Saturday, November 30, 2013

thanksgivingkuh


Thanksgivingkuh

This year, Thanksigiving and Hannukah share a night and the holiday has been named Thanksgivingkuh.  This won’t happen again for another 70,000 years so in its honor I am looking at the physics behind a dreidel.  The question I asked was how much rotational kinetic energy does a small plastic dreidel have when it is initially spun?

KErot = ½ Iω2

To calculate the moment of inertia of the dreidel I approximated it as a solid cylinder which is not entirely correct but I think it was the best option.  The mass of a plastic dreidel  is about 0.00907 kg according to a source online.  The diameter of the dreidel was approximately 1 inch which makes the radius is 0.0127 meters.

I= ½ mr2
I= ½ (0.00907kg)(0.0127m)2
I=7.314x10-7 kgm2

The initial angular velocity of the dreidel--it rotated approximately 3 rotations per second, therefore:

3 rotations x (2π radians / one rotation) = 18.85 radians/second

Therefore the total rotational kinetic energy of the dreidel when it was initially spun is:

KE = ½ (7.314x10-7)(18.85)2 = 1.299 x 10-4 J

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