diff --git a/src/kernel/noise.lisp b/src/kernel/noise.lisp
index 7a6f060..c6002cb 100644
--- a/src/kernel/noise.lisp
+++ b/src/kernel/noise.lisp
@@ -91,6 +91,9 @@
                 (* y1 (+ (* x0 p012) (* x1 p112) (* x2 p212)))
                 (* y2 (+ (* x0 p022) (* x1 p122) (* x2 p222))))))))
 
+(defun float-noise (v)
+  (noise (p! v 0 0)))
+
 (defun turbulence (p n-octaves)
   (let ((sum 0.0)
         (scale 1.0))
@@ -108,6 +111,9 @@
          (dz (- (noise (p! x y (+ z delta))) (noise (p! x y (- z delta))))))
     (p! dx dy dz)))
 
+(defun float-noise-gradient (v &optional (delta 0.01))
+  (noise-gradient (p! v 0 0) delta))
+
 (defun color-noise (p &optional (delta 0.01))
   (let ((pn (p:normalize (noise-gradient p delta))))
     (c! (abs (p:x pn)) (abs (p:y pn)) (abs (p:z pn)))))
diff --git a/src/plugins/force-field.lisp b/src/plugins/force-field.lisp
index 53d0dc3..059b3d4 100644
--- a/src/plugins/force-field.lisp
+++ b/src/plugins/force-field.lisp
@@ -13,7 +13,7 @@
 ;;;; constant-force-field ======================================================
 
 (defclass constant-force-field (force-field)
-  ((force-vector :accessor force-vector :initarg :force-vector :initform (p! 0 -9.81 0))))
+  ((force-vector :accessor force-vector :initarg :force-vector :initform (p! 0 0 0))))
 
 (defmethod field-value ((field constant-force-field) point time)
   (declare (ignore point time))
@@ -33,13 +33,25 @@
         (p! 0 0 0)
         (p:scale dir (/ (magnitude field) (* dist dist))))))
 
-;;;; noise-force-field =========================================================
+;;;; time-varying-force-field ==================================================
 
-(defclass noise-force-field (force-field)
+(defclass time-varying-force-field (constant-force-field)
   ((noise-frequency :accessor noise-frequency :initarg :noise-frequency :initform 1.0)
    (noise-amplitude :accessor noise-amplitude :initarg :noise-amplitude :initform 1.0)))
 
-(defmethod field-value ((field noise-force-field) point time)
+(defmethod field-value ((field time-varying-force-field) point time)
+  (declare (ignore point))
+  (p+ (force-vector field)
+      (p:scale (float-noise-gradient (* time (noise-frequency field)))
+               (noise-amplitude field))))
+
+;;;; 3d-noise-force-field =========================================================
+
+(defclass 3d-noise-force-field (force-field)
+  ((noise-frequency :accessor noise-frequency :initarg :noise-frequency :initform 1.0)
+   (noise-amplitude :accessor noise-amplitude :initarg :noise-amplitude :initform 1.0)))
+
+(defmethod field-value ((field 3d-noise-force-field) point time)
   (declare (ignore time))
   (p:scale (noise-gradient (p:scale point (noise-frequency field)))
            (noise-amplitude field)))
diff --git a/test/demo-misc.lisp b/test/demo-misc.lisp
index dac5e17..aecc8b6 100644
--- a/test/demo-misc.lisp
+++ b/test/demo-misc.lisp
@@ -168,7 +168,7 @@ in this and demos below, update the *EXAMPLE-MOL-FILENAME* for your setup.")
 (with-clear-scene
   (let* ((p-sys (make-particle-system (make-point-cloud (vector (p! 0 0.1 0)))
                                       (p! 0 .2 0) 10 -1 'particle
-                                      :update-angle (range-float (/ pi 8) (/ pi 16))
+                                      :update-angle (range-float 20.0 10.0)
                                       :life-span (rand1 5 10))))
                                         ;    (setf (draw-live-points-only? p-sys) nil)
     (setf (name p-sys) 'p-system)
@@ -187,7 +187,7 @@ in this and demos below, update the *EXAMPLE-MOL-FILENAME* for your setup.")
 (with-clear-scene
   (let* ((p-sys (make-particle-system (make-point-cloud (vector (p! 0 0.1 0)))
                                       (p! 0 .2 0) 10 -1 'particle
-                                      :update-angle (range-float (/ pi 8) (/ pi 16))
+                                      :update-angle (range-float 20.0 10.0)
                                       :life-span (rand1 5 10)))
          (shape (make-point-instancer-group p-sys
                                             (make-octahedron .2))))
diff --git a/test/demo-parametric-curve.lisp b/test/demo-parametric-curve.lisp
index 6e5bbac..55452a7 100644
--- a/test/demo-parametric-curve.lisp
+++ b/test/demo-parametric-curve.lisp
@@ -155,8 +155,10 @@ Make sure you have opened the graphics window by doing:
   (make-spirograph-spring *scene* 2.5 0.7 0.6 t 0.0 2.0 2.0 1.0 0.5
                           :show-assembly? t :color (c! 0 0 1)
                           :do-collisions? nil
-                          :force-fields (list (make-instance 'constant-force-field
-                                                             :force-vector (p! .1 0 0)))))
+                          :force-fields (list (make-instance 'time-varying-force-field
+                                                             :force-vector (p! 0 0 0)
+                                                             :noise-frequency 10.0
+                                                             :noise-amplitude 8.0))))
 ;;; hold down space key in 3D view to run animation
 
