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Table for signs of position, displacement, and velocity

  1. Aug 13, 2015 #1
    1. The problem statement, all variables and given/known data
    SL8oRHo.jpg

    2. Relevant equations
    Velocity = Displacment/Time

    3. The attempt at a solution
    Ok, so I thought this was straightforward at first, but I'm having my doubts. The position column, I filled out with the standard assumption the right and up are positive and left and down are negative. For displacement, I was a bit confused as to what it was asking, so I figured each situation started at a point and wanted the displacement for that point. By that logic I got the signs for displacement which happened to be the same as the signs of position. Finally for velocity, I first thought it was based on whether the object was speeding up or slowing down. But, I thought that shouldn't really matter for the sign of velocity, but for whether it is increasing or decreasing, which the question doesn't care about. I know velocity = displacement/time, and since time is always positive, the sign of velocity should be dependent on displacement. So I put the same signs from displacement into velocity.

    PS, I don't have any background in physics, this is my summer assignment to which they gave no background and no textbook -.-
    The fact that all the signs ended up the same still makes me worried that I am doing/did something wrong.
     
  2. jcsd
  3. Aug 13, 2015 #2
    That is acceleration!

    The sign of velocity indicates its DIRECTION


    edit: read your answers. no problems
     
    Last edited: Aug 13, 2015
  4. Aug 13, 2015 #3
    So the increasing/decreasing stuff was just a red herring. Thanks for your confirmation :)
     
  5. Aug 13, 2015 #4
    well I think it is!

    If a car is going forward at 100 mph one moment, and going forward at 50mph the next moment ; then its velocity is still positive (it is going forward!)

    however, it is (de)accelerating.


    for interest, note that if a car is turning a corner, its velocity IS changing because its direction is changing. your 'ball' is going in a straight line.
     
  6. Aug 13, 2015 #5

    SammyS

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    It seems that none of these specify a position.

    Should there perhaps be another column labeled acceleration. The situations described fairly beg for that.
     
  7. Aug 13, 2015 #6
    yes, that was my first thought, then I edited the post.

    The questions seem to be red herrings - they specifically mention acceleration, but do not give the option. Its almost trying to trick the student, and there should be no trick questions!
     
  8. Aug 13, 2015 #7

    SammyS

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    ... but has the student provided an image which includes the complete table?
     
  9. Aug 13, 2015 #8
    That is the complete table. No acceleration column. iG2GGwr.jpg
     
  10. Aug 13, 2015 #9

    haruspex

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    I agree that logically there should be an acceleration column, but it should be instead of the position and displacement columns. No origin nor initial position are specified, so position and displacement are both indeterminate.
     
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