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When a person swings, why does the amplitude of oscillations increase if the person changes the body position ?
That is, when descending and approaching the vertical position, if the person extend his legs and changes the body position almost horizontally, the amplitude increases.

I can sense that it has to do with the center of gravity, but i don't understand where the added energy comes from ?

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    $\begingroup$ You're feeding energy into the system by changing the effective length of the pendulum (distance from pivot point to your center of mass), thus performing work against the centrifugal force. This video explains it quite well. $\endgroup$ – Jeff Mar 26 '16 at 13:19
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You are not adding energy, but changing the rotational inertia (of the person relativ to the center point of the rotation/swinging).

In the bottom point of the swinging, where the locational energy is 0, the total energy of the system is calculated as rotational inertia * rotational speed, and that is constant (without adding energy through applying forces). If the person moves body parts = mass further away from (or nearer to) the rotational center, the rotational inertia changes, and correspondigly, the rotational speed changes, and the product is constant.

For a detailed explanation of rotational inertia (or moment of inertia), see https://en.wikipedia.org/wiki/Moment_of_inertia

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  • $\begingroup$ You certainly are adding energy... $\endgroup$ – lemon Mar 26 '16 at 17:06

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