Date: Fri, 10 Dec 2010 17:54:12

Author: Keith Heyward

Subject: Re: Rolling Disk, Ring and Sphere


Good advice. Thanks all. Given that the radius and total mass do not
affect the outcome (when the force is gravitational and friction/air
resistance are neglected), any size/mass ball will do. FYI, I did find a
source on-line to buy a 6" solid wooden ball for about $20. Plus John
Hubisz gave me a bocci ball (thanks John!), so I'm covered.

Thanks again for everyone's help!

Keith Heyward

-----Original Message-----
From: [] On
Behalf Of Jerry DiMarco
Sent: Wednesday, December 08, 2010 3:41 PM
Subject: Re: [tap-l] Rolling Disk, Ring and Sphere

We know the mass is not critical, but many students do not - even
after they see the math that proves it. So it helps dispel the myth when
they actually see objects of the same size and mass racing down an
incline. Once they see that demo, then it's easier to introduce objects of
unequal size or mass...


At 12/8/2010 02:35 PM, you wrote:
>Is the mass that critical? We have the ring/disk equal mass equal radius
>set but showing that all solid balls accelerate the same and all hollow
>spheres accelerate the same would show that it is the distribution and not
>the size or mass that determines the acceleration. Racing a small solid
>ball and a large hollow ball, and then a large solid ball and a small
>hollow ball would show that the solid ball always wins. (Or at least
>SHOULD! Everyone knows how some "SHOULD" demos can work out sometimes.
>Gordon Shepherd


Jerry DiMarco
Manager of Lecture Demonstrations and Instructional Labs
Montana State Univ., Physics Dept.

Bozeman, MT

Our Motto: "Find your inner demo."