For a fish swimming at a speed v relative to the water, the energy expenditure p
ID: 2828020 • Letter: F
Question
For a fish swimming at a speed v relative to the water, the energy expenditure per unit time is proportional to v3. It is believed that migrating fish try to minimize the total energy required to swim a fixed distance. If the fish are swimming against a current u (u < v), then the time required to swim a distance L is L/(v - u) and the total energy E required to swim the distance is given by the formula below, where a is the proportionality constant.
E(v) = av^3 L/(v - u)
Determine the value of v that minimizes E. (Note: This result has been verified experimentally.)
Explanation / Answer
v = 3u/2.
To determine the value of v that minimizes E, solve the equation E'(v) = 0.
E(v) = (av^3) * (L/(v - u)).
Use that if f(x) = g(x)*h(x), then f'(x) = g'(x)*h(x) + g(x)*h'(x).
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