Systems of Forces and Moments
•
Moments – rotation at a point due to a force applied at some distance
Sign Convention
Clockwise = negative
Counterclockwise = positive
M
P
= DF

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In terms of components…
F
x
y
y
x
P
F
D
F
D
M
+
=
∑
y
F
x
D
y
If line of action goes through P…
D
x
then M
P
= 0

Problem 4.24
– The tension in the cable is the same on both sides of the pulley.
The sum of the moments about point
A
due to the 800
‐
lb force and the forces
exerted on the bar by the cable at
B
and
C
is zero. What is the tension in the
cable?

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The weights
W
1
and
W
2
are suspended by the cable system shown. The cable
BC
is horizontal. If the largest moment that can be resisted by the pole
DP
at
point
P
is 100 lb
‐
ft, determine
W
1
.