Kinetic energy of the compound pendulum











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Question:



A rigid, uniform rod of length $L$ and mass $M$ is pivoted to the origin $O$ at one end, and is then left to swing freely in a vertical plane.



If the angle the rod makes with the vertical is $theta$, show that the kinetic energy of the rod is $$T = frac 16 ML^2{dot theta} ^2$$





Attempt:



In terms of $theta$, the position of the center of mass is



$$(x,y) = bigg(frac L2 sin theta, -frac L2 cos theta bigg)$$



In a previous part of the question, I have shown that the moment of inertia of the rod about one of its endpoints is $$I = frac 13 ML^2$$



Thus, the total kinetic energy of the rod should be



begin{align}
T & = frac 12 M(dot x^2 + dot y^2) + frac 12 I omega ^2 \
& = frac 12 Mbigg[bigg(frac L2 dot thetacos theta bigg)^2 + bigg(frac L2 dot theta sin thetabigg)^2 bigg] + frac 12bigg(frac 13 ML^2 bigg) dot theta ^2 \
& = frac 18 ML^2 dot theta ^2 + frac 16 ML^2 dot theta ^2 \
& neq frac 16 ML^2 dot theta ^2
end{align}



Is this perhaps not the right way to do this question?










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    Question:



    A rigid, uniform rod of length $L$ and mass $M$ is pivoted to the origin $O$ at one end, and is then left to swing freely in a vertical plane.



    If the angle the rod makes with the vertical is $theta$, show that the kinetic energy of the rod is $$T = frac 16 ML^2{dot theta} ^2$$





    Attempt:



    In terms of $theta$, the position of the center of mass is



    $$(x,y) = bigg(frac L2 sin theta, -frac L2 cos theta bigg)$$



    In a previous part of the question, I have shown that the moment of inertia of the rod about one of its endpoints is $$I = frac 13 ML^2$$



    Thus, the total kinetic energy of the rod should be



    begin{align}
    T & = frac 12 M(dot x^2 + dot y^2) + frac 12 I omega ^2 \
    & = frac 12 Mbigg[bigg(frac L2 dot thetacos theta bigg)^2 + bigg(frac L2 dot theta sin thetabigg)^2 bigg] + frac 12bigg(frac 13 ML^2 bigg) dot theta ^2 \
    & = frac 18 ML^2 dot theta ^2 + frac 16 ML^2 dot theta ^2 \
    & neq frac 16 ML^2 dot theta ^2
    end{align}



    Is this perhaps not the right way to do this question?










    share|cite|improve this question
























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      up vote
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      down vote

      favorite











      Question:



      A rigid, uniform rod of length $L$ and mass $M$ is pivoted to the origin $O$ at one end, and is then left to swing freely in a vertical plane.



      If the angle the rod makes with the vertical is $theta$, show that the kinetic energy of the rod is $$T = frac 16 ML^2{dot theta} ^2$$





      Attempt:



      In terms of $theta$, the position of the center of mass is



      $$(x,y) = bigg(frac L2 sin theta, -frac L2 cos theta bigg)$$



      In a previous part of the question, I have shown that the moment of inertia of the rod about one of its endpoints is $$I = frac 13 ML^2$$



      Thus, the total kinetic energy of the rod should be



      begin{align}
      T & = frac 12 M(dot x^2 + dot y^2) + frac 12 I omega ^2 \
      & = frac 12 Mbigg[bigg(frac L2 dot thetacos theta bigg)^2 + bigg(frac L2 dot theta sin thetabigg)^2 bigg] + frac 12bigg(frac 13 ML^2 bigg) dot theta ^2 \
      & = frac 18 ML^2 dot theta ^2 + frac 16 ML^2 dot theta ^2 \
      & neq frac 16 ML^2 dot theta ^2
      end{align}



      Is this perhaps not the right way to do this question?










      share|cite|improve this question













      Question:



      A rigid, uniform rod of length $L$ and mass $M$ is pivoted to the origin $O$ at one end, and is then left to swing freely in a vertical plane.



