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Physics/Rolling Objects

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Hi,

I'm partaking in a competition for bonus in my class this week In which we are asked to create an object less than 5 lbs with a diameter of less that 5 inches to roll down a slope with a top height of 6 inches. I understand through torque calculations that the mass and radius are actually irrelevant and that inertia is the priority. Knowing that a sphere has 2/5 moment inertia, technically I assumed that it would be the best choice but when compared to a rolling pop can, it was slower. So I began to think of disproportionate mass and/or a system with a hollow sphere in which upon the first rotation a mass would fall of a ledge in the hollow sphere and propel the sphere forward...
Any suggestions?

Thanks you

Answer
There must some problem here because I have done this experiment MANY times and a solid sphere ALWAYS beats a ring or disk. The mass does not matter! The radius does not matter! Only the shape matters and two objects with identical shape will roll to the bottom of an incline in the same time.
Now a hollow sphere is a completely different case since a hollow sphere has a much greater moment of inertia than a solid sphere. Make sure your sphere is a solid such as a pool ball or bowling ball!

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James J. Kovalcin

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I am teaching or have taught AP physics B and C [calculus based mechanics & electricity and magnetism] as well as Lab Physics for college bound students. I have a BS in Physics from the University of Pittsburgh and a Master of Arts in Teaching from same. I have been teaching physics for 34 years. I am constantly updating my skills and have a particular interest in modern physics topics.

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