwritten only.
Closed book; no collaboration. For multiple choice questions, circle the one best answer or letter (unless more
than one answer is asked for).
Constants & Useful Data: g = 9.81 m/s2
Some Moments of Inertia: solid sphere: I = (2/5) M R2
solid cylinder or disk (rotated about axis of symmetry): I = (1/2) M R2
hoop or thin ring (rotated about axis of symmetry): I = M R2
thin rod, rotated about center: I = (1/12) M L2
thin rod, rotated about end: I = (1/3) M L2
1. a. (2 pts.) A mass 2M moving at speed v0 collides head-on with another mass M initially at rest. The two stick
together. If the masses can move only along the x-axis, what is their final speed after collision? (Ignore friction.)
A. 0.333 v0 D. v0
B. 0.5 v0 E. 1.5 v0
C. 0.667 v0 F. 3 v0
b. (1 pt.) The collision in part (a) is:
A. elastic
B. inelastic
2. A 150-gram baseball traveling to the left at 45 m/s is hit by a bat. The ball leaves to the right at high speed.
a. (2 pts.) If the baseball receives an impulse of 17 kg·m/s from the bat, what is the baseball’s final speed?
A. 52 m/s D. 89 m/s
B. 68 m/s E. 98 m/s
C. 75 m/s F. 120 m/s
b. (2 pts.) If the baseball is in contact with the bat for 0.70 ms, what is the average force of the bat on the ball?
A. 390 N D. 5200 N
B. 770 N E. 8900 N
C. 1300 N F. 24,000 N
3. Computer hard disks with operating “speeds” of 7200 rpm (revolutions/minute) are now commonly available.
a. (2 pts.) Laptop manufacturers now advertise computers that can sense when they have been dropped. Before the
laptop strikes the ground, it “parksits hard disk by decelerating it from its normal operating speed to a complete
stop in just 0.25 s. What is the disk’s angular acceleration during this time period (assuming that the deceleration
is constant)?
A. –80. rad/s2 D. –1100 rad/s2
B. –180 rad/s2 E. –3000 rad/s2
C. –500 rad/s2 F. –11,000 rad/s2
b. (3 pts.) How many revolutions does the hard disk complete during the 0.25 s in part (a)?
A. 0.36 rev E. 52 rev
B. 2.4 rev F. 180 rev
C. 9.6 rev G. 320 rev
D. 15 rev H. 610 rev

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4. (2 pts.) An artist constructing a mobile (hanging sculpture)
wants to suspend a lightweight rod of length L with a 2.0-kg
object on one end and a 0.80-kg object on the other. In order
for the rod to be balanced, at what distance from the larger
mass should she suspend the rod? (Ignore the mass of the rod
itself; consider only the two masses.)
A. 0.12 L D. 0.29 L
B. 0.17 L E. 0.35 L
C. 0.24 L F. 0.42 L
5. A clock’s second hand completes exactly one rotation every minute.
a. (2 pts.) What is its angular velocity?
A. 0.10 rad/s D. 6.7 rad/s
B. 0.42 rad/s E. 27 rad/s
C. 1.7 rad/s F. 110 rad/s
b. (1 pt.) In which direction does its angular momentum vector point?
A. toward the “12” D. toward the “9”
B. toward the “3 E. into the clock face
C. toward the “6 F. out of the clock face
6. (1 pt.) The Earth spins into the east. If every person, animal, car, etc., on the surface of the Earth simultaneously
started walking or driving eastward, the Earth’s angular speed would:
A. speed up just a little
B. be unchanged
C. slow down just a little
7. (2 pts.) All of the following are true EXCEPT which one statement?
A. As a spinning ice skater pulls her arms and legs inward, her angular speed increases.
B. As a spinning ice skater pulls her arms and legs inward, her angular momentum remains constant.
C. Examples of gyroscopes include spinning toy tops, bicycle wheels, and the Earth itself.
D. A large rate of precession is a desirable quality in industrial and navigational gyroscopes.
8. (2 pts.) Why does a bicycle at rest fall over easily, while a bicycle in motion does not? (In fact, the faster the
bicycle is going, the more stable it becomes!)
A. Spinning wheels have greater angular momentum than stationary wheels.
B. Spinning wheels have greater moment of inertia than stationary wheels.
C. Spinning wheels have greater torque than stationary wheels.
D. Spinning wheels have a greater rate of precession than stationary wheels.
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