6
6
Chap. 10 HW2 Questions
5.
Figure 10-25
a
is an overhead
view of a
horizontal bar
that can pivot;
two horizontal
forces act on the
bar, but it is
stationary. If the angle between the bar and
F
2
is now decreased from 90° and the
bar is still not to turn, should
F
2
be made larger, made smaller, or left the same?
When the angle is decreased the torque due to
F
2
is now reduced.
Therefore
F
2
should be made larger.
9.
Figure 10-28 shows three flat disks
(of the same radius) that can rotate
about their centers like merry-go-
rounds. Each disk consists of the same
two materials, one denser than the
other (density is mass per unit
volume). In disks 1 and 3, the denser
material forms the outer half of the
disk area.
In disk 2,
it forms the inner half of the disk area.
Forces with identical magnitudes are
applied tangentially to the disk, either at the outer edge or at the interface of the
two materials, as shown.
Rank the disks according to (a) the torque about the disk
center, (b) the rotational inertia about the disk center, and (c) the angular
acceleration of the disk, greatest first.
a) All the
F
’s are the same and are directed tangentially.
The distances
from the center varies leading to different torques.
The ranking is 1 and 2 tie, then 3.
b) More mass further from the center leads to larger moment of inertia.
The ranking is 1 and 3 tie, then 2
c) Since 2 has the smallest moment of inertia and one of the largest torques,
its angular acceleration is largest.
Since 3 has one of the smallest moments of inertia and the the smallest
torque, its angular acceleration is smallest.
The ranking is 2, 1,3

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