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Our physics teacher rolls a giant ball down a ramp at the same time also rolls down a plastic car with four wheels. Why is it that the car reaches the bottom first???

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  • $\begingroup$ What's inside the ball? How much sooner does the car reach bottom? $\endgroup$ – Bob D Oct 29 '19 at 14:38
  • $\begingroup$ If you want to make the principle less mysterious and more transparent you do the "racing wheels" demo which involves several cylinders (a uniform solid one and a shell are easy to get, one that has it's mass concentrated at the center is nice if yuo can rig it) and possibly one or more spherical shapes as well. That way everything is rolling in toto and the finishing order is related to moment of inertia divided by mass and radius-squared (i.e. the coefficient on the moment of inertia). $\endgroup$ – dmckee --- ex-moderator kitten Oct 29 '19 at 14:59
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As both the ball and the car roll down the incline their potential energy is converted into kinetic energy. There are two forms of kinetic energy, linear and rotational kinetic energy. The linear kinetic energy gain determines how soon they reach the bottom of the incline. Now for the ball, all of its mass is involved in the rotation, while only the wheels rotate for the car. Hence a larger fraction of the total kinetic energy is tied up in rotation for the ball than the car. That is why the car reaches the bottom sooner.

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