Ph 111.01
October 22, 2008
PS 8b (21)
M
R
α
Problem 1 = Walker P. 117.
A torque of
1.14
N
·
m is applied to a bicycle
wheel of radius
27
cm and mass
0.66
kg. Treating the wheel as a hoop, find
its angular acceleration.
rad/s
2
Solution.
The equivalent to
F = ma
for rotational motion is
I
τ
α
=
.
We can solve for the angular acceleration.
But first, we need the
moment of inertia.
We use the form that applies to a hoop:
()
(
)
2
22
0.66 kg
0.27 m
0.0481 kgm
IM
R
==
=
Then, the angular acceleration is
( )
2
2
1.14 Nm
23.70 radians/s
0.0481 kgm
I
=
(Sounds fast . . . .)
Problem 2 = Walker P. 1111.
A wheel on a game show is given an initial
angular speed of
1.31
rad/s. It comes to rest after rotating through 3/4 of a
turn.
M
R
α
(a) Find the average torque exerted on the wheel given that it is a disk of
radius
0.78
m and mass 6.4 kg.
N
·
m
(b) If the mass of the wheel is
halved
and its radius is
doubled
, will the
angle through which it rotates before coming to rest increase, decrease, or
stay the same?
increase
decrease
stay the same
Explain. (Assume that the average torque is unchanged.)
Solution.
(a)
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 Fall '08
 Staff
 Acceleration, Force, Mass, Moment Of Inertia, Angular Acceleration, R M, Walker P.

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