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Copyright 2001 Professional Publications, Inc.
Statics Practice Problems  1
STATICS PRACTICE PROBLEMS
1.
All joints in the truss shown connect pinned axial members. Determine the force in member
BD.
(A) 250 N (Compression)
(B) 250 N (Tension)
(C) 450 N (Tension)
(D) 600 N (Compression)
Solution
Take the moments about point C.
()
(
)
(
)
(
)
CA
v
400 N 3 m
6 m
600 N
4 m
0
MR
∑
−+
=
Solve for
Av
R
,
the vertical reaction at point A.
Av
600 N
R
=
Use the method of sections. Cut the truss at members BC, BD, and AD. Treat the force in
member BD as an external force.
Take the moments about point D.
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Statics Practice Problems  2
()
(
)
(
)
(
)
D
400 N 3 m
600 N
6 m
4 m BC
0
M
∑
=−−
=
BC
600 N (Compression)
=−
Take the moments about point B.
(
)
(
)
B
600 N 3 m
4 m AD
0
M
∑
+
=
AD
450 N (Tension)
=
Take the sum of the forces.
BC+AD+BD
600 N
450 N
BD
0
xx
x
F
∑
==
−
+
+
=
BD
150 N (Tension)
x
=
Av
Load
BD
600 N
400 N
BD
0
yy
y
FR
∑
−
=
−
−
=
BD
200 N (Tension)
y
=
Use the Pythagorean theorem.
( )
22
BD
150 N
200 N
250 N
=+
=
Answer is (B).
2
. A 4 m ladder having a mass of 18 kg is placed against a wall as shown. The coefficient of
friction,
m
, between the ladder and the wall is 0.20. When an 80 kg painter reaches a point 2.5 m
from the lower end at A, the ladder begins to slip. Determine the friction coefficient between the
ladder and the floor.
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°
Copyright 2001 Professional Publications, Inc.
Statics Practice Problems  3
(A)0.30
(B) 0.31
(C) 0.33
(D)0.35
Solution
°
µ
Calculate the gravitational forces corresponding to the masses.
()
2
m
18 kg
9.81
176.6 N
s
⎛⎞
=
⎜⎟
⎝⎠
2
m
80 kg
9.81
784.8 N
s
=
Take the sum of the moments about point A.
(
)
(
)
(
)
(
)
A
B
B
176.6 N
2 m sin30
784.8 N
2.5 m sin30
4 m
cos30
0.2 4 m
sin30
0
MN
N
∑
=°
+
°
−
°
−°
=
B
299.6 N
N
=
Sum the forces in the
y
direction.
(
)
AA
176.6 N784.8 N
0.2 299.6 N
0
FN
∑
=−
+
=
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Statics Practice Problems  4
A
901.5 N
N
=
Sum the forces in the
x
direction.
AB
0
x
FN
N
µ
∑
=−
=
299.6 N
0.33
901.5 N
==
Answer is (C).
3.
The crate shown has a mass of 20 kg and is resting on a 30
°
incline. The coefficient of static
friction between the crate and the incline is 0.35. What is the minimum force
P
that will push
the crate up the plane?
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This document was uploaded on 12/23/2010.
 Spring '10

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