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Homework Help: Basic physics question about average velocity

  1. May 2, 2017 #1
    1. The problem statement, all variables and given/known data
    a car starts from hither , goes 50km in a straight line to yon , immediately turns around, and turns to hither . the time for this round trip is 2 hours . the average speed of the car for this round trip is ?

    2. Relevant equations

    average velocity = disrance covered /time
    3. The attempt at a solution
    as the body comes back to its initial position so average velocity is zero ...... but answer key shows the right answer is 50 km/hr .............I cant understand ..... what you think what is correct answer
    Last edited by a moderator: May 2, 2017
  2. jcsd
  3. May 2, 2017 #2


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    Staff: Mentor

    It sounds like they are asking for average speed, not average velocity. Your equation you list is for average speed, not average velocity.

    And 50km/hr looks to be a wrong answer, since the time elapsed is 2 hours. Are you sure you have written the problem and answer correctly?

    EDIT -- I misread the problem, and now see why they got 50km/hr.
    Last edited: May 2, 2017
  4. May 2, 2017 #3
    ohhhh yesh its my mistake I write wrong equation .......average speed = total speed /time ....... what you think now my answer is correct or wrong ?????????
  5. May 2, 2017 #4
    The car travels 100 km in 2 hr, so its average speed is 50 km/hr. But, as berkeman points out, the question asks for the average speed, not the average velocity.
  6. May 2, 2017 #5


    Staff: Mentor

    This equation -- average speed = total speed /time -- is meaningless (and wrong). There is no such thing as "total speed".
    The corrected equation is speed = (total distance)/(elapsed time).
    For velocitiy, it is velocity = (total displacement)/(elapsed time).
    In your calculation, since the car started from and returned to the same point, the total displacement is 0, so the velocity is 0. That's not what they asked for, though.
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