擺線與軌跡

作者:王一哲
日期:2020/10/29

原理

假設有一個半徑為

r 的小球在水平地面上純滾動,球心O點向右前進的速度為
v
,小球繞O點順時鐘方向轉動的角速度為
ω
,由於小球與地面接觸點沒有相對滑動,因此

v=rω

則圓周上的某個點P移動的軌跡為擺線 (cycloid),若用參數式可以表示為

x=r(tsint)     y=r(1cost)

以下是使用 GeoGebra 繪製擺線的方法及成果。

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使用 GeoGebra 繪製的擺線動畫

繪圖步驟

  1. 輸入指令設定半徑r、速度v、角速度、週期T。

    ​​​​r = 1
    ​​​​v = 1
    ​​​​ω = v/r
    ​​​​T = 2*π/ω
    
  2. 輸入指令設定控制時間 time 的數值拉桿,再開啟屬性選單,將動畫設定為遞增(一次)

    ​​​​time = Slider(0, 3*T, 0.01*T)
    
  3. 輸入指令圓心位置x坐標值

    xO

    ​​​​x_O = v*times
    
  4. 輸入指令畫出圓心O點以及半徑為r的圓形c。

    ​​​​O = Point({x_O, r})
    ​​​​c = Circle(O, r)
    
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繪圖步驟1 ~ 4

  1. 使用點工具於圓形c上新增點A,再將點A拉到圓形c的正下方。

  2. 輸入指令畫出繞著O點順時鐘方向旋轉的P點,再隱藏點A。

    ​​​​P = Rotate(A, (-ω*time/π*180)°, O)
    
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繪圖步驟5、6

  1. 於P點上按滑鼠右鍵選取顯示移動軌跡或使用以下指令設定。
    ​​​​SetTrace(P, True)
    
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繪圖步驟7

  1. 輸入指令用參數式畫出擺線。
    ​​​​a = Curve(r*(t - sin(t), r*(1- cos(t))), t, 0, 6*π)
    
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繪圖步驟8

  1. 於數值滑桿 time 上按滑鼠右鍵選取開始動畫,點P的移動軌跡與使用參數式繪製的擺線會重疊。
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繪圖步驟9

  1. 若要清除移動軌跡,可以從選單點選檢視清除所有痕跡 或按快速鍵 Ctrl + F
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繪圖步驟10

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使用 GeoGebra 繪製的擺線動畫

相關指令的官方說明書

  1. 數值滑桿 https://wiki.geogebra.org/en/Slider_Command
  2. https://wiki.geogebra.org/en/Point
  3. https://wiki.geogebra.org/en/Circle_Command
  4. 旋轉 https://wiki.geogebra.org/en/Rotate_Command
  5. 設定軌跡 https://wiki.geogebra.org/en/SetTrace_Command
  6. 曲線 https://wiki.geogebra.org/en/Curve_Command


tags:GeoGebra