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At noon, ship A is 10 nautical miles due west of ship B. Ship A is sailing west at 16 knots and ship B is sailing north at 21 knots. How fast (in knots) is the distance between the ships changing at 4 PM? (Note: 1 knot is a speed of 1 nautical mile per hour.)

Sagot :

At [tex]$5 \mathrm{pm}$[/tex] the distance between the ships is changing at the speed of [tex]$\mathbf{3 0 . 2 1}$[/tex] knots

Firstly we need to an equation to represent both ships and the distance between each. A is moving [tex]$25 \mathrm{knots}$[/tex] west and [tex]$B$[/tex] is moving [tex]$17 \mathrm{knot}$[/tex] north. [tex]$D$[/tex]will be the distance between the two. Drawing it, you'll notice that it creates a right triangle.

So we use the Pythagorean Theorem:

[tex]$D^{\wedge} 2=A^{\wedge} 2+B^{\wedge} 2$[/tex]

Differentiate in relation to time:

[tex]$2 \mathrm{D}(\mathrm{dD} / \mathrm{dt})=2 \mathrm{~A}(\mathrm{dA} / \mathrm{dt})+2 \mathrm{~B}(\mathrm{~dB} / \mathrm{dt})$[/tex]

Now we must find all of our variables.

[tex]& A=\text { time(speed })+\text { original distance }=5(25)+10=135 \\[/tex]

[tex]& B=5(17)+0=85 \\[/tex]

[tex]& D=V\left(A^{\wedge} 2+B^{\wedge} 2\right)=159.53 \\[/tex]

[tex]& d A / d t=25 \\[/tex]

[tex]& d B / d t=17[/tex]

Plug in all your variables and solve for

[tex]$(\mathrm{dD} / \mathrm{dt})$ :[/tex]

[tex]& 2 \mathrm{D}(\mathrm{dD} / \mathrm{dt})=2 \mathrm{~A}(\mathrm{dA} / \mathrm{dt})+2 \mathrm{~B}(\mathrm{~dB} / \mathrm{dt}) \\[/tex]

[tex]& 2(159.53)(\mathrm{dD} / \mathrm{dt})=2(135)(25)+2(85)(17) \\[/tex]

[tex]& 319.061(\mathrm{dD} / \mathrm{dt})=9640 \\[/tex]

[tex]& \mathrm{dD} / \mathrm{dt}=9640 / 319.061 \\[/tex]

[tex]& \mathrm{dD} / \mathrm{dt}=30.21 \text { knots }\end{aligned}[/tex]

At [tex]$5 \mathrm{pm}$[/tex] the distance between the ships is changing at the speed of [tex]$\mathbf{3 0 . 2 1}$[/tex] knots

To know more about Pythagoras Theorem

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