Question
Energy Expended by a Fish it has been conjectured that a fish swimming a distance of $L \mathrm{ft}$ at a speed of $\nu$ ft/sec relative to the water and against a current flowing at the rate of $u$ ft/sec $(u<\nu)$ expends a total energy given by$$E(\nu)=\frac{a L \nu^{3}}{\nu-u}$$where $E$ is measured in foot-pounds (ft-lb) and $a$ is a constant. Find the speed $\nu$ at which the fish must swim in order to minimize the total energy expended. (Note: This result has been verified by biologists.)
Step 1
The function is a quotient, so we will use the quotient rule for differentiation, which states that the derivative of $\frac{f(x)}{g(x)}$ is $\frac{f'(x)g(x) - f(x)g'(x)}{[g(x)]^2}$. Show more…
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It has been conjectured that a fish swimming a distance of $L \mathrm{ft}$ at a speed of $\nu \mathrm{ft} / \mathrm{sec}$ relative to the water and against a current flowing at the rate of $u \mathrm{ft} / \mathrm{sec}(u<\nu)$ expends a total energy given by $$ E(\nu)=\frac{a L \nu^{3}}{\nu-u} $$ where $E$ is measured in foot-pounds (ft-lb) and $a$ is a constant. Find the speed $\nu$ at which the fish must swim in order to minimize the total energy expended. (Note: This result has been verified by biologists.)
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Energy Expended by a Fish Suppose a fish swimming a distance of $L$ ft at a speed of $v \mathrm{ft} / \mathrm{sec}$ relative to the water and against a current flowing at the rate of $u \mathrm{ft} / \mathrm{sec}(u<v)$ expends a total energy given by $$E(v)=\frac{a L v^{3}}{v-u}$$ where $E$ is measured in foot-pounds (ft-lb) and $a$ is a constant. a. Evaluate $\lim _{v \rightarrow y^{+}} E(v),$ and interpret your result. b. Evaluate $\lim _{v \rightarrow \infty} E(v),$ and interpret your result.
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Migrating Fish A fish swims at a speed $v$ relative to the water, against a current of 5 $\mathrm{mi} / \mathrm{h} .$ Using a mathematical model of energy expenditure, it can be shown that the total energy $E$ required to swim a distance of 10 mi is given by $$E(v)=2.73 v^{3} \frac{10}{v-5}$$ Biologists believe that migrating fish try to minimize the total energy required to swim a fixed distance. Find the value of $v$ that minimizes energy required. [Note: This result has been verified; migrating fish swim against a current at a speed 50$\%$ greater than the speed of the current.]
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