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Quick question

  1. Aug 30, 2007 #1
    1. The problem statement, all variables and given/known data

    A horse canters away from its trainer in a straight line, moving 130 m away in 18.0 s. It then turns abruptly and gallops halfway back in 5.2 s. Average speed is 8.41 m/s
    Calculate its average velocity for the entire trip, using "away from the trainer" as the positive direction.

    2. Relevant equations

    velocity equation and speed equation
     
  2. jcsd
  3. Aug 30, 2007 #2

    berkeman

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    Staff: Mentor

    Welcome to the PF, lostfan. You need to show us some of your own work, before we can offer tutorial advice. Them's the PF rules.

    So please show us the equations that you allude to, and take a try at applying them to this problem.
     
  4. Aug 30, 2007 #3
    well i tried 120 - (and +) 65 over 18 - (and +) 5.2

    i kept getting the wrong answers ( i check if its wrong or right cause im using webassign.net)
     
  5. Aug 30, 2007 #4

    berkeman

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    Staff: Mentor

    Sorry, I'm not tracking what you are saying. The positive velocity for the first part is calculated based on 130m in 18s, right? What is that? And the negative velocity for the second part is what? And remember that the total average will be whatever velocity vector would have gotten you to that same final spot in the same total amount of time...
     
  6. Aug 30, 2007 #5
    i dont understand what you are saying sorry i only took phys for 3 days now
     
  7. Aug 30, 2007 #6

    berkeman

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    Staff: Mentor

    Let's take a simpler example. Let's say you drive a car to the right (call that the + direction) at 30mph for an hour, and then turn around and drive back half the way at that same 30mph (which would take half an hour, right?).

    Velocity is a vector with speed (the scalar part) and a direction. So the velocity for the first part of the trip was +30mph for 60 minutes, and the velocity vector for the second part was -30mph for 30 minutes. Now think about what the average velocity vector would be.... It's the vector that would get you from your starting place to the finishing place in 90 minutes, right?

    Now see if that helps your thinking about the problem that you are working on. Just spend some time thinking it through. And review the parts of your textbook that talk about vectors and velocity and stuff like that.

    I have to bail here pretty soon, but if you still have problems after a while, post more of your work and you should get some help from one of the other folks.
     
  8. Aug 30, 2007 #7
    thx for helping but i dont think its goin at a constant speed
     
  9. Aug 30, 2007 #8

    berkeman

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    Staff: Mentor

    The way the problem is worded, the speed is one constant value for the outbound portion, and a different constant speed for the inbound portion. I think you have what you need to figure it out. Good luck!
     
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