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3 Components of forces
3.1 Components and addltlon of forces
Imagine that a friend is pushing my car to help to start it. If he did not push exactly horizontally, how much f o r a would act horizontally? Figure
35 shows my friend applying the f o r a F, which in this case is 400N
inclined at U)" above the horizontal. I have chosen a co-ordinate system with X horizontal and y vertical.
Figure 35
You saw in the previous section how to add forces together to find the resultant. In this case we are going to do the opposite: replace the single
force (resultant) by two (the components) that add up to make it. As you
will see, this is particularly useful when forces do not act conveniently in
simple vertical or horizontal diitions. Any f o r a Facting in a direction B
above the horizontal can be replaced by two forces, F, and F,, which act at right angles to each other; F, in the horizontal & i o n and F, in the
vertical direction. The two foras add vcctorially to make F, as you can
easily confirm (Figure 36). and the magnitudes are F, = F cos 8 and F, = F cos(90° - 8) = F sin 8
I hope you will agree that a f o r a acting vertically is not going to move my
car along the road. F. is called the mu? of Fresolved in the x direction, or
more briefly the x-&mponent o i F. The part of the force that acts horizontally in this case is, therefore,
F, = 400 cos 30" = 346 N or F, = 346 N+
Another friend takes over pushing my car, and applies a force G = 500 N.
inclined at 20" above horizontal. Evaluate C..
U we are interested in the distana that the back of the car was k i n g
moved up or down by the pushing, we would be interested in, amongst other things, the vertical components of F and C, in this case F, and G,. For example F,=400cos6OQ=200N F 9 = 2 0 0 N t As indicated above, we could also calculate F, as
SA0 Sl Find the vertical component of the force applied by my second friend.
F m s (80 - 0 )
Figure 36 Fwae
Now, what if we want to find the total f o r a exerted by the two people in the hoiizontal d i i t i o n when they are pushing together? Say we find the
resultant of the two foras R = F+ C. Then we could resolve that total
force horizontally. Can you see an easier way with the information you
already have?
R. Flgure 38 Direction ojresultanr
Figure 37
The easier way is simply to add the horizontal components of the separate forces. Look at the vector diagram in Figure 37. This diagram should make it clear to you that for a vector addition
It is also true that
R.=Fx+G,
and
R,=Fy+Gv
We can evaluate each of these components from the information given:
and R y = F , + G , = 2 0 0 + 1 7 1 = 3 7 1 N
As we now have the magnitudcs of the two rectangular components of R
we can immediately write that, for the right-angled triangk in Figure 38,
R' = R,' + R:
and that the angle 8, Figure 38, is given by
8 = tan-' (Rv/R,)
In this casc
R = (816' + 371')'1a = 896 N
SA0 Sd
At what angle does R act?
What you have just been doing is to add the form using their compo-
nents. This is the method that I should like you to uae, in p n f m n a
to drawing. With practice and a suitabk calculator it is quick and
accurate. The component method can also bc used for any numtcr of
forces. Several other jobs are also easily done by components, so it is a
technique that you must become familiar with.
In this case the total force is acceptable (leas than 5 kN), but the X-
component is not.
The d i i t i o n of this force is given by tan-'(R,/R,). Finding
tan-' (-197814094) on my calculator gives an answer -26" (to two X (^) sigoificant figurn). This is to be expected. R, is negative and R, is positive,
R,= + W N so the resultant^ R^ will^ be^ in^ the fourth quadrant (Figure 41). Angles^ am
measured anticlockwise from the positive x-axis, so -26" indicates an
h=- 1 9 7 8 ~ angle^ 26"^ clockwise^ from that^ axis.^ But is this the only answer?^ In^ fact^ then
are two angles whose tangent is -0.4831 (i.e. -1978/4094) and your calculator has only given you one of them. The other is 180" larger than Fimre 40
the one given by a calculator. The tangent of 154" (ie. 180"- 26") is also
-0.4831. Try it. Simply relying on tan-' (RJR,) to find the direction of the resultant might mislead you because it will have two answers and your calculator will only give one of them. You have to look at directions of R, and R, to determine which quadrant the resultant is in.
Sign of resultant Resultant is components in quadrant: R. RY
@ -.m
positive positive (^) POSII~VO negative positive 2 trig~mmnric negative negative functions 4
lrnnernonic C A S T 3 ) oositive nepative Figure 4 I If you can do the following SAQ you can be reasonably conMent about components.
SA0 86
A pylon supports a number of cables (Figure 42) with the horizontal forces shown being exerted on the pylon. (a) Find the resultant (magnitude and direction). Your specification of direction should include the sense.
(b) If an extra cable is to be added, to balance the existing ones, specify the
direction and tension renuired. F,-7WN F , = 2 W N Figure 42 Pylon, plan view The extra force required to bring a set of forces into equilibrium, as in SAQ
- 36, is called the equilibrant.
3.2 Flndlng unknown forces
Consider the parking of a car facing down a hill. How strong must the handbrake be to prevent the car running away? To be more specific, my car has a mass of about 1100 kg. If the manufacturers intend to design for
the handbrake to hold on a hill of slope 20" (about 1 in 3). what forward
force must the handbrake resist? First note (Figure 43) that on a slope of U)", the angle between the weight vector and the perpendicular to the road
is also 20". I am interested in potential movement along the road so I shall
choose my X co-ordinate down the road, parallel to the surface, and y
Figure 43 perpendicular to x (Figure 44). So what force tries to push the car down
the road? It is the component of W in that direction - along X.
W,= Wcos 70" = 1100 kg X 9.81 N kg-' X cos 70°= 3691 N
= 3.69 kN
This is the force that the brake has to be able to resist. That was an example of a very simple problem in which the idea of
components was useful. Are components useful h the analysis of more
complicated problems, such as pin-jointed structures? They certainly are.
