or u can write it as 21.1 m/s downwards ... so both are correct .
h = V1t + 1/2gt^2 = 1.5(2) + 4.9 (4) = 22.6 m downwards
2) V2 = 1.5 - 9.8 x 2 = - 18.1 m/s
and h = 1.5(2) - 4.9 x 4 = - 16.6 m which means 16.6 m downwards
-
Mailbag
======
vi = 1.5 m/s
a = 9.81 m/s^2
t = 2.00 sec.
d = ???
vf = ???
d = vi * t + 1/2 a t^2
d = 1.5 m/s * 2sec + 1/2*9.81 * 2^2
d = 22.62 meters
vf^2 = vi^2 + 2 * a * d
vf^2 = 1.5^2 + 2*9.81 * 22.62
vf^2 = 2.25 + 443.8
vf^2 = 446.05
vf = sqrt(446.05) = 21.12 <===== Speed if the helicoper is going down.
The distance between them is not 22.62 because after the mailbag is released, the helicopter is still moving down so ...
d = r * t
r = 1.50 m/s
t = 2.00 sec
d = 1.5 * 2 = 3 m
The difference is 22.62 - 3.00 m = 19.6 m/s < ====== Distance difference helicopter going down.
Part C
=====
If the helicopter is going up, we have an entirely different problem. Somehting has to be minus. We will make anything going up a minus. This time
vi = - 1.5 m/s
t = 2.00 sec
a = 9.81 m/s^2
d =
vf =
d = vi * t + 1/2 a t^2
d = -1.5 * 2 + 1/2 * 9.81 * 2^2
d = - 3 + 19.62
d = 16.62
Now the trick is that you are falling 16.62 m while the helicopter is going up. In two seconds the helicoper will go up 2 * 1.5 = 3 meters.going up.
The distance separating them is 16.62 - (-3) = 19.6
===========
Comment. The reason your teacher is getting a minus answer for the first one is the direction you take to be minus.
The problem might work out quite differently if you take up as plus and down as minus. Most people tend to do that because they are thinking of the coordinate axis (the -y axis goes down). I am one of those people ordinarily and your teacher is another. I have learned on this board that most people consider down as plus and up as minus which feels backwards to me, but one gets used to it.
A minus velocity just means you are going in the opposite direction to the one you assumed was positive.
If you the state which way is plus and which way is minus, you can NEVER be marked wrong. Famous last words, right?
