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Simple Motion Along a Straight Line Problem

  1. Sep 18, 2010 #1

    MysticDude

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    Simple "Motion Along a Straight Line" Problem

    Hi new here to PF. I have a simple physics problem but I was confused when the teacher was explaining it (he went a little too fast on this part) so I was hoping you physicians can help me out here.

    1. The problem statement, all variables and given/known data
    An automobile travels on a straight road for 40km at 30 km/hr. It then continues in the same director for another 40 km at 60 km/hr. (a) What is the average velocity of the car during this 80km trip?(Assume that it moves in the positive x direction.) (b) What is the average speed?

    Given:
    Trip was 80km.
    First 40 km were traveled at 30km/hr
    The second 40km were traveled at 60km/hr

    2. Relevant equations
    Average velocity = (Delta)x / (Delta) t where x is for position and t is for time.
    Average speed = total distance / (Delta) t


    3. The attempt at a solution
    Okay so I was able to get that for the first 40km it took (4/3) hours by dividing (40km)/(30km/hr). For the next 40km I was able to get that it took (2/3 hours) by dividing (40km)/(60km/hr). In other words, 80km in 2 hrs. So I think that the average speed should be 80km/2hrs = 40km/hr. But I just don't know how to get the average velocity. Should I do the same thing again or something else. I know it is easy but I'm new to physics.
     
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  3. Sep 18, 2010 #2
    Re: Simple "Motion Along a Straight Line" Problem

    Hi MysticDude

    Based on your equation : Average velocity = Δx / Δt

    What is the value of Δx ? In other words, how far the automobile from its starting point?
    What is the value of Δt ? How long did it take to reach its last position?
     
  4. Sep 18, 2010 #3

    MysticDude

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    Re: Simple "Motion Along a Straight Line" Problem

    Well, Δx = x(final) - x(initial) so it should be 80km - 0 km = 80km.
    Δt = t(final) - t(initial) so it should be 2hrs - 0hrs = 2hrs.
    so Δx/Δt = 40 km/hr
    So does this mean that the average velocity is also equal to the average speed?
     
  5. Sep 18, 2010 #4
    Re: Simple "Motion Along a Straight Line" Problem

    Yes :biggrin:

    The average velocity and average speed will be different if the object moves in different direction than the previous one, like it moves 40 km to the east, then 10 km to the west.
    The distance = 40 + 10 = 50 km
    The displacement or position = 40 - 10 = 30 km
     
  6. Sep 18, 2010 #5

    MysticDude

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    Gold Member

    Re: Simple "Motion Along a Straight Line" Problem

    Thanks a bunch! I knew it had to be a simple problem since it was only the first week of school, but for some reason I got confused. Once again, thanks a lot for helping a new guy out. I got into AP Physics so I signed up for help and you sir just gave it to me :P.
     
  7. Sep 19, 2010 #6
    Re: Simple "Motion Along a Straight Line" Problem

    You're welcome. I also got helped by many members here. Glad I can help others :smile:
     
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