Lecture notes SWK122 Theme 2

Moment of force

Moment M of a force about point P is a vector
Magnitude
direction
rotational sense

IF
d 5




Rotational sense
coplanar forces all forces lie on same plane



clockwise moment axis to planeandlieson A


p

anticlockwise




MO rXF r FITZ rXFI TXFz


crossproduct between 2 vectors

AxB ABSINO



Mo rxF
FxFy Fz

Fyfe rafy I Irsefz rz.fr j rxfy ryf.scE

, FxF rFSino ID
M Fasino
1



resultant moment

MR 0
Fixed ITS Fs


FxF Fx FitFz

, SWK 122 Statics
Theme 2


Lecture Unit 2.1

Question 1


1.1 Hibbeler edition 15 Problem F4.3 page 140.

(a) Calculate the moment arm of the
60 kN force about point O.
(b) Calculate the moment of the 60 kN
force about point O.
(Recall that moments are vectors,
hence you have to specify the direc-
tion of the moment as well.) k

100 N
3 5

1.2 Hibbeler edition 15 Problem F4.1 page 140. c
4


(a) Calculate the moment arm of the OA 2m
100 N force about point O. O
α
(b) Calculate the moment of the 100 N
force about point O. 5m


1.3 Hibbeler edition 15 Problem F4.4 page 141. 50 N
100 mm
60o
Neglect the thickness of the member and
B
(a) calculate the moment arm of the 50 N
force about point O.
OB µ
(b) calculate the moment of the 50 N
m




45o
m




O
0




force about point O.
20




100 mm
d
n




1

, Pricipal of moments
1.1 a d 4 360545 1 5.121m Mo Foxy Fyxx
b
ME 8 5.1217 307.13KNM t anticlockwise



1.29 OA 1272 1512 Eg
sky 50

M ruxxtf.NU10015112
a 4 im
ÑÉ 1003 5
Ñm clockwise
M Fd 100714.67 460Nm clockwise




1.3a
or MEETYITY.to
i o.zcos4s to.n
501056010.25in45
a
11.25NM
9 100 100 20010545
314.42MM
D 200sinus
141.42MM
OB a b
369.551813




M dF 50110.2249

11.24 NM