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EASY Question from Physics Beginner

  1. May 26, 2005 #1
    I am going to be taking Intro to Physics in the Fall when I go back to college. I've always been terrible in mathematics and have never taken physics before!

    I found a cool little website called physicsclassroom.com. In an effort to get a head start on what will most likely be my toughest subject, I have printed out Lesson One which reads like Greek to me. I am positive someone can answer this question though.

    Regarding acceleration, the tutorial states "In Example D, the object is moving in the negative direction (i.e. has a negative velocity) and is speeding up." I understand WHAT it is saying (i.e., I would be able to answer a question on a test) but I do not understand WHY, or HOW, rather, something can move in a "negative direction." To me, there is up,down, left and right -all of which are positives. If something is moving, it's going somewhere tangible. To move in a negative direction makes me assume it is going someone INtangible. Is the object going to Hell?

    Can someone give me a real-life example of something moving in a negative direction?
  2. jcsd
  3. May 26, 2005 #2
    Acceleration is a Vector quantity. Vectors have both magnitude and direction. Therefore, some direction is chosen as positive, and the 'other' direction is negative.
  4. May 26, 2005 #3


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    We talk about negatives and positives when we setup our problem on the basis of a coordinate system. The Y and X axis is a good place to start. Remember that? Regular ol plain graph. Well, at 0,0, or the origin (center), nothing is happening yet. When you start going into a "negative" direction however, you are going in the -x or -y and since there is an origin designated as 0,0, it is obvious that there indeed is a "negative" direction. Theres no "negative" movement in real life but for the sake of doing calculations and graphing and working with problems, there is according to our graphs.
  5. May 26, 2005 #4
    That's what I thought, but I wanted to be positive (no pun intended!). I figured it had something to do with that darn X & Y axis!!!

    Thanks though!!! I'm sure I'll become a regular here!
  6. May 27, 2005 #5
    welcome physicsvirgin
  7. May 27, 2005 #6

    I'm sure you'll be seeing a lot more of me in the future.
  8. May 27, 2005 #7


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    It's more than just about the X and Y axis though. When doing experiments, you have to pick some quantity by which all else can be referenced. When if are experimenting with, for example, motion and friction of a moving object (such as a toy car), you will call its forward direction positive. Friction (i.e. brakes), rather than "decelerating" the car, apply a "negative acceleration".
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