Can explain what this do? I can't make sense of it<br /><br />Zero condition<br /><br />if(vy>0) return 1;&nbsp; // particle is not falling&nbsp; [b][color=red]{what does return 1 mean?}[/color][/b] [b][color=purple]which i found out later means do nothing[/color][/b]<br /><br /><br /><br />return y;&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;// displacement of particle above floor [b][color=red]{why return y?}[/color][/b] [color=purple][b]which i found out later means check for y = 0 to action [/b][/color]<br /><br /><br /><br />Action&nbsp; &nbsp; &nbsp; Check box End step at event <br /><br />// action if particle is falling and below the floor<br />vy = Math.abs(vy)*coef_of_restitution;&nbsp; [b][color=blue]{to return the value of Math.abs(vy)*coef_of_restitution to variable vy to simulate hitting n bouncing up like collision}[/color][/b]<br /><br /><br />ODE [color=blue][b]{Ordinary Differential Equations}[/b][/color] page is<br /><br />dy/dt = vy<br /><br />and <br /><br />dvy/dt = - g<br /><br /><br />thx for reply!