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1. The rod on the power control mechanism for a business jet is subjected to a force of
80 N. Determine the moment of this force about the bearing at A.
60cos15.020sin8060sin15.020cos80 yx FF
AM
mNM A 71.7
(counterclockwise - ccw)
+
Problems (on Moment)
Fx
Fy
2. In order to raise the lamp post from the position
shown, force F is applied to the cable. If F=200 N,
determine the moment produced by F about point A.
q
mNM A 768.471615.23sin200
+
(counterclockwise - ccw)
B
A
Geometry:
6 m
3 m
C3 m
6 mCosine law:
m.BCcosBC 37710563263 222
Sine law:o15.23
sin
3
105sin
37.7 q
q
200sinq
200cosq
Problems (on Moment)
3. Calculate the moment Mo of the 250 N force about the base point O of the robot.
Problems (on Moment)
Fx = 250sin40 NFy = 250cos40 N
Fx
Fy40o40o50o
30o
0.4cos30+0.3cos40
0.5
0.4
sin
30
+0
.3si
n4
0
40sin3.030sin4.05.040sin25040cos3.030cos4.040cos250 oM
mNM o 55.189
+
(counterclockwise - ccw)
A
O
20o
60o
500 mm
400 mm
300 mm
4. The towline exerts a force of P=4 kN at the end of the 20-m-long crane boom. If x=25 m, determine
the position of the boom so that this force creates a maximum moment about point O. What is this
moment?
A
B
1.5
m
25 m
O
mkNM o 80204max
cos:/20cos5.1sin25805.1cos425sin4
q
𝑀𝑎𝑥𝑖𝑚𝑢𝑚 𝑚𝑜𝑚𝑒𝑛𝑡, ABOB
22sec4005.1tan25sec205.1tan25
075.397tan75tan225tan140025.2tan75tan625 222
2252
75.39722547575075.39775225tan
2
2,1
2 mmmm
om 426.561733.1
5067.1tan2,1
o574.33426.5690 q
5. The small crane is mounted along the side of a pickup bed and facilitates the handling of
heavy loads. When the boom elevation angle is q=40o, the force in the hydraulic cylinder BC is
4.5 kN, and this force applied at point C is in the direction from B to C (the cylinder is in
compression). Determine the moment of this 4.5 kN force about the boom pivot point O.
6. The 120 N force is applied as shown to one end of the curved wrench. If a=30o, calculate
the moment of F about the center O of the bolt. Determine the value of a which would
maximize the moment about O; state the value of this maximum moment.
A
O (70+150+70)=290 mm
F = 120 N
(25
+7
0+
70
+2
5)
=1
90
mm
30oIf a=30o, MO=?
190290
2570702530sin120701507030cos120
xy FF
oM
mNmmNM o 5.4141500
+
(clockwise - cw)
Determine the value of a which would maximize the moment about O. MOmax=?
A
O(70+150+70)
=290 mm
F = 120 N
(25
+7
0+
70
+2
5)
=1
90
mm
a
mNmmNM o 6.4185.41603290190120 22
aFor MOmax, F must be perpendicular to OA.
o2.33290
190arctan
a
a
aaaa
aa
o
o
o
FFd
dM
FFM
2.33
29.0
19.0tan19.0cos29.0sin0
19.0sin29.0cos
7. Assume that the hydraulic cylinder AB exerts a force Ԧ𝐹 of constant magnitude 2.5 kN
as the bin is elevated. Determine the moment of Ԧ𝐹 about the point O as a function of qfor the range 0≤ q ≤90o. At what angle q is this moment a maximum and what is themaximum moment?
Problems (on Moment)
8. The spring-loaded follower A bears
against the circular portion of the cam
until the lobe of the cam lifts the
plunger. The force required to lift the
plunger is proportional to its vertical
movement h from its lowest position.
For design purposes determine the angle
q for which the moment of the contact
force on the cam about the bearing O is
a maximum. In the enlarged view of the
contact, neglect the small distance
between the actual contact point B and
the end C of the lobe.
Problems (on Moment)
9. The pipe assembly is
subjected to an 80 N force.
Determine the moment of this
force about point A.
Also determine the minimum
distance from point A to the line
of action of force Ԧ𝐹.
