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- Thread starter Ali Hamaiz
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Svein

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g is a *vector *(pointing to the center of the Earth).

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CWatters

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What sign did you use for the initial velocity? ditto the displacement/height?I threw a ball vertically upward the value of g at its max height (vi=0) ,what will be the value of g +10 or -10 m/s^2. I think it can be both if you consider the cartisean plane's cordinate , it depends upon refrence.

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This means with the sign of Vi you decide the value of g to be positive or negative.

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CWatters

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Correct. It depends which direction (up or down) you define as positive.

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jtbell

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If position, velocity, and acceleration are defined to be positive in the upwards direction and negative in the downwards direction, then the (y-component of) acceleration of a freely-falling object is ##a = -g## (more precisely ##a_y = -g##) and its position as a function of time is ##y = y_0 + v_{0y} t + \frac 1 2 a_y t^2 = y_0 + v_{0y} t - \frac 1 2 gt^2##.

If position, velocity, and acceleration are defined to be positive in the downwards direction and negative in the upwards direction, then the (y-component of) acceleration of a freely-falling object is ##a = +g## (more precisely ##a_y = +g##) and its position as a function of time is ##y = y_0 + v_{0y} t + \frac 1 2 a_y t^2 = y_0 + v_{0y} t + \frac 1 2 gt^2##.

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sophiecentaur

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Soon your diagram can be in your head.

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I got it thanks .

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If you call the directional sense of gravitational force negative, then

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Best practice is to define your coordinate system on the paper with y clearly pointing up. But if you point y down, gravitational acceleration is going to be positive.

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A.T.

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g is avector(pointing to the center of the Earth).

It basically boils down to what you mean by "g".gis defined as apositivenumber, themagnitudeof the (vector)

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The whole exercise was meant to show that the location on where we define to be "zero" and the orientation of our coordinate axis are really arbitrary, and that no matter how we do this, it should not affect the final answers. Mother Nature doesn't give a hoot on which direction we call "positive" and "negative", and thus, your answer shouldn't either.

Zz.

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CWatters

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...as long as they are consistent in their definition.The whole exercise was meant to show that the location on where we define to be "zero" and the orientation of our coordinate axis are really arbitrary, and that no matter how we do this, it should not affect the final answers. Mother Nature doesn't give a hoot on which direction we call "positive" and "negative", and thus, your answer shouldn't either.

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