PHY126 HW Set 2 (Chap.11)
Problem 11.42
Description:
A physics student is standing on an initially motionless, frictionless turntable with
rotational inertia 0.31
. She's holding a wheel of rotational inertia 0.22
spinning at 130
about a vertical axis, as in the figure. When she turns the wheel upside down, student and turntable
begin rotating at 70
.
Part A
What is the student's mass, considering her to be a cylinder 30
in diameter?
Express your answer using two significant figures.
Part B
How much work did she do in turning the wheel upside down? Neglect the distance between the axes
of the turntable and wheel.
Express your answer using two significant figures.
Solutions
(a) Since the turn table is frictionless, there are no external torques about its axis, and the z component of
angular momentum is conserved. Initially this is all spin of the wheel,
When the wheel is inverted, the student and turrable acquire anugular momentum,
such that
or
rpm).
130
)(
m
kg
22
.
0
(
2
0
⋅
=
L
z
L
0
0
L
L
L
z
=
−
.
2
0
L
L
z
=
If
is the total moment of inertia about the z-axis,
or
Now
tot
I
rpm),
130
)(
m
kg
2(0.22
rpm)
70
(
2
⋅
=
=
tot
z
I
L
.
m
kg
817
.
0
2
⋅
=
tot
I
tot
I

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