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Simple problem I thought

  1. Sep 21, 2005 #1
    I assigned a couple of problem to my students today without realizing that these would be too difficult for high school students. However, after starting the problem, I got stumped. I know I am missing something, but what is it? The problem reads as follows:

    5. A person walks first at a constant speed of 5.00 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 3.00 m/s. What is
    a. her average speed over the entire trip and
    b. her average velocity over the entire trip?

    Part b is simple, but 'a' is where I got stuck. You know delta x is the same, and you know that t1 and t2 will be different as well.

    So, delta x = v1*t1, and x=v2*t2.

    However this leaves us with three unknowns and two equations.

    What am I missing for my third equation?

    Thanks!
     
  2. jcsd
  3. Sep 21, 2005 #2
    x= distance traveled from A to B
    v_1=5 m/s
    v_2=3 m/s

    x=5t_1
    x=3t_2

    t_1=x/5
    t_2=x/3

    avg. speed = distance/total time= 2x/(t_1 +t_2)=(2x)/(x/3+x/5)=3.75 m/s

    one of your unknowns simply cancels out.

    since the average speed will be the same no matter what the distance, it is unimportant what the distance between a and b are.
     
    Last edited: Sep 21, 2005
  4. Sep 22, 2005 #3
    Thanks,

    I knew it was something like that.
     
  5. Sep 22, 2005 #4

    HallsofIvy

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    Staff Emeritus
    Science Advisor

    Okay, so we are all relieved that the problem has a well-defined answer!

    Are you telling us that you assigned this problem to your students without knowing how to do it yourself?


    I agree that this problem is not too difficult for high school students- but I'm wondering about you!
     
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