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Homework Help: Frames of reference and vectors problem

  1. Sep 21, 2015 #1
    1. The problem statement, all variables and given/known data
    A car is moving toward north with a speed of 35 km/h. A truck travels toward west with a speed of 42 km/h.
    Which is the speed of the caravan according to the car's driver?

    2. Relevant equations

    3. The attempt at a solution
    The solution given by the book is 15 m/s with direction south-west, but I get the solution of 15 m/s with direction north-west and I cannot get the reason.
    Can you help me with this?
  2. jcsd
  3. Sep 21, 2015 #2


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    Gold Member

    Please show or describe your calculation/drawing, which gave you the results (15 m/s, NW).
  4. Sep 21, 2015 #3
    I used Pitagora theorem to get the value of 54 km/h that is 15 m/s, but the direction of the vector is north-west and not south-west.
  5. Sep 21, 2015 #4


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    Gold Member

    You've already got the velocity correct in the first place, the direction is correct like the book says (SW). Did you try to solve the (direction-)problem graphically or numerically? Please show me your attempt to be able to help you.
  6. Sep 21, 2015 #5


    Staff: Mentor

    Does "caravan" refer to the truck? In British English, a caravan is a trailer. In American English, a caravan is a collection of vehicles (also animals) that are traveling as a group.

    The truck is roughly SW of the car. Perhaps the question could be rephrased as "how fast is the distance from the car to the truck changing?"
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