Muddy Card Responses Lecture M2
Can you describe what the physical meaning of a moment is, and how do you choose r in
rxF.
A moment (or more correctly a moment of a force) is the effect of a force acting about an
axis to cause an angular acceleration (or rotation). r is to some extent arbitrary.
It is the
position vector of some point on the line of application of the force F.
What are i and j in the force moment equations, why do you use this notation:
R
ij
is the
force acting on particle i due to particle j. I use the superscripts because I reserve a subscript
to define a tensor quantity (which we will see later in the term). r
i
is the position vector of
point i.
i
When we write
∑
r
i
×
F
+
M
i
=
0
, what is M
I
? It is a pure moment. In reality a pure
moment usually results from a force couple (i.e. two equal and opposite forces with parallel
lines of action). Given this it can be argued that the M
I
is superfluous. Note in the particular
example I did on the board there were no pure moments applied to the system, so there were
no M
I
terms.
I missed the derivation that resulted in the sum of moments in a system being equal to the
sum of the external moments?
Confused as to why moments cancel?
Please explain why.
r
1
×
R
1
3
+
r
3
×
R
3
1
=
0
?
Given that
R
13
=
−
R
31
and that the two forces have the same line of action this implies that
r
1
×
R
1
3
=
−
r
3
×
R
3
1
.
Note that this is only true when the particles in the system are connected
by the internal reactions. It is not true of an arbitrary system of particles where there are no
This preview has intentionally blurred sections. Sign up to view the full version.
View Full Document
This is the end of the preview.
Sign up
to
access the rest of the document.
 Winter '12
 CharlesColeman
 Force, Aeronautics, Astronautics, Righthand rule

Click to edit the document details