# Homework Help: Projectile Motion

1. Dec 30, 2009

### majormuss

1. The problem statement, all variables and given/known data

Guys, this question is really frustrating me. Ho can I can I Find time when magnitude of acceleration when time is not giving, and also the object is not moving vertically(so I cant use a=9.81m/s?... I tried using (Change in velocity)V= Acceleration* Time,but I eventually get stuck!!
Can anyone help Clarify things to me.
2. Relevant equations
(Change in velocity)V= Acceleration* Time

3. The attempt at a solution

2. Dec 30, 2009

### tiny-tim

Welcome to PF!

Hi majormuss! Welcome to PF!

(what exactly is the question? )

If the projectile starts at an angle to the vertical, then you need two equations, one for the vertical component of motion with a = -9.81 m/s2, and one for the horizontal component with a = 0.

3. Dec 30, 2009

### majormuss

Re: Welcome to PF!

thnx for the response, but when I assume Acceleration is 0 for horizontal motion my equation ends up... V(f)-V(i)/a... then i get a denominator of 0, which makes it undefined. I cant find time

4. Dec 30, 2009

### pgardn

Re: Welcome to PF!

Well Vf and Vo should be the same for the horizontal because it does not change while in the air (assume no air friction). The previous poster stated there is no acceleration in the horizontal.

In many of these projectile problems, time is usually determined by info. given about motion in the vertical in which case you use your -9.81 m/s/s as your acceleration.

5. Dec 31, 2009

### tiny-tim

As pgardn says, Vix and Vfx are the same.

I see now that the problem is that you're only using one of the constant acceleration equations …
There are two others!