 ;;; run simulation
@@ -168,8 +170,11 @@ Make sure you have opened the graphics window by doing:
   (make-spirograph-spring *scene* 2.5 0.7 0.6 t 0.0 2.0 2.0 1.0 0.5
                           :show-assembly? t :color (c! 0 0 1)
                           :do-collisions? t
-                          :force-fields (list (make-instance 'constant-force-field
-                                                             :force-vector (p! .1 0 0)))))
+                          :force-fields (list (make-instance 'time-varying-force-field
+                                                             :force-vector (p! 0 0 0)
+                                                             :noise-frequency 10.0
+                                                             :noise-amplitude 8.0))))
+
 ;;; hold down space key in 3D view to run animation
 
 ;;; run simulation
diff --git a/test/demo-particle.lisp b/test/demo-particle.lisp
index 6da336e..cf1ad66 100644
--- a/test/demo-particle.lisp
+++ b/test/demo-particle.lisp
@@ -194,7 +194,7 @@ force field.
                  'dynamic-particle
                  :life-span -1 ;infinite life-span
                  :do-collisions? nil
-                 :force-fields (list (make-instance 'noise-force-field
+                 :force-fields (list (make-instance '3d-noise-force-field
                                                     :noise-frequency 0.2
                                                     :noise-amplitude 0.2)))))
     (add-shape *scene* shape)
@@ -348,6 +348,36 @@ Create a row particle systems with varrying attributes.
 (update-scene *scene* 30)               ;do update for batch testing
 
 
+#|
+(Demo 14 particle) dynamic particle system =====================================
+
+Dynamic particles growing from a superquadric with a time-varying force field.
+|#
+
+(format t "  particle-system 14...~%") (finish-output)
+
+(with-clear-scene
+  (let ((p-source (freeze-transform (make-superquadric 16 8 2.0 1.0 1.0))))
+    (setf (point-source-use-face-centers? p-source) t) ;comment out to use poly points
+    (let ((p-sys (make-particle-system-from-point-source
+                  p-source
+                  (lambda (v) (p:scale v 0.2))
+                  'dynamic-particle
+                  :life-span -1 ;infinite life-span
+                  :do-collisions? nil
+                  :force-fields (list (make-instance 'time-varying-force-field
+                                                     :force-vector (p! 0 -0.0 0)
+                                                     :noise-frequency 100
+                                                     :noise-amplitude 4.0)))))
+      (add-shape *scene* p-source)
+      (add-shape *scene* p-sys)
+      (add-motion *scene* p-sys))))
+;;; hold down space key in 3D view to run animation
+
+;;; for automated testing
+(update-scene *scene* 30)
+
+
 #|
 END ============================================================================
 |#
diff --git a/test/demo-sweep-mesh.lisp b/test/demo-sweep-mesh.lisp
index ec462d8..77aa04a 100644
--- a/test/demo-sweep-mesh.lisp
+++ b/test/demo-sweep-mesh.lisp
@@ -83,7 +83,7 @@ with the particle system.
                                                         (lambda (v) (p:scale v 0.2))
                                                         'particle
                                       :life-span (round (rand2 5 10))
-                                      :update-angle (range-float (/ pi 16) (/ pi 32))))
+                                      :update-angle (range-float 20.0 10.0)))
          (sweep-mesh-group (make-sweep-mesh-group (make-circle 0.1 6) p-sys
                                                   :taper 1.0 :twist 0.0)))
     (add-shape *scene* p-source)
@@ -107,7 +107,7 @@ The mesh automatically grows with the particle system.
          (p-sys (make-particle-system-from-point (p! 0 3.0 0) 1 (p! -.5 0 -.5) (p! .5 0 .5)
                                                  'climbing-particle
                                                  :support-polyh shape
-                                                 :update-angle (range-float (/ pi 8) (/ pi 16))
+                                                 :update-angle (range-float 20.0 10.0)
                                                  :life-span -1))
          (sweep-mesh-group (make-sweep-mesh-group (make-circle 0.2 6) p-sys
                                                   :taper 1.0 :twist 0.0)))
diff --git a/test/demo-uv-mesh.lisp b/test/demo-uv-mesh.lisp
index 8c0e40c..594e882 100644
--- a/test/demo-uv-mesh.lisp
+++ b/test/demo-uv-mesh.lisp
@@ -150,7 +150,7 @@ Create a group of UV-MESH shapes along a PARTICLE-SYSTEM.
 
 (with-clear-scene
   (let* ((p-sys (make-particle-system-from-point (p! 0 0 0) 10 (p! -.2 .2 -.2) (p! .2 .5 .2) 'particle
-                                                 :update-angle (range-float (/ pi 8) (/ pi 16)))))
+                                                 :update-angle (range-float 20.0 10.0))))
     (setf (name p-sys) 'p-system)
     (add-shape *scene* p-sys)
     (add-motion *scene* p-sys)))