1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
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: 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?

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



    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

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


    User Avatar
    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!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook