Chapter 11
4.
Picture the Problem
: The arm extends out either horizontally and the weight of the crab trap is
exerted straight downward on the hand.
Strategy:
The torque equals the moment arm times the force according to equation 113.
In this case
the moment arm is the horizontal distance between the shoulder and the hand, and the force is the
downward weight of the crab trap.
Solution:
Multiply the moment arm by the
weight:
Insight:
If the man bent his elbow and brought his hand up next to his shoulder, the torque on the
shoulder would be zero but the force on his hand would remain 35 N or 7.9 lb.
14.
Picture the Problem
: The fish exerts a torque on the fishing reel and it rotates with constant
angular acceleration.
Strategy:
Use table 101 to determine the moment of inertia of the fishing reel assuming it is a
uniform cylinder (
2
1
2
MR
).
Find the torque the fish exerts on the reel by using equation 111.
Then
apply Newton’s Second Law for rotation (equation 114) to find the angular acceleration and
equations 102 and 1010 to find the amount of line pulled from the reel.
Solution:
1. (a)
Use table 101 to find
I
:
(
29
(
29
2
2
2
1
1
2
2
0.84 kg
0.055 m
0.00127 kg m
I
MR
=
=
=
×
2.
Apply equation 111 directly to find
τ
:
(
29
(
29
0.055 m
2.1 N
0.12 N m
r F
=
=
=
×
3.
Solve equation 1114 for
α
:
2
2
0.12 N m
92 rad/s
0.0013 kg m
I
×
=
=
=
×
4.
(b)
Apply equations 102 and 1010:
(
29
(
29
(
29
(
29
2
2
2
1
1
2
2
0.055 m
92 rad/s
0.25 s
0.16 m
s
r
r
t
θ
=
=
=
=
Insight:
This must be a small fish because it is not pulling very hard; 2.1 N is about 0.47 lb or 7.6
ounces of force.
Or maybe the fish is tired?
20.
Picture the Problem
: The person lies on a
lightweight plank that rests on two scales as
shown in the diagram at right.
Strategy:
Write Newton’s Second Law in the
vertical direction and Newton’s Second Law
for rotation to obtain two equations with two
unknowns,
m
and
cm
x
.
Solve each to find
m
and
cm
x
.
Using the left side of the plank as
the origin, there are two torques to consider:
1. (a)
Write Newton’s
Second
Law in the vertical direction to
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 Fall '08
 Ashkenkai
 Physics, Angular Momentum, Force, Work, Moment Of Inertia, Rotation

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