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Just a quickie!

  1. May 24, 2008 #1
    Um this isn't really a homework question, just a general question in relation to the sign you put in front of 9.8 m/s^2?
    I don't know, when do i put a negative or positive in front of it when doing projectile motion and straight line motion questions?

    Do you put negative in front of 9.8 m/s^2 when the object is being thrown up or do you put positive?

    I just don't know.
    If anyone could help me I'd be ever so grateful:)
    Thankyou very much:)
  2. jcsd
  3. May 24, 2008 #2


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    Homework Helper

    It depends on how you model the question. Do you take direction upwards to be positive or negative? If let the skyward direction be positive, then you take -g and +g when you take downwards to be positive. Do this consistently throughout the question, don't switch signs.
  4. May 26, 2008 #3
    This all depends on the coordinate system that you choose. A good idea before starting a question is to set up your coordinate system such that it will make your solution simpler. Make sure that you be consistent though in your signs and it helps to draw a diagram to be reminded of your choice.

    With that being said, lets assume you let the y-axis point up, with an object falling down, you'd put in the negative sign (and positive if thrown up). However, if you choose your axis to point down, a falling object will have a postive sign and negative if thrown up.

    So once again it all has to do with your coordinate system and how you set it up.

    Hope this helps.
    Cheers CB
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