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Gravity = 9.80 m/s^2

  1. Jul 6, 2013 #1
    What is the general rule for knowing if g is negative or not? In some formulas, its already negated so i just leave it as is. In this circular motion problem, g is not negative for tangential acceleration= gsin(x).
  2. jcsd
  3. Jul 6, 2013 #2


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    Staff: Mentor

    'g' is an unsigned constant. Any sign given to it is used to denote direction according whatever coordinate system you choose to use. For example, for a given problem you might choose "down" to be positive, in which case the acceleration due to gravity is +g. Or, if you choose "up" as positive, then -g is the acceleration.
  4. Jul 6, 2013 #3

    Doc Al

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    Whether the acceleration due to gravity is negative or not just depends on your coordinate system and sign convention. Generally, the constant g = 9.8 m/s^2 is always taken as a positive value. That represents the magnitude of the acceleration due to gravity (at the earth's surface).
  5. Jul 6, 2013 #4
    g is just a symbol. It can mean anything, but in physics it often means the free-fall acceleration at the surface of the Earth, or its magnitude. Acceleration is a vector, so it has a direction; when the problem is confined to one dimension, the direction is usually denoted by the sign; then the sign of g will depend on what direction is "positive" or "negative". On the other hand, magnitude of any vector is positive; so when g is a magnitude, it is always positive.

    Ultimately, it is all a matter of choice. Sometimes you are the one who makes the choice; sometimes the choice is made by the author of a problem.
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