1
PS ID _______________ LAU 1321 Quiz 3 Name (Last, First) __________________
Instructions: No SCANTRON will be used for this quiz. Mark your answers clearly in the quiz. Each
problem is worth 1 point. You don’t need to write down your CougarNet ID.
Given: The moment of inertia of a solid cylinder of mass M radius R is MR2/2.
The moment of inertia of a solid sphere of mass M radius R is 2MR2/5.
Acceleration due to gravity g=9.80 m/s2
1) An ice skater doing a spin pulls in her arms, decreasing her moment of inertia by a factor of
two. How does her rotational kinetic energy change?
A) It is reduced by a factor of two.
B) It is reduced by a factor of four.
C) It increases by a factor of two.
D) It increases by a factor of four.
E) It doesn't change.
Answer: C
Solution
22
2
( / 2)
11
22
2 2 2
i i f f
ii
f i i i
fi
i
f i i i
II
II
II
I
KI
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2) A uniform ball with a mass of 125 g is rolling without slipping along the horizontal surface of
a table with a speed of 4.50 m/s when it rolls off the edge and it falls towards the floor, 2.20 m
below. What is the rotational kinetic energy of the ball just before it hits the floor?
A) 0.506 J
B) 0.729 J
C) 1.05 J
D) 2.61 J
E) This question cannot be answered
without knowing the radius of the ball.
Answer: A
Solution
The rotational energy does not change after the ball rolled off the table.
2 2 2 2 2 2
2
1 2 / 5( ) 2 / 5( ) ( / ) 2 / 5( ) (0.125)(4.5) 0.506
2 2 2 2 5
m r m r r m
K I J
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3) A 6.0-m radius playground merry-go-round with a moment of inertia of 2000 kg∙m2 is
rotating freely with an angular speed of 2.0 rad/s. Two people, each having a mass of 50 kg, are
standing right outside the edge of the merry-go-round and step on it with negligible speed. What
is the angular speed of the merry-go-round right after the two people have stepped on?
A) 0.20 rad/s
B) 0.40 rad/s
C) 0.60 rad/s
D) 0.80 rad/s
E) 0.71 rad/s
Answer: E
Solution