Problems (on Moment)
FxFy
Fz
𝒓𝑪/𝑨
NFxy 28.6930cos80
NFF xyx 53.4440sin28.6940sin
NFF xyy 07.5340cos28.6940cos
NFz 4030sin80
kjiF
4007.5353.44
Fxy
Force in vector form
Position vector from point A to point C
ikijr AC
25.02.03.04.0/
kjir AC
2.04.055.0/
Moment of force 𝑭 about point A
FrM ACA
/
kjikjiM A
4007.5353.442.04.055.0
ijikjkM A
61.1091.81681.172219.29 kjiM A
38.1109.1339.5
𝒓𝑪/𝑨
Magnitude of the moment
FrM ACA
/
mNM A 16.1838.1109.1339.5 222
d: minimum distance
minimum distance from point A to the line of action of force 𝑭
q
q sin// FrFrM ACACA
q sin/ ACrd
F
A FdM
8016.18 d
𝒅 = 𝟎. 𝟐𝟐𝟕𝒎
kjiM A
38.1109.1339.5
10. The pipe assembly is secured on the wall by the two brackets. Determine,
a) the moment of force Ԧ𝐹 about point A,
b) the moment of force F about line AO,
c) the minimum distance from point B to the line AO
Problems (on Moment)
11. The spring which connects point B of the disk and
point C on the vertical surface is under a tension of 500 N.
Write this tension as it acts on point B as a force vector
and determine the moment Mz of this force about the shaft
axis OA.
𝑭spring𝑭spring=Fspring𝒏C/B
Coordinates : 𝐵 −150𝑠𝑖𝑛30, 150𝑐𝑜𝑠30, 600 𝑩 −𝟕𝟓, 𝟏𝟐𝟗. 𝟗, 𝟔𝟎𝟎𝑪(𝟑𝟓𝟎, 𝟑𝟎𝟎, 𝟎)
kjikji
Fspring
5.3977.11257.281
69.754
6001.170425500
Position vector from point O to point C (It may be written the position vector from point O to point B)
jir OC
3.035.0/
Moment of force 𝑭𝑠𝑝𝑟𝑖𝑛𝑔 about point O springOCO FrM
/
kjikjijiM O
02.4525.13925.1195.3977.11257.2813.035.0
Moment about the shaft axis mNkMnMM OOAOOA 02.45
12. The arms AB and BC of a desk lamp lie in a vertical plane that forms an angle of 30o with xy
plane. To reposition the light, a force of magnitude 8 N is applied as shown. Determine,
a) the moment of the force about point A,
b) the moment of the force about the axis of arm AB,
c) the angle between the line of action of force and line AB,
d) the perpendicular distance between the line AB and line of action of force.
30o
20o45o50o
45o
150 mm
AC
D
z
x
y
Direction CD is parallel to the z-axis and
lies in a parallel plane to the horizontal
plane.
mmBCmmAB 325,450
mmBCmmAB 325,450
k.j.i.F
N.CosCosF
N.SinF
N.SinCosF
kFjFiFF
z
y
x
zyx
325665931
32520458
665458
93120458
a)
k.j.i.
k.j.i.k.j.i.M
FrM
k.j.i.r
kSinSin.Sin.jCos.Cos.iCosSin.Sin.r
?M
A
A/CA
A/C
A/C
A
572163192
325665931283501104910
283501104910
3050325045450503250454503050325045450
mmBCmmAB 325,450
k.j.i.k.j.i. 1.81 M
Nm.k.j.i. k.j.i. M
k.j.i.r
rn
k.j.i.r
kSinSin.jCos.iCosSin.r
n M M
?M
AB
AB
AB
ABAB
AB
AB
ABAAB
AB
64028111350710610
811350710610572163192
350710610
159031802750
3045450454503045450
b)
c)
63114833
18350710610325665931
.Cos.
Cosk.j.i.k.j.i.
CosnFnF ABAB
q
q
m..Sin
.
