Projectile Motion Animation SketchUp

cannon.rb

class Cannon
  def initialize(bearing, elev, velocity)
    @item = Sketchup.active_model.entities[0]
    @t = 0
    @v = velocity.to_f
    @angle = elev.to_f
    @bearing = bearing.to_f
    @vy = @v * Math::cos(@angle.degrees)
    @vz = @v * Math::sin(@angle.degrees)

    #@zi = @item.bounds.center.z
    @zi = @item.transformation.to_a[-2]
    @last_pt = nil
  end
  def nextFrame(view)
    x = 0
    y =  @vy * @t
    z = (@vz * @t) + 0.5 * -10.0 * (@t ** 2)
    pt = Geom::Point3d.new(x, y, z)
    t = Geom::Transformation.new(pt)
    @item.transformation =  t
    if @last_pt
    #Sketchup.active_model.entities.add_cpoint @item.bounds.center
    Sketchup.active_model.entities.add_line pt, @last_pt
    end
    @last_pt = pt
    @t += 0.2
    if z < 0
      return false
    else
      return true
    end
  end
end

def fire(bearing, angle, velocity)
  Sketchup.active_model.active_view.animation = Cannon.new(bearing, angle, velocity)
  nil
end