Look at the simple PJS in Figure 45, the top of a hoist. It is static and,
therefore, in equilibrium. All the forces on the pin at E are balanced: the resultant is zero. We are faced with the problem of determining the forces
on the members, given as usual that the applied force W, the weight of the
load, makes the members' own weight insignificant, so that the members exert forces directed along their centre-lines. (I shall come back to this later in the Unit.) The key to the use of components here is that if the pin at
E is in equilibrium, then the resultant of the forces acting on the pin is zero,
so the component of the resultant in any direction must be zero. How can we apply this knowledge in this case? Member 1 is horizontal, and therefore exerts a horizontal force on the pin at E. Now please consider the following questions (before looking at the answers).
Quearion: (a) What is the vertical component of the force exerted by member l? (b) What do you know about the total of all the vertical components on the pin at E? (c) What can you deduce about the vertical component of the force exerted by member 2?
Answer: (a) Zero^ -^ a horizontal force has zero vertical component.
(b) The vertical components must sum to zero (equilibrium).
(c) Member 2 must have a vertical component which exactly
balances the weight W- it gets no help from member 1 in the
vertical direction. You should be able to see that member 2 must balance the weight by pulling upwards. Because it is inclined it must also he pulling to the left, so member 1 must push to the right to complete the balance.
SA0 37
Figure 46 shows the free-body diagram for the pin at E in Figure 45.
(a) Write down the equation relating F, to W which represents the
vertical equilibrium of the pin.
(b) Express the magnitude F, in terms of W.
(c) Write the equation representing horizontal equilibrium of the pin at E.
(d) Express F, in terms of F,.
(e) Express F, in terms of W.
(I) State whether the members are in tension or compression.
You will notice that in SAQ 37 it was possible to find two unknown forces
in terms of a third. Actually you found two magnitudes - the orientations
were already known. This is related to the fact that a point (the pin) has
two degrees of freedom in a plane - the methods of simple statics can be
used on a pin to find two unknowns, which may either be two magnitudes or a magnitude and a diction.
SA0 56
Can you see some other combination of two unknowns?
Figure 46
What you are really learning to do in this section is to solve those two problems for concurrent forces. If the unknowns are a magnitude plus direction then you simply find the resultant of the known forces, and the unknown one must be equal and opposite to it. The other problem, when the directions are known but magnitudes unknown, is the essence of solving pin-joint structures. In fact more complicated PJS problems really
(a) F, was positive, F, was negative. What is the meaning of these signs? 57.7 N
(b) What a n the forces in members 1 and 27
(C) If the load W was doubled, what would happen to F, and F,?
Figure 50 shows the actual forces acting.- NW
The example above should have made the method reasonably clear. Compare the example with the procedure given below and make sure that 51.7^ N each step is clear. The method I am showing you is not the only one, but I believe it is the one that you will find most convenient. I suggest that you
perform step 9 to 1WN m .,-:;:- Figure 50
Ploccdmsfor &&g mo anmown force mngoihnia crhen tbe forces '
e c t i q r t a p o h t u c i n ~
1 Draw the appropriate free-body diegram with known fo
2.,lx:akark.,.,,,,,,. .....' : ~ l eW&..-&& .,,z brcw (pd&tg .,..,,...:.. corre8pgi&g to tensi
.;;:Mum&..... .. the :-bers fOf&',';;::
3 Choose the first axis X along one unknown for&, and mark a
positive direction (If the= is a horizontal or vertical force, use
it,) 4iii!~howo,..,.,, ,, .... t k. & y d: .: 2 ,..,,... axis y p.qqd@lar- ,,'.'.. to the..&&.. ..... ,,. F ."-M&all rekg+,$9ngks. this stop4i,rbnc to error;)
6 Along the positiie diredim of & second exin, s'A components
to ztro. Evaluate one unknown force.
7 , ,Along the positiv~direction of thejirst exk,S
.';:;::^ i-ro, evaluate, ,,,,^ ....&other: unkno& , , ...::.^ force. ,. 8"::ira* ,... .. kiii'gZforcsa teil& ...::.: ,,,, w@jye ...
dew, .sketoh;frscbody d i a p m showing the
the actual direction with n W c a l valuds.
9 Check the solution by venioal and horbmtal equilibrium of I
SA0 40
Sketch the f o r m that would appear on the free-body diagram of each of
the pins in Figure 51.
SA0 41
For the figures of SAQ 40, choose the best first axis orientation.
Now here are three 'complete' SAQs. You are expected to do some simple free-body diagrams of your own. Please use the procedure steps. If you are stuck, see the SAQ solution for that step only, them try to carry on.
SA0 42
F i d the foam in the members of Figure 52, supporting a hoist and load totalling 75 kg.
\
'.
Figure 54
SA0 43
Figure 53 shows a car sissor jack supporting half the weight df a typical
car (total mass 1101 kg) on a slope. Members 3, 4 and 5 form a rigid
base for 1 and 2. Find the forces in the members 1 and 2.
Figure 53
SA0 U
Figun 54 shows a pin connecting thm members. Member 1 is known to be in wmpnssion, exerting a force of 200 N. The other two members maintain equilibrium; find the forces within them.
3.3 Summary The use of components provides a very useful approach to problem solving. Concurrent resultants may be found by summing rectangular components. If a set of wncumnt f o r m is known to be in equilibrium, then it is possible to determine either one force (magnitude and direction) since it must be the equilibrant to the resultant of the known forces, or two forces
(magnitude and sense) if the orientations are known, such as in pin-joint
structure models.
F o r e components also help to provide physical insight into equilibrium, and are also important ideas in the solution of dynamic problems (Block 4).
