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I need things cleared up when it comes to acceleration.

Is acceleration negative when it points in a certain direction...or when an object is slowing down?

I was working on a free falling bodies word problem:

A diver springs upward with an initial speed of 1.8 m/s from a 3.0-m board.

(a) find the velocity with which he strikes the water.[Hint: when the diver reaches the water, his displacement is y=-3.0 meters (measured from the board), assuming that the downward direction is chosen as the negative direction.]

(b) What is the highest point he reaches above the water?

I can get the final velocity from part (a) alright,

To find the distance he jumps up, if I use a positive acceleration in the equation:

Vf^2=Vi^2+2ad

then I get a negative distance...which I shouldn't if hes traveling UPWARD as he jumps.

I use the same equation to get the final velocity as he hits the water:

Vf^2=vi^2+2ad

and I used both a negative displacement vector(-3.17m) and a negative acceleration vector(-9.8 m/s^2) and got + or - 7.9 m/s as the answer(I used the minus one, and it was the right answer)

but my question is: when is acceleration negative?

Thanks

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# Homework Help: Acceleration, screwed up vector?

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