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Homework Help: Related rates and sailing ship

  1. Oct 28, 2011 #1
    1. The problem statement, all variables and given/known data
    At 9 a.m ship A is 50 km [E] of ship B. Ship a is sailing [N] at 40 km/h and ship B is sailing at 30 km/h. How fast is the distance between them changing at 12 am?

    2. Relevant equations
    x^2 + y^2 = z^2
    dz/dt [t=3h] = ?
    db/dt = 30 km/h
    da/dt = 40 km/h

    3. The attempt at a solution
    My instructor made me draw out a diagram
    the diagram helped me determine that dy/dt = 70 km/h and that x = 50 km
    so: x^2 + y^2 = z^2
    50^2 + y^2 = z^2
    2y(dy/dt) = 2z (dz/dt)
    2y(70) = 2z (dz/dt)

    Not sure if i'm doing this right/what to do next.
  2. jcsd
  3. Oct 28, 2011 #2


    Staff: Mentor

    This seems like the right approach, but it's hard to follow, since you haven't identified what x, y, and z represent.

    What is the problem asking you to find?

    BTW, your instructor is doing you a favor by making you draw a diagram...
  4. Oct 28, 2011 #3

    Heh, didn't mean to make it seem like my instructor forced me to draw a diagram.
    Anyway, I believe the problem is asking for dz/dt [t=3hours] I'm not sure what you mean by identify x, y, and z.
  5. Oct 28, 2011 #4


    Staff: Mentor

    By "identify" I mean what do x, y, and z represent in your drawing?
  6. Oct 28, 2011 #5
    I'm not entirely sure.
    Just know that the x = 50 km the distance between A and B.
    Here's the diagram:

    Attached Files:

  7. Oct 28, 2011 #6


    Staff: Mentor

    Well, isn't z the distance between the two ships?
  8. Oct 28, 2011 #7
    Well, yes. Hmm.
    I guess x is the distance between A-B at 9 am
    I'm still not sure what y is.
  9. Oct 28, 2011 #8


    Staff: Mentor

    There's no point in calling that distance x, since it is known (50 km). Also, adding the ships' speeds doesn't do you any good, since they are moving opposite directions on different tracks.

    At any time t after 9:00AM, ship A will be 30t (km) south of its starting point, and ship A will be 40t (km) north of its starting point. Call the distance between the two ships D1 + D2, where D1 is the length of the hypotenuse of the left triangle (with vertices ship B, its starting point, and a point on the line connecting the two starting points) and D2 is the hypotenuse of the right triangle (with vertices ship A, its starting point, and a point on the line connecting the two starting points).

    The two triangles are not congruent, because the ships are going different speeds, and the straight line between ships A and B does not hit the midpoint of the line that joins the two starting points. However, even though the triangles have different sizes, they are similar, meaning that their corresponding sides are proportional.

    Use this information to identify all three sides of each triangle at any time after 9AM, and find d/dt(D1 + D2) at 12:00 noon.
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