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Homework Help: Average speed for round trip

  1. Jan 22, 2016 #1
    1. The problem statement, all variables and given/known data
    A car is moving in a road and its average speed is 40 km/h. Then it moves back in that road by 60 km/h speed. What is the average speed of this car? ( The answer is not 50 km/h )

    2. Relevant equations
    Average speed= d/t

    3. The attempt at a solution

    I can't solve this. I get 50 km/h every time. I do this: (60+40)/2
    Because I don't have time or distance.

    Please Help
  2. jcsd
  3. Jan 22, 2016 #2


    Staff: Mentor

    But you know that it goes the same distance for each part of the trip.
    Let D = the distance for each leg of the trip.
    Calculate the time it takes for the first leg at 40 km/hr.
    Calculate the time it takes for the second leg at 60 km/hr.
    You now have enough information to calculate the average speed for both legs.

    BTW, in future threads, try to come up with a better thread title. "I can't solve this problem" is NOT a good thread title. I changed the title for you to something more informative.
  4. Jan 23, 2016 #3

    But I can't calculate the time. Maybe I don't have enough knowledge.

    Can you show me the solution?

    Thanks a lot
  5. Jan 23, 2016 #4


    User Avatar

    Staff: Mentor

    When there is something in a problem that you don't know but think you need, try replacing it with a variable (i.e. an "unknown"). Then proceed to work out what you want using that variable in place of the unknown quantity. If it turns out that you didn't need it after all, it will cancel out (disappear) along the way to your solution. Working with variables and applying the rules of algebra to solve problems is an important skill to master.

    Hint: For this problem, try letting the length of the road be represented by the variable D.
  6. Jan 23, 2016 #5


    Staff: Mentor

    For each leg of the trip you have the distance (D) and the rate (40 km/hr for the first leg and 60 km/hr for the second leg). That's enough information to get an expression for the time of each leg.
    No, that's not how it works here. The rules of this forum don't allow us to work a problem for you. See https://www.physicsforums.com/threads/physics-forums-global-guidelines.414380/ (under Homework Guidelines).
  7. Sep 15, 2017 #6
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