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: Stupid Riddle that is driving me mad

  1. Oct 24, 2005 #1
    You drive a car at a speed of 40 km/hr to a place and then 60 km/hr back... what is the average speed of the car.

    I feel really stupid for asking this but its really making me mad..
     
  2. jcsd
  3. Oct 24, 2005 #2
    hahaha as soon as i posted it it was so obvious...

    Set distance equal to one

    t1=1/40 t2=1/60

    [tex]\frac{\Delta d}{\Delta t}=avg vel[/tex]

    [tex]\frac{2}{\frac{1}{40}+\frac{1}{60}} [/tex]
     
  4. Oct 24, 2005 #3
    Which produces 48 km/hr as answer
     
  5. Oct 25, 2005 #4

    TD

    User Avatar
    Homework Helper

    You are correct. Often people make the obvious mistake to take the arithmetic mean, being (40+60)/2 = 50 but in this case, you need the harmonic mean :smile:
     
  6. Oct 28, 2005 #5
    harmonic mean could be calculated slightly easier:
    [tex]\frac{v_1 v_2}{\frac{v_1 + v_2}{2}}[/tex]
     
    Last edited: Oct 28, 2005
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook