EJSS 2 Cannon aim at each other Projectile Motion Model m=1kg, g=-9.81m/s^2

[lookang]

EJSS 2 Cannon aim at each other Projectile Motion Model m=1kg, g=-9.81m/s^2


http://weelookang.blogspot.sg/2015/02/ejss-2-cannon-aim-at-each-other.html
different trails from y1=0 to 10 showing the deterministic collision always occur
run: https://dl.dropboxusercontent.com/u/44365627/lookangEJSworkspace/export/ejss_model_projectile2cannon/projectile2cannon_Simulation.xhtml
offline: https://dl.dropboxusercontent.com/u/44365627/lookangEJSworkspace/export/ejss_model_projectile2cannon.zip
scr: https://dl.dropboxusercontent.com/u/44365627/lookangEJSworkspace/export/ejss_src_projectile2cannon.zip
author: lookang, Fu-Kwun Hwang
author EJSS 5.0: Francisco Esquembre

Simulation embedded
sound file taken from http://www.freesfx.co.uk/sfx/bouncing
referencing from http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=144.0
based on a eariler model i made http://weelookang.blogspot.sg/2014/09/ejss-projectile-customized-after.html

Question:

do you know why the cannonball always hit each other provided they point at each other and velocities are pointed towards each other and greater than zero?
Answer :

is conceptually in the green line being without the effects of gravity, the cannons ball will meet as predicted by Newton's 1st Law of motion without net external force.
now, adding the same gravitational acceleration means both cannon balls will move down at the same rate ( pink lines).
therefore, the 2 cannon balls has to collide .