# Homework Help: A projectile problem

1. Feb 8, 2012

### jysim

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

A cannon is fired at an angle of 30 degrees above the horizontal from a cliff that is 20m above a flat river bottom. What is the initial speed of the projectile if it is found to land 40m from the base of the cliff?

2. Relevant equations
g = 9.8ms^-2
Vi = initial velocity

3. The attempt at a solution
I know that Vi cos 30t = 40, but does the "t" represent the time taken for the entire projectile motion, or just the upper symmetrical parabolic trajectory? I'm stuck at Vi cos 30t = 40 because what I did after simply didn't make any sense at all.

I've been stuck on this question for days on end, please help me with it! Thanks! :D

2. Feb 8, 2012

### JHamm

The $t$ represents the entire time of flight, from your equation you can actually solve for it. Now how about a second equation for the vertical part of the flight?