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Homework Help: Positive or negative velocity and acceleration

  1. Sep 13, 2009 #1
    1. The problem statement, all variables and given/known data
    A motion detector is placed on the top of a ramp sloping down. A cart is placed at the other end of the ramp (bottom of the ramp). The cart is pushed up the ramp towards the motion detector (top of ramp), and stopped at it's highest point.

    2. Relevant equations
    1. Is the object speeding up or slowing down?
    2. Is the velocity of the object positive or negative?
    3. Is the velocity getting more positive/less positive/more negative/less negative?
    4. Does the object have a constant positive or negative acceleration?

    3. The attempt at a solution

    1. When we push the cart, it's speeding up, but it'll slow down over time because it's going up slope, so is it speeding up or slowing down?
    2. I can't determine whether it's positive or negative because the cart is moving toward the motion detector, which would make it negative...right? But it's initial position is at the bottom of the ramp, which makes it negative.
    #3 and #4 are the same as above. I can't tell which one is negative or positive.

    Also, the graphs from the motion detector shows:
    Position vs. time: sloping down
    Velocity vs. time: sloping up

    I've tried to read the textbook, but I'm still confused. I know that direction of an object determine whether its velocity is negative or positive.
    Up, right: positive.
    Down, left: negative.
    In the textbook, there's a chart like this:
    Initial velocity--------------Accelearation-----------------Motion
    (+)-------------------------(+)-------------------------speeding up
    (-)-------------------------(-)--------------------------speeding up
    (+)-------------------------(-)--------------------------slowing down
    (-)-------------------------(+)--------------------------slowing down
    (-)or(+)--------------------(0)--------------------------constant velocity
    (0)-------------------------(-)or(+)---------------------speeding up from rest
    (0)-------------------------(0)--------------------------remaining at rest

    EDIT: Thanks in advance.
    Last edited: Sep 13, 2009
  2. jcsd
  3. Sep 15, 2009 #2


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    Staff Emeritus
    Science Advisor
    Homework Helper

    Welcome to PF :smile:

    If you have these graphs, don't they indicate whether velocity etc. are positive or negative?
  4. Sep 16, 2009 #3
    Thank you for your answer, but I've already turned in and graded my lab. I kinda understand it now. The thing that messed me up is the direction.

    In the text book, it says "down" direction is negative, and "up" direction is positive. Since the detector is on top of the ramp, I would assume that the direction away from the detector, which is going DOWN the ramp is negative, but it turns out, AWAY from the detector is positive. >.<

    Thanks for your answer. ^-^
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