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Homework Help: Velocity+avg. speed probs help Pleassse!

  1. Sep 21, 2004 #1
    velocity+avg. speed probs....help Pleassse!

    hey i'm confused with these problems. :bugeye: could anyone do a good deed and help me out? My teacher spends more time watching family guy than teaching us. thanks

    1. a person walks first at a constant speed of 5.0 m/s along a straight line from point A to point B and then back along the line from B to A at a constant speed of 3.0 m/s (a) what is her average speed over the entire trip? (b) her average velocity over the entire trip?

    2. A car makes a 200 km trip at an average speed of 40 km/h. A second car starting 1.0 h later arrives at their mutual destination at the same time. what was the average speed of the second car for the period that it was in motion?

    thanks again. i don't really understand the first one--is it as simple as it seems? do you just divide 8.0/2 to get 4 m/s???? for average speed?
  2. jcsd
  3. Sep 21, 2004 #2
    1. (a) Call the distance between A and B [tex]x[/tex].

    Then, the time taken to travel from A to B = Distance/Speed = [tex]\frac {x}{5}[/tex]

    The time taken to travel from B to A = [tex]\frac {x}{3}[/tex].

    The total time taken to travel from A to B and back = [tex]\frac {x}{3} + \frac {x}{5} [/tex]

    The total distance travelled = [tex]2x[/tex]

    Average Speed = Total Distance/Total Time

    Work it out yourself. :smile:

    (b) Since the person's displacement from his starting point is zero by the end of the journey, what do you think his velocity is? Have a look at your textbook for the definition of velocity.

    2. First find the time taken to make a 200 km trip at 40 km/h. Then note that the second car takes 1 hour less than this. Remember that the distance travelled by both cars are equal.
    Last edited: Sep 21, 2004
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