FSin
Md
dFSinM
AB
AB
2490631148
811
q
q
d) If the force is resolved into two components parallel and normal to line AB, only normal component
generate a moment. The parallel component of a force to a line does not generate moment with respect to
that line.
z
13. Strut AB of the 1 meter diameter hatch door exerts a force of 450 N on point B. Determine
the moment of this force about point O. Line OB of the hatch door lies in the yz plane.
y
z
x
B
O
A
30o
30o
0.5 m
𝐴 0.5𝑠𝑖𝑛30, 0.5 + 0.5𝑐𝑜𝑠30, 0𝐵 0, 1𝑐𝑜𝑠30, 1𝑠𝑖𝑛30𝑨 𝟎. 𝟐𝟓, 𝟎. 𝟗𝟑𝟑, 𝟎𝑩(𝟎, 𝟎. 𝟖𝟔𝟔, 𝟎. 𝟓)
kji
kjiF
79.40184.5389.200
5.0067.025.0
05.0933.0866.025.00450
222
𝑭
y
z
x
B
O
A
30o
30o
0.5 m
kjiF
79.40184.5389.200
Position vector from point O to point B (It may be written the position vector from point O to point A)
kjr OB
5.0866.0/
kjikjM O
79.40184.5389.2005.0866.0
FrM OBO
/
kjiM O
97.17945.10086.374
! Obtain the same result using position vector 𝒓𝑨/𝑶
jiijjr OA
933.05.05.030cos5.05.0/
FrM OAO
/
Minimum distance from point O to line AB, d:
𝐹 = 450 𝑁 , 𝑀𝑂 = 𝑀𝑂 = 374.862 + 100.452 + 179.972 = 427.84 𝑁.𝑚
𝑑 =𝑀𝑂
𝐹= 0.951 𝑚
𝑭
14. Concentrated force is acting perpendicular to the crank arm BC at point C. For
the position q=70°, what is Mx , the moment of about the x axis? At this instant,
My =20 N·m and Mz =37.5 N·m.
150 mm
z x
200 mm
Aq
100 mm
B
O
y
P
C
Problems (on Moment)
q=70°, My =20 N·m and Mz =37.5 N·m Mx=?
zx
200 mm
Aq
B
O
y
P
200 mm
z
y
O, A
B, C
P
70o
kPjPP
cossin
kjirr OC
70cos20070sin200250/
kjir
4.6894.187250
kPjPkjiM o
cossin4.6894.187250
PrM o
iPiPjPkPM o
sin4.68cos94.187cos250sin250
C
q=70°, My =20 N·m and Mz =37.5 N·m Mx=?
mNmmNPPM x 294.2525294sin4.68cos94.187
mmNPM y 31020cos250 1
2
2
1
mmNPM z 3105.37sin250
3
3
1020
105.37
cos250
sin250
P
P 875.1tan
93.61
NP 170
iPiPjPkPM o
sin4.68cos94.187cos250sin250
15. Rectangular plate ADCF is held
in equilibrium by string AB which
has a tension of T=14 kN.
Determine,
a) The magnitude of the moment of
about the axis DC,
b) The perpendicular distance
between lines AB and DC.
z
x
y
A (0, 5, 3) m
D (0, 2, 2)
C (4, 6, 0) m
B (6, 3, 0) m
FO
Problems (on Moment)
z
x
y
A (0, 5, 3) m
D (0, 2, 2)
C (4, 6, 0) m
B (6, 3, 0) m
O kjikji
n
kjiT
kjinTT
DC
T
3
1
3
2
3
2
6
244
6412
7
32614
a) The magnitude of the moment of about the axis DC
m kN..- M
kjikjinMM
kji
kjijiTrM
M
DC
DCCDC
CBC
DC
6710339812
3
128
3
212
3
218
3
1
3
2
3
2 281218
281218
641232
?
/
b) The perpendicular distance between lines AB and DC
67.58
52.042
22cos ,cos)1(14
3
22
3
22
3
16
3
24
3
212
cos
3
22 , 6412
q
qDCDC
DC
nTnT
kjinkjiT
The parallel component of a force to a line does not generate moment with respect to that line.
If is resolved into two components parallel and normal to line DC, thenT
mθT
MθTM DC
DC 89.067.58sin14
67.10
sind , dsin
The magnitude of moment MDC is equal to the normal component of to line DC and the
perpendicular distance between and line DC.
T
T
z
x
y
A (0, 5, 3) m
D (0, 2, 2)
C (4, 6, 0) m
B (6, 3, 0) m
O