ENGR 30
STATICS
Discussion Session 5&6
Winter 2018
1

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Two-force member
•
Requiring that the sum of forces and total moment
in any direction be zero leads to the conclusion that
F
1
and
F
2
must have equal magnitude but opposite
sense, and on the same line.
Three-force member
•
Since the rigid body is in equilibrium, the sum of the
moments of
F
1
,
F
2
, and
F
3
about any axis must
be zero. It follows that the moment of
F
3
about
D
must be zero as well and that the line of action of
F
3
must pass through
D
.
The
lines of action of the three forces must
be concurrent or parallel.
7
Fundamental Concepts

Sample Problem
A
slender
rod
of
length
L
is
attached to collars that can slide
freely
along
the
guides
shown.
Knowing
that
the
rod
is
in equilibrium,
derive
an
expression for the angle
θ in terms of
the angle β.
W
A
G
B
θ
β
β
A
B
O
W
A
B
tan
L
cos
2
L
sin
2
tan
GB
L
sin
2
GO
L
cos
Therefore
:
tan
GO
Where
:
1
1
1
Fromthe force
geometry
:
GB

Sample Problem
Two force member, the force at reactions D and C must pass points
DC
B
x
B
y

Fundamental Concepts
•
Idealized 3D structural supports
5

Fundamental Concepts
•
Idealized 3D structural supports
(for some problem, the couple-moment must be assumed zero to provide statical determinacy, if
there are no active forces on that directions)
6

Fundamental Concepts
•
Idealized 3D structural supports
7