Quadruple somersault

During a jump to his partner, an aerialist is to make a quadruple somersault lasting a time t = 1.87 s t = 1.87 s . For the first and last quarter-revolution, he is in the extended orientation shown in the figure, with rotational inertia I 1 = 19.9 k g m 2 I_{1} = 19.9 kgm^{2} around his center of mass (the dot). During the rest of the flight he is in a tight tuck, with rotational inertia I 2 = 3.93 k g m 2 I_{2} = 3.93 kgm^{2} What must be his angular speed ω 2 \omega_{2} in r e v / s rev/s around his centre of mass during the tuck?


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The answer is 3.23.

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1 solution

Dhruva Patil
Jan 11, 2015

S i n c e t h e o n l y f o r c e a c t i n g o n h i m ( g r a v i t a i o n a l ) i s a c t i n g o n t h e C . O . M , a n g u l a r m o m e n t u m i s c o n s e r v e d 1. I 1 ω 1 = I 2 ω 2 L e t t h e t i m e t a k e n t o p e r f o r m t h e f i r s t q u a t e r + l a s t q u a r t e r = t 1 N u m b e r o f r e v o l u t i o n s i n t 1 = 1 2 L e t t h e t i m e t a k e n t o p e r f o r m t h e r e s t o f t h e j u m p = t 2 N u m b e r o f r e v o l u t i o n s i n t 2 = 7 2 2. 1 2 r e v = I 2 ω 2 I 1 × t 1 ( ω 1 = I 2 ω 2 I 1 f r o m a n g u l a r m o m e n t u m c o n s e r v a t i o n ) 3. 7 2 r e v = ω 2 × t 2 D i v i d i n g t h e a b o v e t w o e q u a t i o n s , w e g e t 4. t 2 7 I 2 I 1 t 1 = 0 G i v e n : 5. t 1 + t 2 = 1.87 S o l v i n g t h e a b o v e t w o l i n e a r e q u a t i o n s , w e g e t t 1 = 0.785 s t 2 = 1.085 s I n s e r t i n g t h e a b o v e v a l u e s i n e q 3 , w e g e t ω 2 = 3.23 r e v s 1 Since\quad the\quad only\quad force\quad acting\quad on\quad him\quad (gravitaional)\quad is\\ acting\quad on\quad the\quad C.O.M,\quad angular\quad momentum\quad is\quad conserved\\ 1.\quad { I }_{ 1 }{ \omega }_{ 1 }={ I }_{ 2 }{ \omega }_{ 2 }\\ Let\quad the\quad time\quad taken\quad to\quad perform\quad \\ the\quad first\quad quater\quad +\quad last\quad quarter\quad =\quad { t }_{ 1 }\\ Number\quad of\quad revolutions\quad in\quad { t }_{ 1 }=\quad \frac { 1 }{ 2 } \\ Let\quad the\quad time\quad taken\quad to\quad perform\quad \\ the\quad rest\quad of\quad the\quad jump\quad =\quad { t }_{ 2 }\\ Number\quad of\quad revolutions\quad in\quad { t }_{ 2 }=\quad \frac { 7 }{ 2 } \\ 2.\quad \frac { 1 }{ 2 } rev=\frac { { I }_{ 2 }{ \omega }_{ 2 } }{ { I }_{ 1 } } \quad \times \quad { t }_{ 1 }\quad \quad \quad \quad \quad \\ ({ \omega }_{ 1 }=\frac { { I }_{ 2 }{ \omega }_{ 2 } }{ { I }_{ 1 } } \quad from\quad angular\quad momentum\quad conservation)\\ \\ 3.\quad \frac { 7 }{ 2 } rev=\quad { \omega }_{ 2 }\quad \times \quad { t }_{ 2 }\\ Dividing\quad the\quad above\quad two\quad equations,\quad we\quad get\\ 4.\quad { t }_{ 2 }-\frac { { 7I }_{ 2 } }{ { I }_{ 1 } } { t }_{ 1 }\quad =\quad 0\\ Given:\quad \\ 5.\quad { t }_{ 1 }\quad +\quad { t }_{ 2 }\quad =\quad 1.87\\ Solving\quad the\quad above\quad two\quad linear\quad equations,\quad we\quad get\\ { t }_{ 1 }=0.785\quad s\\ { t }_{ 2 }=1.085\quad s\\ Inserting\quad the\quad above\quad values\quad in\quad eq\quad 3,\quad we\quad get\\ { \omega }_{ 2 }=\boxed { \boxed { 3.23\quad rev\quad { s }^{ -1 } } } \\ \\

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