Steps 4 and 5 were covered in detail in Unit 3.

You now know somethingof

the four types of force that are required for the Statics part of this course
(weight, stiRncss, hydrostatic, friction), so I shall now look in more detail
at the first three steps.
Step 1 is very important. It is easy to misunderstand or to be confused by
any problem. That may be because the question is ambiguous, but is more
likely to be because you have not read the question carefully and
considered it. I usually find that it helps to make a sketch and to list the
relevant types of force. If you are unsure how to start, then draw a picture.
Personally, I would say that a problem was 'absorbed' when you could
describe the problem fully to someone else without reference (but without
needing to remember the numbers). Rapid and reliable understanding of
problems only comes with experience - in other words, when you have
done many similar problems before. This is why you can only become
good at solving Engineering Mechanics problems by practising problem
solving, and not just by reading the course Units.
Now, what about Steps 2 and 31 How do you choose the free body, and
how can you ensure that the forces are correctly shown?The best choia of
the free body depends upon foreseeing the forces that will act, so let me
comment on the forces first.
In a real engineering problem the significance of the various forces would
be assessed by e x p r i m or by making some preliminary calculations.
When you are faced with a 'papcr'question the situation is rather different.
You can no longer refer to reality, you only have the information that you
are given. The presentation of information in the question is the major clue
as to what is required of you. Also you should be aware of the 'nonnal'
procedure, for example neglecting friction at the pins of a pin-jointed
ture.

WydrwtlHe t o m If thoe are due to gas (ag air) then they an

nonnally negligible. except in obvious spedal cans such as baUoonr
and airships If they a n due to liquid (e.g. water) then they s h o d
m be negbted (tor cxamplc, water buoyancy should always In
included).
Wdgbtf- Tbemmg&cted in a n u m b a o f 'standard
problnns, sucb as the manben of a PJS. If appropriate infmatiol
is not given (mass, wdgbt or h) tben o & d y weight foras arc tc
b e o d d
St&rf- lbse are usually obvious in the mm that r
phydcsl member M d y then for the spec& purpose of pmvidinl
a force.
Meriooforec. Wbm motion betwesll two sudaco8 M posaibk
then friction M a possibility. Howcycl, it is oRen acgbcted (fo~
exampk, at pi~llof a PJS). QUC(I~~OBS requiring the Mumon o.
friction usually make this fairly expiicit by menticning the codkient
iriction or 0 t h relevant terms.
C
To determine the forces on a free body, I suggest that you consider first
the weight. The others will then be found by checking over the surface
of the free body for anything that it is touching. Figure 44 summarizes
the stiffness and friction forces that are 'discovered' when a free body is cut
out from its environment. The table may look rather complicated but it
is quite easy to decide what forces are required in any particular case.
It simply depends upon the movement that is possible at the joint. If
movement in one direction is prevented, then there will be a force in the
opposite direction to maintain equilibrium. However, if a joint allows
motion then them will be a friction force, although it may be possible to
neglect it because it is so small.
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