An arrow is shot straight up in the air at an initial speed of 15.0 m/s. After how much time is the arrow moving downward at a speed of 8.00 m/s?

Question

anonymous · Answer

final velocity = 0 

0 = 15 + (-9.8)(t) 
t = 1.53 s

That is only to go up. 

Coming down, 
initial velocity = 0 
final velocity = 8
a = 9.8

t = (8-0)/9.8
=0.816s

Add them up = 2.346s

anonymous · Answer

Awesome thanks!

anonymous · Answer

Alright quick questions, when I am working out the downward time I am getting a negitive number. Does that just indicate the direction of movement like with acceleration?

anonymous · Answer

gravity! set it's direction right.

anonymous · Answer

Time will ALWAYS be positive, no matter what. You need to cancel out the -ve signs using acceleration and velocity.

anonymous · Answer

Ok so when i'm going down gravity is positive then if i'm fighting gravity its negitve correct?

turingtest · Answer

You can define either direction as positive so long as you are consistent in your work

anonymous · Answer

Yes. That is how most people learn it.

anonymous · Answer

Awesome guys thanks a ton!