# Projectile motion problem

1. Feb 1, 2009

### estie

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

A particle leaves its initial position x_0 at time t=0 moving in the positive x direction with speed v_0 but undergoing acceleration of magnitude a in the negative x direction. Find expressions for (a) the time when it returns to the position x_0.

2. Relevant equations

v=v_0-at
x=x_0+v_0*t

3. The attempt at a solution

i know that v=v_0, but that gives me a time of 0, which doesn't make sense. i'm also not supposed to use x_0 in the solution. otherwise, i think the solution should be (v-v_0)/-a.

2. Feb 1, 2009

### Delphi51

This formula is for uniform motion (zero acceleration).
You need to use x = Vo*t + .5*a*t^2.
When you set x = 0, you will have a quadratic equation with two solutions. One will be t=0 of course and the other greater than zero.

3. Feb 1, 2009

### Mk

By the way estie, if you wish to use TeX-style notation on this forum you can wrap [ tex] [/tex] tags around your equations so it'll look more like this:
$$x=x_0+v_{0}t$$

Welcome to PF!