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Homework Help: Help me complete this question please!

  1. Jan 4, 2009 #1
    This is from the M1 review excercise Edexcel. I've managed to do a) b) and c) and but now I'm completely stuck. I'm no particular wizz at mechanics and getting stuck at d) means I'm suddenly uncertain about e) and f) :(.

    Here is the question:
    In this question i and j are horizontal unit vectors and at right angles to each other.
    At 12:00 a helicopter A sets out from its base 0 and flies with speed 120 km h-1 in the direction of the vector 3i+4j.

    a) Find the velocity vector of A
    At 12:20 that day another helicopter B sets out from O and flies with speed 150 km h -1in the direction of the vector 24i + 7j.
    b) Find the velocity vector of B
    c) Find the position vectors of A and B at 13:00
    d) Calculate the distance of A from B at 13:00
    At 13:30 B makes an emergency landing. A immediately changes direction and flies at 120 km h-1 in a straight line to B.
    e) Find the position vector of B from A at 13:30
    f) Determine the time when A reaches B.

    The answers I have so far are:
    a) 72i +96j
    b) 144i +42j
    c) A = 72i +96j B = 96i +28j

    The answers I'm supposed to get for the rest are:
    d) 72.1km
    e) 60i-95j
    f) 14 26
  2. jcsd
  3. Jan 4, 2009 #2


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    Homework Helper

    Why are you getting stuck at d)? It's the length of the difference between the two position vectors in part c).
  4. Jan 4, 2009 #3
    d) The distance between two points d,

    [tex] d = \sqrt{ (x_2-x_1)^2 + (y_2-y_1)^2 } [/tex]

    Co-ordinate geometry, I assume that if you're doing M1 then you have done Co-ordinate geometry in C1 this year?

    e) For this find the position vectors for both like you did in question c, then use the method in c to find the distance between the two points.

    f) The question says that B becomes stationary, use the distance you find in e and the velocity of 120 kms-1 to determine the time it will take for A to reach B.
  5. Mar 18, 2009 #4
    thank you so much!
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