Scientific problems are approached by building ‘models’. Not little cardboard models, but models made from mathematical tools. We can play with the symbolic models and adjust them until they start to behave in a way that resembles the way we expect them to, in order to be able to make projections. When we have done this, we get an understanding of the things we care about which is much deeper than we could ever get if we only use words and pictures. Mathematical models do not replace words and pictures, they sharpen them. So models deepen our understanding of ‘systems’, whether we are talking about a mechanism, a robot, a chemical plant, an economy, a virus, an ecology, a cancer or a brain. And it is necessary to understand something about how models are made.
In the following tasks, we are going to use such approach, in which we apply calculus to solve these tasks, and try to understand to behaviour of the functions asserted, or evolved from our analytical approach.
Task 1:
Given that the Rock ’n’ Roll coaster given over interval of 8 seconds, then by inspection:
h8=–483+4682–118×8+48=0
Also, given that the Colossos coaster is modelled by:
ft=t3–13t2+44t–32
Which is also modelled for 8 seconds, so:
f8=83–1382+44×8–32=0
Hence, by the Factor Theorem, (t-8) must be a factor of both f(t) and h(t).
First, we divide h(t) by (t-8) as follows:
–4t2+14t–6
t–8
–4t3+46t2–118t+48
–4t3+32t2
14t2–118t+48
14t2–112t
–6t+48
–6t+48
0
Therefore:
ht=t–8–4t2+14t–6
=–2(t–8)(2t–1)(t–3)
Which shows that the Rock ‘n’ Roll coaster will reach the ground level at t=0.5s, t=3s, and t=8s.
Similarly, dividing f(t) by (t-8) gives:
t2–5t+4
t–8
t3–13t2+44t–32
t3–8t2
–5t2+44t–32
–5t2+40t
4t–32
4t–32
0
Which means that:
ft=(t–8)(t2–5t+4)
ft=(t–1)(t–4)(t–8)
Hence the Colossos reaches the ground level at t=1s, t=4s, and t=8s.
To find maximum and minimum heights we use the first and second derivatives as follows:
1) The Rock ‘n’ Roll coaster:
h‘t=–12t2+92t–118
h‘t=0
–12t2+92t–118=0
t=–92±2800–24=236∓56√7
So, for:
t1=236–567≈1.63s
we have
ht1=–39.45
And for:
t2=236+567≈6.04s
we have
ht2=132.04
To determine the nature of these stationary points we apply the second derivative test as follows:
h‘‘(t)=–24t+92
so, at the point (1.63, -39.45) we have a minimum since:
h‘‘1.63=–24×1.63+92=52.88>0
Similarly, at the point (6.04, 132.04) the coaster reaches a maximum point since:
h‘‘6.04=–24×6.04+92=–52.96<0
2) The Colossos coaster:
f‘t=3t2–26t+44
f‘t=0
3t2–26t+44=0
t=26±1486=133±373
So, for:
t1=133–37/3≈2.31s
we have
ft1=12.60
And for:
t2=133+37/3≈6.04s
we have
ft2=–20.75
To determine the nature of these stationary points we apply the second derivative test as follows:
f‘‘t=6t–26
so, at the point (2.31, 12.60) we have a maximum since:
f‘‘2.31=2.31×6–26=–12.14<0
Similarly, at the point (6.36, -20.75) the coaster reaches a minimum point since:
f‘‘6.04=6×6.36–26=12.16>0
Table of values for both graphs are as follows:
t
h(t)
0
48
0.5
-28
1
0
2
-36
3
0
4
56
5
108
6
132
6.5
126
7
104
7.5
63
8
0
t
f(t)
0
-32
1
0
2
12
3
10
4
0
5
-12
6
-20
7
-18
8
0
And the graphs of both h(t) and f(t) are shown below:
h(t)
f(t)
The above graphs clearly show that the Rock ‘n’ Roll coaster is bigger than the Colossos coaster.
Finally, to determine which coaster has the larger underground area, we calculate the areas underneath the x-axis for both functions as follows:
The Rock ‘n’ Roll coaster:
∫123–4t3+46t2–118t+48dt=–t4+463t3–59t2+48t(12 ,3)≈65.1
∫01t3–13t2+44t–32dt+∫48t3–13t2+44t–32dt=t44–133t3+22t2–32t0,1+t44–133t3+22t2–32t4,8≈67.42
Therefore, we can deduce that the Colossos coaster has larger underground area than the Rock ‘n’ Roll coaster.
Task 2
To find the maximum speed we use the first derivative in order to find the local points, as follows:
v‘t=3–0.4t
And setting the first derivative equal to zero gives:
3–0.4t=0
Or
t=30.4=7.5s
Which gives:
v7.5=37.5–0.27.52=11.25ms–1
And to prove that this speed is a maximum speed we apply the second derivative test as follows:
v‘‘t=–0.4<0
So, the stationary point found is a maximum.
And a graph of the given velocity model is shown below:
Task 3
r
h
Let the proposed can be as shown in the figure above, where r is the radius of the base, and h is the height, then we deduce that the volume of the can V is given by:
V=πr2h
Or
πr2h=330
(1)
Also, the total surface area of this can A is given by:
A=2πr2+2πrh
(2)
From relation (1) we have
h=330πr2
which when substituted in relation (2) we obtain:
A=2πr2+2πr×330πr2
Or
A=2πr2+660r
(3)
Hence to find the minimum area of material needed, we should check whether A given by equation (3), attains a minimum value for some value of r.
Taking the first derivative of this function gives:
A‘=4πr–660r2
Then setting A’ equal to zero gives:
4πr–660r2=0
So
r=165π3≈3.745cm
To determine the nature of this local point we apply the second derivative test:
A‘‘=4π+1320r3
Which takes positive values for
r=165π3
since:
A‘‘=4π+1320165π=12π>0
Therefore, the minimum area of material needed can be achieved by setting the radius of the can to 3.745cm, and the height h=7.49cm, which gives the minimum surface area of
A≈264.357cm2
Task 4
To find the minimum and maximum values of p, we use the first derivative as follows:
p‘=3×2–36x+105
Then
p‘=0
gives:
3×2–36x+105=0
3×2–12x+35=0
(x–5)(x–7)=0
Hence:
Either
x=5 ⇒p=112p
Or
x=7 ⇒p=108p
To determine the nature of these two points, we apply the second derivative text:
p‘‘=6x–36
So, at the point (5,112) we have:
p‘‘=6×5–36=–6<0
Which implies that the function has a maximum at this point.
Similarly, at the point (7,108) the function has a minimum since:
p‘‘=6×7–36=6>0
A graph of the function is as follows:
Task 5
Since the horizontal distance AB is the sum of the two segments AD and DB, we can find these two distances by applying Pythagoras Theorem to the triangles ACD and CDB respectively, thus:
From triangle ACD we have:
AC2=AD2+CD2
So
AD2=552–202=2625
Which gives
AD≈51.24m
Similarly, applying Pythagoras Theorem on triangle CBD gives:
CB2=CD2+BD2
Hence
BD2=302–202=500
Which gives
BD≈22.36m
Therefore,
AB=AD+BD≈51.24+22.36=73.60m
Conclusion:
The above tasks, and our techniques in approaching them, have shown how powerful a technique calculus can be, by enabling us to understand the behaviour of the functions obtained in these models we applied to solve these tasks.
References:
An Introduction To Mathematical Modelling by Michele D Alder 2001.
Mathematical Techniques: An Introduction for the Engineering, Physical, and Mathematical Sciences, D W Jordan and P Smith.
Desmos.com for graph sketching.
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