# Projectile motion help

1. Oct 9, 2006

### pmtecra

a cannon is launched @ 30deg angle above the horizontal with an unknown velocity the projectile is observed to hit a vertical wall at a horizontal distance of 100.0m and it hits a height of 15.0m above the ground, find the initial velocity.

the following is what i did to solve it, but the fact that i didnt even use the 30deg at all throughout the solution has me second guessing my solution, so i was wondering if anyone could point out what i did wrong

any help is appreciated

thanks

http://desi.thriceshy.com/images/1440scan.jpg [Broken]

Last edited by a moderator: May 2, 2017
2. Oct 9, 2006

### Astronuc

Staff Emeritus
The height of the wall 15m divided by distance 100 m = 0.15 and tan-1 (0.15) = 8.53°, which means that the projectile is fired above that angle and therefore the projectile must be traveling downward in its parabolic trajectory.

Neglecting air resistance, the trajectory is parabolic, and one must find the function y(x) which describes the parabolic trajectory and find v which gives y=15 m at x=100 m.

Something like this - http://hyperphysics.phy-astr.gsu.edu/Hbase/traj.html#tra8

Last edited: Oct 9, 2006