      If the angle the rod makes with the vertical is $theta$, show that the kinetic energy of the rod is $$T = frac 16 ML^2{dot theta} ^2$$





      Attempt:



      In terms of $theta$, the position of the center of mass is



      $$(x,y) = bigg(frac L2 sin theta, -frac L2 cos theta bigg)$$



      In a previous part of the question, I have shown that the moment of inertia of the rod about one of its endpoints is $$I = frac 13 ML^2$$



      Thus, the total kinetic energy of the rod should be



      begin{align}
      T & = frac 12 M(dot x^2 + dot y^2) + frac 12 I omega ^2 \
      & = frac 12 Mbigg[bigg(frac L2 dot thetacos theta bigg)^2 + bigg(frac L2 dot theta sin thetabigg)^2 bigg] + frac 12bigg(frac 13 ML^2 bigg) dot theta ^2 \
      & = frac 18 ML^2 dot theta ^2 + frac 16 ML^2 dot theta ^2 \
      & neq frac 16 ML^2 dot theta ^2
      end{align}



      Is this perhaps not the right way to do this question?







      classical-mechanics






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      asked Nov 15 at 20:21









      glowstonetrees

      1,822315




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          1 Answer
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          If you are including the motion of the CM is because you are considering the motion of each part of the rod as composition of two motions: the motion of each part around the CM and the motion of the CM with respect to our frame (in wich $O$ is at rest). In this case, the moment of inertia has to be around the CM: $I = dfrac 1{12} ML^2$ leading to



          $$T=frac 18 ML^2 dot theta ^2 + frac 1{24} ML^2 dot theta ^2=frac 16 ML^2 dot theta ^2$$



          We have two contributions: kinetic energy of rotation around the CM and kinetic energy of translation of the CM.



          If we choose to consider the parts of the rod as simply moving around $O$ we have to use the moment of inertia around $ O$ because there are no composition of motions for any part and there is only energy of rotation: $T=dfrac 16 ML^2 dot theta ^2$






          share|cite|improve this answer























          • Sorry, can you explain what you mean by "composition of movement"?
            – glowstonetrees
            Nov 16 at 18:16










          • Sorry, english is not my native language and I fell victim of a "false friend". I meant "composition of motions", so is, the motion is the vectorial sum of motions, in our case, the motion of the CM and the motion around the CM.
            – Rafa Budría
            Nov 16 at 19:33











          Your Answer





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          1 Answer
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          active

          oldest

          votes








          1 Answer
          1






          active

          oldest

          votes









          active

          oldest

          votes






          active

          oldest

          votes








          up vote
          1
          down vote



          accepted










          If you are including the motion of the CM is because you are considering the motion of each part of the rod as composition of two motions: the motion of each part around the CM and the motion of the CM with respect to our frame (in wich $O$ is at rest). In this case, the moment of inertia has to be around the CM: $I = dfrac 1{12} ML^2$ leading to



          $$T=frac 18 ML^2 dot theta ^2 + frac 1{24} ML^2 dot theta ^2=frac 16 ML^2 dot theta ^2$$



          We have two contributions: kinetic energy of rotation around the CM and kinetic energy of translation of the CM.



          If we choose to consider the parts of the rod as simply moving around $O$ we have to use the moment of inertia around $ O$ because there are no composition of motions for any part and there is only energy of rotation: $T=dfrac 16 ML^2 dot theta ^2$






          share|cite|improve this answer























          • Sorry, can you explain what you mean by "composition of movement"?
            – glowstonetrees
            Nov 16 at 18:16










          • Sorry, english is not my native language and I fell victim of a "false friend". I meant "composition of motions", so is, the motion is the vectorial sum of motions, in our case, the motion of the CM and the motion around the CM.
            – Rafa Budría
            Nov 16 at 19:33















          up vote
          1
          down vote



          accepted










          If you are including the motion of the CM is because you are considering the motion of each part of the rod as composition of two motions: the motion of each part around the CM and the motion of the CM with respect to our frame (in wich $O$ is at rest). In this case, the moment of inertia has to be around the CM: $I = dfrac 1{12} ML^2$ leading to



          $$T=frac 18 ML^2 dot theta ^2 + frac 1{24} ML^2 dot theta ^2=frac 16 ML^2 dot theta ^2$$



          We have two contributions: kinetic energy of rotation around the CM and kinetic energy of translation of the CM.



