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Extremely annoying question, average velocity related

  1. Dec 28, 2006 #1
    1. The problem statement, all variables and given/known data
    Ok so i have this simple question on a physics practice sheet, and because i don't get how the professor got his answers (answer are given) i have become confused and created this question into a difficult one. I am very annoyed when i can't get the exact same answer as the professors answers, and i have followed every step. Here is the question

    A truck drivers 12km east in 20 minutes and then drives 4 km north in 10 minutes. What is the average velocity of the truck for the entire trip?

    3. The attempt at a solution
    So i draw it all...in the notes, it basically says, average velocity = (delta)x/time
    so i get my delta x by subtracting the 12km-4km = 8km (i'm not going to convert here). It says average velocity for the entire trip so i gather you subtract/add the numbers...and also convert the 20minutes and 10minutes to hour seeing how you're working with km here...30min=0.5hr.
    Now i put that all into the equation, av.= 8/0.5 = 16??

    But the answers say, 24km/hr east and 8km/hr north (6.7m/s east, 2.2m/s north) ??

    I know how those numbers came about...12km+12km=24, 4km+4km=18 and the m/s ones came from the conversions of 12km=3.33m/s and 4km=1.11m/s so you just double those...i know i'm doing something wrong here, i've made it seem too difficult when it's only supposed to be a simple exercise...

    And also, the question after that one, is a bit similar, except that there is no addition or subtraction of any numbers, it just basically gives you the over all x and time which you divide both and i get the same answer...i'm going to go back and read over my notes because i think i've missed something...any help would be great thanks!
  2. jcsd
  3. Dec 28, 2006 #2


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    Staff Emeritus
    Science Advisor

    If you draw the diagram, you will see that the displacement is the hypotenuse of the triangle with sides 12 and 4.

    Here, you've just tried to put together numbers to get the answer given on the solution sheet! This is not the way to go about problems. Adding together two distances, namely 12km+12km will not give a velocity of 24km/h!

    Start by obtaining the magnitude of the resultant velocity, then find the angle in order to resolve into east and north components.

    You'll have to post some details in order to be helped/
  4. Dec 28, 2006 #3
    Wait i think i finally got it...jeez i don't know why i make things like this so hard...the times, 20min and 10min, well i added them to get 30min and converted it to km/hr so i got 0.5...and that would end up being the time right? coz if you divide 12 by 0.5 you get 24 and 4 by 0.5 is 8, which is what the answers say....so have i done this properly?
    And also if u divide 3.33m/s by 0.5 you get 6.6 which is what the answers were for the m/s one...but i'm not sure if it's right to divide by 0.5 because that's in km/hr??
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