EF 151 Exam #4, Spring, 2011
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Name:
Section: ________
Guidelines:
• Assume 3 significant figures for all given numbers unless otherwise stated
• Show all of your work – no work, no credit
• Write your final answer in the box provided
• Include units for all answers
• Include directions for all vectors
• Please remain seated if there are less than 5 minutes to go in the exam so as not to
disturb those still trying to finish
Constant Acceleration
Uniform Circular Motion
an – centripetal acceleration
ȡ – radius of curvature
Ȧ – rotational speed
T – period
f – frequency
– angle
EF 151 Exam #4, Spring, 2011
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1. (4 pts) A business major tries to loosen a bolt by pushing as shown. What is
the direction of his moment vector?
A. -cos(50)i, -sin(50)j
B. +cos(50)i, +cos(50)j
LJ
C. +k
&
D. –k
Parallel Axis Theorem
2. (4 pts) You sit on a rotating platform with your arms drawn in. What happens when you stretch your arms
outward?
Torque and Acceleration
A. The rotation speed is unchanged
dž
B. The rotation speed decreases
Torque
C. The rotation speed increases
(any curve)
Center of Mass
Impulse / Momentum
3. (4 pts) A hollow sphere and a solid sphere are released from the same height at the same time and roll down a
hill. Which one will reach the bottom first?
A. The hollow sphere
Work, Energy, Power
B. The solid sphere
C. Can’t tell – depends upon the radius of each one
D. Can’t tell – depends upon the mass of each one
E. Can’t tell – depends upon the radius and the mass of each one
4. (4 pts) Which value for ș will cause the greatest torque about point A?
A. 0°
B. 30°
C. 60°
ș
D. 90°
E. 120°
60°
F, 150°
A
G. 180°
5. ( 4 pts) A rotating object has a angular speed of Ȧ. What is its speed if you double its kinetic energy?
A. 4Ȧ
B. 2Ȧ
C. 1.4Ȧ
D. Ȧ
E. 0.7Ȧ
F. 0.5Ȧ
G. 0.25Ȧ
EF 151 Exam #4, Spring, 2011
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5. (16 pts) An electric motor spins at a constant speed of 250 rpm while lifting a 0.50 kg weight a distance of
1.2 meters in 7.0 seconds. What is the torque exerted by the motor?
EF 151 Exam #4, Spring, 2011
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7. (16 pts) A tennis ball (hollow sphere) starts from rest at
point A and rolls without slipping down a ramp as shown.
What is the speed of its center of mass at point B?
(Hint: you do not need the mass or radius for this problem)
A
/
B
2.5m
0.8m
2.5m
6. (16 pts) What is the value for Y if the center of mass of the
object is at point C?
(assume uniform thickness and density)
30 N
12.12m
8. (16 points) An Easter egg shaped plate can rotate
freely about its center of mass located at the origin of
the coordinate system. It has 3 forces applied as
shown. The egg’s mass moment of inertia about its
2
center of mass is 4.2 kgym . The point coordinates for
the tail of each force vector is given in meters. What is
its angular acceleration? (Use CCW as positive)
C
2m
80 N
Y
25°
y
[0,1.4]
[-1.2, 0]
x
[2.5, 0]
1.5m
7.53m
60 N
EF 151 Exam #4, Spring, 2011
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9. (16 pts) A car starts from rest and drives around a circular track, increasing its speed at a constant rate. It
takes 45 seconds to complete 2 laps with a final speed of 88 ft/s. What is the radius of the track?
0.5 m
10. (16 pts) Determine the mass moment of inertia about
point A for this 18 kg sign that is a rectangular plate of uniform
thickness and density.
A
1.6 m
2.4 m