/* model of a bouncing ball.  the ball can be elastic, in which case
   it loses half its velocity at each bounce, or inelastic, in which 
   case it behaves like a spring with spring constant k and damping
   constant e.  The model selection can be done by setting Compress or
   NoCompress in the basic model, which also incorparates the dynamics
   of the ball in the air.
*/
    
    
ht = 10, ht' = 0,
sample(ht), 
always {m = 1, g = 10, k = 10000, e = 10},
always { Compress,
         cont(ht),
         if ht > 0 then ht'' = -g   // free fall
         else Bounce()
},
always {
    Bounce = (){
         if Compress then SpringBounce(),
         if NoCompress then InelasticBounce()
    },
    InelasticBounce = (){
          ht' = - 0.5 * prev(ht')
    },
    SpringBounce = (){
          ht'' + e * ht' + k * ht + g = 0
    }
}