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# Physics/Viscous Flow

Question
If you have 2 plastic balls with the same outer surface but one is filled with water and the other one is empty and you released them, from rest, down the same slope, the ball with the water would accelerate more due to viscous flow right? If that is the case then would a ball that is fairly(but not completely) filled with sand accelerate faster than the empty ball and why? would there be a similar effect?
Thanks

No, that's not it. A solid sphere has a lower moment of inertia than a shell. Classic physics problem. The higher moment of intertia is always slower. Same mass, harder to turn. If you fill a sphere with anything, the gravitational force is higher, increasing the applied torque. But the moment of inertia doesn't increase as much, since you have more mass near the center. It spins easier, like a figure skater who pulls their arms close. So filling = faster.

Physics

Volunteer

#### Dr. Stephen O. Nelson

##### Expertise

I can answer most basic physics questions, physics questions about science fiction and everyday observations of physics, etc. I'm also usually good for science fair advice (I'm the regional science fair director). I do not answer homework problems. I will occasionally point out where a homework solution went wrong, though. I'm usually good at explaining odd observations that seem counterintuitive, energy science, nuclear physics, nuclear astrophysics, and alternative theories of physics are my specialties.

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I was a physics professor at the University of Texas of the Permian Basin, research in nuclear technology and nuclear astrophysics. My travelling science show saw over 20,000 students of all ages. I taught physics, nuclear chemistry, radiation safety, vacuum technology, and answer tons of questions as I tour schools encouraging students to consider careers in science. I moved on to a non-academic job with more research just recently.

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Ph. D. from Duke University in physics, research in nuclear astrophysics reactions, gamma-ray astronomy technology, and advanced nuclear reactors.