# Frames of reference and vectors problem

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1. Sep 21, 2015

### notsoclever

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?
Thanks

2. Sep 21, 2015

### stockzahn

Please show or describe your calculation/drawing, which gave you the results (15 m/s, NW).

3. Sep 21, 2015

### notsoclever

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.

4. Sep 21, 2015

### stockzahn

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.

5. Sep 21, 2015

### 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?"