Physics Challenge

On Nov 7, 7:50*am, Helpful person wrote:
On Nov 6, 12:11*pm, Mike Collins wrote:



Spin them?
The hollow one should spin for longer

No. *The hollow one will spin for less time. *There is more energy
stored in the solid spinning ball as it has a larger moment of
inertia. *Energy drain is similar for both.

http://www.richardfisher.com

Sorry you are right. I misread the question.

On Nov 6, 10:48*am, Sam Wormley wrote:
You are given two spheres that are identical in size, weight, appearance, and touch but one sphere is hollow while the other is solid. (As an example, the solid sphere could be made out of a light wood and the hollow sphere made out of a denser wood, then both spheres carefully painted to look and feel the same.) Using only simple items that you might find at home (no fancy equipment, no drills, no X-ray machines), determine which sphere is hollow.

A Lazy Susan would be the thing to use; the hollow sphere would make
the better flywheel, having more angular momentum for the same
rotational speed. So it would be harder to start or stop spinning.

John Savard

On Nov 7, 8:50*am, Helpful person wrote:

No. *The hollow one will spin for less time. *There is more energy
stored in the solid spinning ball as it has a larger moment of
inertia. *Energy drain is similar for both.

Huh? Make both spin at the same angular velocity. Since the hollow one
has more of its mass more actively rotating, while more of the solid
one's mass is in the center, where it doesn't move as much, the hollow
one of equal mass (and denser material) has more angular momentum *and
more energy*, and, thus, with the same energy drain, it will stay
spinning longer.

John Savard

On Nov 7, 8:51*am, Helpful person wrote:

Sorry you are right. *I misread the question.

Oh, that was you correcting yourself, not the person you replied to
acknowledging you were right.

John Savard

"Quadibloc" wrote in message ...
On Nov 6, 10:48 am, Sam Wormley wrote:
You are given two spheres that are identical in size, weight, appearance, and touch but one sphere is hollow while the other is solid. (As an example, the solid sphere could be made out of a light wood and the hollow sphere made out of a denser wood, then both spheres carefully painted to look and feel the same.) Using only simple items that you might find at home (no fancy equipment, no drills, no X-ray machines), determine which sphere is hollow.

A Lazy Susan would be the thing to use; the hollow sphere would make
the better flywheel, having more angular momentum for the same
rotational speed. So it would be harder to start or stop spinning.

John Savard
===============================================

What happens if the hollow space is an offset cube, cylinder, pyramid or other shape? There is nothing in the wormlet’s description to the contrary. Would it then be a “better� flywheel? And have you ever compared a raw egg to a hard-boiled egg by spinning them?
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On Nov 10, 3:29*am, Quadibloc wrote:
On Nov 6, 10:48*am, Sam Wormley wrote:

You are given two spheres that are identical in size, weight, appearance, and touch but one sphere is hollow while the other is solid. (As an example, the solid sphere could be made out of a light wood and the hollow sphere made out of a denser wood, then both spheres carefully painted to look and feel the same.) Using only simple items that you might find at home (no fancy equipment, no drills, no X-ray machines), determine which sphere is hollow.

A Lazy Susan would be the thing to use; the hollow sphere would make
the better flywheel, having more angular momentum for the same
rotational speed. So it would be harder to start or stop spinning.

John Savard

The more optimum design for a flywheel seems to be a disk which is
thick in the center, thin in the periphery:

http://garage.cse.msu.edu/demos/index.html

A flywheel used as a electric-car-battery-substitute might benefit
from being able to be spun up to max speed quickly, especially since a
flywheel probably can't hold a "charge" very long. Distance between
charging stations might come into play; a flywheel that might take
longer to spin, might propel the vehicle a longer distance.

On Nov 6, 2:50*pm, Chris Schram wrote:
In article ,
*Sam Wormley wrote:

On 11/6/12 11:48 AM, Sam Wormley wrote:
Physics Challenge

You are given two spheres that are identical in size, weight,
appearance, and touch but one sphere is hollow while the other is
solid. (As an example, the solid sphere could be made out of a light
wood and the hollow sphere made out of a denser wood, then both
spheres carefully painted to look and feel the same.) Using only
simple items that you might find at home (no fancy equipment, no
drills, no X-ray machines), determine which sphere is hollow.

* * Oops, wrong newsgroup. Sorry about that!

Wrong newsgroup? Yes. Provocative problem? Definitely.

--
is a filtered spam magnet. Email replies WILL be lost.
Try http://public.xdi.org/=chris.schram instead.

The solid sphere should roll down an inclined plane faster. I don't
have any planks lying around, so I might take the expedient solution
of trying to smash the spheres open with a hammer. I'd be sure to
wear a safety visor and keep people and pets at a safe distance.

On Nov 10, 7:53*am, wrote:

The more optimum design for a flywheel seems to be a disk which is
thick in the center, thin in the periphery:

That doesn't make it a more efficient flywheel - if your goal is to
minimize the required RPM for a given energy storage.

The thing is, though, you are at liberty to make a flywheel spin as
fast as you like... _as long as it doesn't fall apart_... and thus,
the fact that this design can safely spin much faster outweighs the
fact that it stores less energy for a given angular velocity.

John Savard

Den lørdag den 10. november 2012 15.56.54 UTC+1 whensenseless muttered darkly:

I might take the expedient solution

of trying to smash the spheres open with a hammer. I'd be sure to

keep people and pets at a safe distance.

Wooden balls will not break with a hammer.

Wouldn't you just employ somebody inferior to do it for you? Then all your responsibilities to society, the world and fluffy animals can safely be ignored. (Just like rottenApple and their countless iSlaves)

Meanwhile, back at the ranch: What about testing your balls for differential thermal inertia?

Hang your balls out overnight and observe how they behave as the dew or frost lifts in morning sunshine.

The solid ball should cling to its coating of moisture or frost for slightly longer than a hollow sphere. The hollow sphere should show early drying at the sun facing surface.

In climes less given to frost, or dew, one could wet your balls and watch how evaporation takes place in warm sunshine. There ought to be a bias of fastest drying towards the centre of the sunny side.

I offer the sun only because we are not allowed advanced tools.. like fire. (or iSlaves)

"Chris.B" wrote in message ...

Wooden balls will not break with a hammer.
============================================
Bwhahahahaha! Of course they will, you senseless and useless moron!
I suppose you’d fire wooden cannon balls at iron ships.
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