a submarine was situated 450 feet below the sea level if decents @ 8 feet per minute what will be its position after 15 minutes
Answers
Let's take sea level as our horizon: if you are on the water surface, your height is h=0. Also, let's assume a vertical axis pointing upwards. This means that the submarine, being 800 feet away from the horizon in the opposite directionwith respect to the axis, will have a negative height of h0=−800.
If it ascends 250 feets, it gains altitude, so the new height is h1=−800+250=−550 feet.
Note that this problem heavily depends on the coordinate system you adopt. In fact, I may have chosen an axes whose origin is set at the initial position of the submarine (which would have become the h0=0), pointing upwards, and the new position would have been h1=0+250=250 feet.
The only things that won't change is that the new position will be 250 "more" than the old one. But if you want to compute absolute values, you need to specify a coordinate system, otherwise you may have as many answers as you want.
u=450
t=15min
speed=d/t
8=450+v/15
8*15=450+v
120-450=v
-330=v
v=-330