The question is: A ball is thrown directly downward with an initial speed of 8.0

Question

The question is:

A ball is thrown directly downward with an initial speed of 8.00m/s from a height of 30.0m. After what time interval does it strike the ground.

So I went through the problem and got the answer 1.79 sec. but after reviewing some notes from my teacher, I got the idea that in equations where an object is dropped down, the height has to be counted as negative. so I went through the problem again using -30m, but now my answer doesn't check out when plugged back in to x - x0 = V0 (t) + 1/2 at^2

Is there something I'm missing about this idea that height or distance is negative in drop-down questions?

Explanation / Answer

As long as your coordinate system the answer should be consistent. In this case, you did change 30m to -30m. But did you also change 8m/s to -8m/s? The same applies to the acceleration.
It's really only a matter of reference point
I think my teacher was trying to describe an object moving downward, but in that case, just picturing it, the height should be positive, as well as the acceleration (because as it falls, it covers more distance each second), etc.

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