Animate a pendulum: Difference between revisions

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@@ Line 2,876: / Line 2,876: @@
 (define (pendulum)
-  (define (accel ?) (- (sin ?)))
+  (define (accel θ) (- (sin θ)))
-  (define ? (/ pi 2.5))
+  (define θ (/ pi 2.5))
-  (define ?' 0)
+  (define θ′ 0)
-  (define ?'' (accel (/ pi 2.5)))
+  (define θ′′ (accel (/ pi 2.5)))
-  (define (x ?) (+ 200 (* 150 (sin ?))))
+  (define (x θ) (+ 200 (* 150 (sin θ))))
-  (define (y ?) (* 150 (cos ?)))
+  (define (y θ) (* 150 (cos θ)))
-  (? (n)
+  (λ (n)
-    (define p-image (underlay/xy (add-line (empty-scene 400 200) 200 0 (x ?) (y ?) "black")
+    (define p-image (underlay/xy (add-line (empty-scene 400 200) 200 0 (x θ) (y θ) "black")
-                                 (- (x ?) 5) (- (y ?) 5) (circle 5 "solid" "blue")))
+                                 (- (x θ) 5) (- (y θ) 5) (circle 5 "solid" "blue")))
-    (set! ? (+ ? (* ?' 0.04)))
+    (set! θ (+ θ (* θ′ 0.04)))
-    (set! ?' (+ ?' (* (accel ?) 0.04)))
+    (set! θ′ (+ θ′ (* (accel θ) 0.04)))
     p-image))