Irregular areas, volumes and mean values of waveforms 205
Problem 2. A river is 15m wide. Soundings of
the depth are made at equal intervals of 3m across
the river and are as shown below.
Depth (m) 0 2.2 3.3 4.5 4.2 2.4 0
Calculate the cross-sectional area of the flow of
water at this point using Simpson’s rule.
From para. (d) above,
Area=^13 ( 3 )[( 0 + 0 )+ 4 ( 2. 2 + 4. 5 + 2. 4 )
+ 2 ( 3. 3 + 4. 2 )]
=( 1 )[0+ 36. 4 +15]=51.4m^2
Now try the following exercise
Exercise 82 Further problemson areas of
irregular figures
- Plot a graph ofy= 3 x−x^2 by completing
a table of values ofyfromx=0tox=3.
Determine the area enclosed by the curve, the
x-axis and ordinatex=0andx=3by(a)the
trapezoidal rule, (b) the mid-ordinate rule and
(c) by Simpson’s rule. [4.5square units] - Plot the graph ofy= 2 x^2 +3 betweenx= 0
andx=4. Estimate the area enclosed by the
curve, the ordinatesx=0andx=4, and the
x-axis by an approximate method.
[54.7square units] - The velocity of a car at one second intervals is
given in the following table:
timet(s) 0 1 2 3 4 5 6
velocity
v(m/s)
0 2.0 4.5 8.0 14.0 21.0 29.0
Determine the distance travelled in 6seconds
(i.e. the area under the v/t graph) using
Simpson’s rule. [63.33m]
- The shape of a piece of land is shown in
Fig. 19.4. To estimate the area of the land,
a surveyor takes measurements at intervals
of 50m, perpendicular to the straight portion
with the results shown (the dimensions being
in metres). Estimate the area of the land in
hectares (1ha= 104 m^2 ). [4.70ha]
50 50 50 50
140 160 200 190 180 130
50 50
Figure 19.4
- The deck of a ship is 35m long. At equal
intervals of 5m the width is given by the
following table:
Width (m) 0 2.8 5.2 6.5 5.8 4.1 3.0 2.3
Estimate the area of the deck. [143m^2 ]
19.2 Volumes of irregular solids
If the cross-sectional areasA 1 ,A 2 ,A 3 ,...of an irregular
solid bounded by two parallel planes are known at equal
intervals of widthd(as shown in Fig. 19.5), then by
Simpson’s rule:
volume,V=
d
3
[(A 1 +A 7 )+4(A 2 +A 4
+A 6 )+2(A 3 +A 5 )]
A 1 A 2 A 3 A 4 A 5 A 6 A 7
dd d d d d d
Figure 19.5
Problem 3. A tree trunk is 12m in length and has
a varying cross-section. The cross-sectional areas at
intervals of 2m measured from one end are:
0.52, 0.55, 0.59, 0.63, 0.72, 0.84, 0.97m^2
Estimate the volume of the tree trunk.