#  Rotational Dynamics (moment of inertia and the action of torques) 

 



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### Parallel-Axis Theorem

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The period of oscillation of an object doesn't change for different suspension points, as long as they're the same distance from the COM.

\[[In-Depth Description](/presentations/parallel-axis-theorem)\]

### Hula Hoop Rotational Inertia

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A suspended hula hoop has the same period of oscillation as a pendulum whose length is equal to the diameter of the hoop.

\[[In-Depth Description](/presentations/hula-hoop-rotational-inertia)\]

### Tennis Racquet Flip

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Demonstration of stable and unstable axes of rotation.

\[[In-Depth Description](/presentations/tennis-racquet-flip)\]

### Spinning Football/Coconut

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Preferred axis of rotation for stability.

### Rotational Stability

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A chain or rope (lariat), when rotated fast enough, assumes an axis about which its moment of inertia is maximum.

### Angular Impulse

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Wheel is made to rotate by falling weight attached by string.

### Tippy-Top

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A top that flips upside down *raising* its center of gravity and *reverses* spin.

### Rattleback

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This object, when given a spin, seems to defy the laws of angular momentum as it stops spinning, rocks in place, and proceeds to spin in the opposite direction.

### Torsion Pendulum

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Determination of the moment of inertia by period measurements.  
  
\[[In-Depth Description](/presentations/torsion-pendulum)\]

### Yo-yo

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A very large cable spool (or smaller version) is made to roll in either direction or slide, depending on the angle of pull; action of a torque.  
  
\[[In-Depth Description](/presentations/yo-yo)\]

### Which Way Will the Bike Move?

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A puzzler concerning torques - pull on a rope attached to the pedal and figure out the consequences.

### Rolling Down an Incline

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Various objects are raced down an inclined plane to verify the effect of their moments of inertia.

\[[In-Depth Description](/presentations/rolling-down-incline)\]

### Torque Stick

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Have students literally feel the increase in torque as the lever arm is increased.

### Rotating Rod

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A rod with adjustable weights (moment of inertia) is made to rotate by a falling weight attached to the axle via string.

\[[In-Depth Description](/presentations/rotating-rod)\]

### Center of Percussion

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The motion (or lack of motion) of the suspension point of an object is observed when the object is struck a blow.  
  
\[[In-Depth Description](/presentations/center-percussion)\]

### Breaking Broomstick on Wineglasses

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Two wineglasses support a broomstick which is given a swift blow with a pipe. The broomstick, but not the glasses, breaks.

### Falling Faster than 'g'

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Allow a stick to rotate under the force of gravity and the free end will accelerate at a rate greater than *g*. Relation between angular acceleration and linear acceleration seems to give free-fall paradox.  
  
\[[In-Depth Description](/presentations/falling-faster-g)\]

### Contact Us

**Mailing Address**: Lecture Demonstration Services, Science Center, Rm B-08A, 1 Oxford Street, Cambridge, MA 02138  
**Campus Location**: Science Center B-08A | **Tel**: (617) 495-5824 | **Email**: scidemos-at-fas.harvard.edu