          If we choose to consider the parts of the rod as simply moving around $O$ we have to use the moment of inertia around $ O$ because there are no composition of motions for any part and there is only energy of rotation: $T=dfrac 16 ML^2 dot theta ^2$






          share|cite|improve this answer























          • Sorry, can you explain what you mean by "composition of movement"?
            – glowstonetrees
            Nov 16 at 18:16










          • Sorry, english is not my native language and I fell victim of a "false friend". I meant "composition of motions", so is, the motion is the vectorial sum of motions, in our case, the motion of the CM and the motion around the CM.
            – Rafa Budría
            Nov 16 at 19:33













          up vote
          1
          down vote



          accepted







          up vote
          1
          down vote



          accepted






          If you are including the motion of the CM is because you are considering the motion of each part of the rod as composition of two motions: the motion of each part around the CM and the motion of the CM with respect to our frame (in wich $O$ is at rest). In this case, the moment of inertia has to be around the CM: $I = dfrac 1{12} ML^2$ leading to



          $$T=frac 18 ML^2 dot theta ^2 + frac 1{24} ML^2 dot theta ^2=frac 16 ML^2 dot theta ^2$$



          We have two contributions: kinetic energy of rotation around the CM and kinetic energy of translation of the CM.



          If we choose to consider the parts of the rod as simply moving around $O$ we have to use the moment of inertia around $ O$ because there are no composition of motions for any part and there is only energy of rotation: $T=dfrac 16 ML^2 dot theta ^2$






          share|cite|improve this answer














          If you are including the motion of the CM is because you are considering the motion of each part of the rod as composition of two motions: the motion of each part around the CM and the motion of the CM with respect to our frame (in wich $O$ is at rest). In this case, the moment of inertia has to be around the CM: $I = dfrac 1{12} ML^2$ leading to



          $$T=frac 18 ML^2 dot theta ^2 + frac 1{24} ML^2 dot theta ^2=frac 16 ML^2 dot theta ^2$$



          We have two contributions: kinetic energy of rotation around the CM and kinetic energy of translation of the CM.



          If we choose to consider the parts of the rod as simply moving around $O$ we have to use the moment of inertia around $ O$ because there are no composition of motions for any part and there is only energy of rotation: $T=dfrac 16 ML^2 dot theta ^2$







          share|cite|improve this answer














          share|cite|improve this answer



          share|cite|improve this answer








          edited Nov 16 at 19:36

























          answered Nov 16 at 17:12









          Rafa Budría

          5,2951825




          5,2951825












          • Sorry, can you explain what you mean by "composition of movement"?
            – glowstonetrees
            Nov 16 at 18:16










          • Sorry, english is not my native language and I fell victim of a "false friend". I meant "composition of motions", so is, the motion is the vectorial sum of motions, in our case, the motion of the CM and the motion around the CM.
            – Rafa Budría
            Nov 16 at 19:33


















          • Sorry, can you explain what you mean by "composition of movement"?
            – glowstonetrees
            Nov 16 at 18:16










          • Sorry, english is not my native language and I fell victim of a "false friend". I meant "composition of motions", so is, the motion is the vectorial sum of motions, in our case, the motion of the CM and the motion around the CM.
            – Rafa Budría
            Nov 16 at 19:33
















          Sorry, can you explain what you mean by "composition of movement"?
          – glowstonetrees
          Nov 16 at 18:16




          Sorry, can you explain what you mean by "composition of movement"?
          – glowstonetrees
          Nov 16 at 18:16












          Sorry, english is not my native language and I fell victim of a "false friend". I meant "composition of motions", so is, the motion is the vectorial sum of motions, in our case, the motion of the CM and the motion around the CM.
          – Rafa Budría
          Nov 16 at 19:33




          Sorry, english is not my native language and I fell victim of a "false friend". I meant "composition of motions", so is, the motion is the vectorial sum of motions, in our case, the motion of the CM and the motion around the CM.
          – Rafa Budría
          Nov 16 at 19:33


















           

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