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Extremely annoying question, average velocity related 
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#1
Dec2806, 01:40 AM

P: 29

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 12km4km = 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
Dec2806, 07:39 AM

Mentor
P: 8,325

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


#3
Dec2806, 09:01 PM

P: 29

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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