# Homework Help: Projectile Motion: Finding Time

1. Nov 19, 2007

### degrass1fan

1. The problem statement, all variables and given/known data Ok, so i'm having alot of trouble finding time when it is not given in a projectile motion problem. Here's what I have set up:

X
A=O
Vi=100.0
VF=100.0
D=?
T=?

Y
A=-9.8
Vi=O
Vf=?
D=8848m
T=?

2. Relevant equations

Distance(d=1/2at2=vit)

3. The attempt at a solution
Ok, here's where my problem is, I can never get the time right. I have failed 2 tests and numerous homework assigments because something is wrong. Here's my attempt at it:

8848=1/2(-9.8)t2

I divide both sides by 8848 then square root it.

I ALWAYS get the wrong time and in turn that gives me the wrong distance and in turn I fail the test.

Last edited: Nov 19, 2007
2. Nov 19, 2007

### Hells_Kitchen

The general idea to the projectile motion problems is:

1. Find time using the relationship of displacement in the y-direction.
2. FInd displacement in the x-direction x= v*t keeping in mind that in the x direction the velocity will be constant.

Now in this particular case i believe that you only have to worry about the first step.

y = .5 gt^2 => t = sqrt(2y/g)