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Homework Help: Velocity at different times

  1. Jul 16, 2012 #1
    1. The problem statement, all variables and given/known data

    You jog at 9.8 km/ h for 8.0 km, and then you jump in the car and drive an additional 16km. With what average speed must you drive the car if your avg speed for the entire 24 km is to be 22km/ h

    2. Relevant equations
    Average speed = distance/ time

    3. The attempt at a solution

    I found the time it took for my jog, which was approx. .84 miles.
    Then I made an equation using some facts :

    22= 24/ .84+ x

    X is about .25 hours

    So now that I know this I plug it into my equation that will solve the problem: 16/.25

    My final answer is about 64 km/ hr.

    My teacher says its 50. Where did I go wrong?
  2. jcsd
  3. Jul 16, 2012 #2
    You made an error calculating x. If 22 = 24 / (.84 + x), we know that the denominator for that equation must be equal to 24 / 22. Calculate that, then subtract .84 to get the time in the car. Then go from there.
  4. Jul 17, 2012 #3
    Mark, i am sure x is .25. I've done the calculations several times. I would like an answer soon because today's test is modeled after the homework
  5. Jul 17, 2012 #4


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    Staff: Mentor

    0.8163 hrs

    I say it's about midway between those values. Your method is correct, just need to retain more sig figs.
  6. Jul 17, 2012 #5


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    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    Why would you use miles to solve any part of this problem?
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