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Average velocity just simply be the average speed

  1. Sep 9, 2011 #1
    Hi peeps, i just started intro physics but finding it sorta difficult, and attempting to do some questions but just not quite sure if im doing it right. would so appreciate any kind of help, my teacher doesn't seem to break it down enough for me lol. can somebody just check my work, and let me know if im right or wrong?:)
    1)A car is driven 40km at 30km/h. it then continues in the same direction for another 40km at 60km/h. what is the average velocity of the car during this 80 km trip?
    In this question, would the average velocity just simply be the average speed, as its just going in one positive direction? As the 80 km took 120 minutes all up, it is just displacement/time, and the answer is 40km/h?

    2) average velocity of these two cases:
    a) you walk 73.2m at a speed of 1.22m/s, and then run 73.2m, at a speed of 3.05m/s along a straight track.
    73.2/1.22=60m/s
    73.2/3.05=24m/s
    60+24/2= 42m/s= average velocity


    b)You walk for 1minute at a speed of 1.22m/s, and then run for 1minute at 3.05m/s
    60 x 1.22= 73.2m
    60 x 3.05=183m
    73.2+183=256.2-total displacement
    120 seconds=total time
    so 256.2/120=2.135m/s = average velocity

    please let me know if im right, id really appreciate some help:), i know theyre easy questions but im not very confident with physics at all!!!:(
     
  2. jcsd
  3. Sep 9, 2011 #2

    Doc Al

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    Staff: Mentor

    Re: Velocity/acceleration/displacement

    Good!

    Redo this one. You're mixing up speed and time. Solve it just like the others: total displacement over total time.


    Good!
     
  4. Sep 9, 2011 #3
    Re: Velocity/acceleration/displacement

    ohh i see, yeah thats definately not right:) thanks for pointing it out. so total time is 73.2/1.22m/s= 60 sec
    73.2/3.05m/s=24 seconds
    total distance = 73.2 x 2=146.4m
    velocity is 146.4/84=1.743m/s..hope thats right! thankyou for your input it really helps:)
     
  5. Sep 9, 2011 #4

    Doc Al

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    Staff: Mentor

    Re: Velocity/acceleration/displacement

    Now you've got it. :approve:
     
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