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5
Chap. 11, HW2
Questions
7.
The angular momenta
()
t
l
of a particle in four situations are
(1)
3
:
4
3
=
=
+
=
dt
dl
t
l
τ
(2)
t
dt
dl
t
l
12
:
6
2
−
=
=
−
=
(3)
0
:
2
=
=
=
dt
dl
l
(4)
2
/
4
:
/
4
t
dt
dl
t
l
−
=
=
=
In which situation is the net torque on the particle
(a) zero:
(3)
(b) positive and constant:
(1)
(c) negative and increasing in magnitude (
t
> 0):
(2)
(d) negative and decreasing in magnitude (
t
> 0):
(4)
8.
Figure 1128 gives the angular momentum
magnitude
L
of a wheel versus time
t
.
Rank the four
lettered time intervals according to the magnitude of
the torque acting on the wheel, greatest first.
Since
dt
dl
=
, the torque is equal to the slope
Ranking: D,
B,
then A and C tie at 0 .
10.
A rhinoceros beetle rides the rim of a horizontal disk rotating
counterclockwise like a merrygoround. If the beetle then walks along the rim in
the direction of the rotation, will the magnitudes of the following quantities (each
measured about the rotation axis) increase, decrease, or remain the same (the disk
is still rotating in the counterclockwise direction):
(a) the angular momentum of the beetle–disk system,
Stays the same.
There are no external torques
(b) the angular momentum and angular velocity of the beetle
Increases.
The beetle is going faster
(c) the angular momentum and angular velocity of the disk?
Since the beetle angular momentum increases, the disk angular momentum
decreases
(d) What are your answers if the beetle walks in the direction opposite the
rotation?
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 Spring '08
 oshea

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