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Ship problem,related time change

wolfsprint

New member
Dec 31, 2012
17
A ship sailing west at 9am with velocity 20km/h. after one hour another ship sailed from the same port with velocity 40km/h at 60 north of west . find the rate of increase of the distance between the two ships at 11am

Can someone please show me how to solve this? i have no idea
 
Last edited:

CaptainBlack

Well-known member
Jan 26, 2012
890
Re: Ship problem , related time change

A ship sailing west at 9am with velocity 20km/h. after one hour another ship sailed from the same port with velocity 40km/h at 60 north of west . find the rate of increase of the distance between the two ships at 11am

Can someone please show me how to solve this? i have no idea
Write the equations of the position of the two ships as a function of time. Now write the square of the distance between them as a function of time, and differentiate...

CB
 

soroban

Well-known member
Feb 2, 2012
409
Hello, wolfsprint!

Did you make a sketch?


A ship sailing west at 9 am with velocity 20 km/h.
After one hour, another ship sailed from the same port with velocity 40 km/h at 60o north of west .
Find the rate of increase of the distance between the two ships at 11 am.
Code:
                 C
                  o
                .   *
           x  .       * 40t
            .           *
          .               *
        .               60[SUP]o  [/SUP]*
    B o  *  *  o  *  *  *  *  o P
         20t   A     20
Ship #1 leaves port $P$ at 9 am at 20 km/hr.
In one hour, it reaches point $A:\:pA = 20.$
In the next $t$ hours, it reaches point $B:\:AB = 20t.$

In the same $t$ hours, ship #2 leaves point $P$ at 40 km/hr.
. . and reaches point $C:\:pC = 40t.$

Let $x = BC.$


Law of Cosines:

. . $x^2 \;=\;(40t)^2 + (20t+20)^2 - 2(40t)(20t+20)\cos60^o$

. . $x^2 \;=\;1600t^2 + 400t^2 + 800t + 400 -800t^2 - 800t$

. . $x^2 \;=\;1200t^2 + 400$


Differentiate with respect to time:

. . $2x\dfrac{dx}{dt} \;=\;2400t \quad\Rightarrow\quad \dfrac{dx}{dt} \;=\; \dfrac{1200t}{x}$


When $t=1,\:x = 40:$

. . $\dfrac{dx}{dt} \;=\;\dfrac{1200(1)}{40} \;=\;30$


The ships are separating at 30 km/hr.