Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Average Speed

  1. May 23, 2007 #1
    1. The problem statement, all variables and given/known data
    A person travels by car from one city to another with different constant speeds between pairs of cities. She drives for 15.0 min at 65.0 km/h, 20.0 min at 75.0 km/h and 60.0 min at 40.0 km/h, and spends 15.0 min eating lunch and buying gas.

    Determine the average speed for the trip.

    2. Relevant equations
    Average Speed = total distance/total time

    3. The attempt at a solution
    (65+75+80)/(15+20+60+15)=1.64 which isn't logical. I don't understand how to compute the total distance???
    Last edited: May 23, 2007
  2. jcsd
  3. May 23, 2007 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Well, the trip is naturally broken up into segments. So don't worry about calculating the total distance if that confuses you. Calculate the distance travelled in each segment and then add them up. For example, first she travels for 15 min at 65 km/h. Okay, how far did she travel in the 15 minutes?
  4. May 23, 2007 #3
    I took each time divided by 60 to give me hours then multiplied that number by the km/h. Added them up and got 81.25. What am I doing wrong?
  5. May 23, 2007 #4


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    What are the units? If you included the units you would see that this is not an average speed. What you got is the total distance. To find the average speed, you must still divide by the total time (in hours if you want the average speed in km/hr). Don't forget to include the 15 minutes break time.
  6. May 24, 2007 #5
    So after getting 81.25 which would be my total distance I divide by the total time which would be 110. This gives me .738636. This doesn't seem logical but is what I am doing correct?
  7. May 24, 2007 #6
    You really would make your life a lot easier if you just carry the units in your calculations.

    The total distance travelled is 81.25km as you said

    The total time taken for the trip is 110min just as you said

    So you have [tex]\frac{81.25}{110} km/min[/tex]

    Can you convert that to km/h ?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook