Physics simple clarification question
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Physics simple clarification question

[From: ] [author: ] [Date: 11-09-16] [Hit: ]
8 m/s^2. How long will it take for the rock to reach the ground, after it is dropped (to the nearest 0.the professor used -g (with up as the + dir) but I dont understand why. Isnt g negative when an object is going up??......
This is from a past exam:

"An airplane is flying horizontally at a constant height h=300 m, with speed v = 50m/s. At time t = 0, a rock is dropped from the airplane when it is directly over a tower so that the rock’s initial velocity is vox = 50 m/s and voy = 0. Assume that air resistance is negligible, and use g = 9.8 m/s^2. How long will it take for the rock to reach the ground, after it is dropped (to the nearest 0.1 s)"

the professor used -g (with up as the + dir) but I don't understand why. Isn't g negative when an object is going up???

-
The acceleration g always points down. Whether one wants to subtract a positive g or add a negative g is partially a matter of style, but treating g as negative automatically points it in the right direction on an x-y coordinate grid.

When an object is going down ballistically its acceleration is downward. When a object is going up ballistically its acceleration is still downward as it is slowing over time.
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