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| * | * Be careful not to drop the 5 kg mass on your foot...ouch! |
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| Discuss the physics behind the demonstration, explaining some of the various steps of the demonstration when appropriate. | For the purposes of this discussion we will consider the open Hoberman Sphere as a thin-walled, hollow sphere of radius R,,open,, and the closed Hoberman Sphere a solid sphere of radius R,,closed,,. The moment of inertia of each of these configurations is then * I,,open,, = (2/3)*M*R,,open,,^2^ * I,,closed,, = (2/5)*M*R,,closed,,^2^ Clearly, since M is constant and R,,open,, > R,,closed,,, I,,open,, > I,,closed,,. The magnitude of the angular momentum of the rotating sphere is given by L = I*ω where I is the moment of inertia and ω is the angular speed. Since angular momentum is conserved, when the weight drops causing I,,open,, -> I,,closed,,, ω must increase which is what we observe. |
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[:MEEquipmentList: List of Mechanics Equipment & Supplies] |
[:Demonstrations:Lecture Demonstrations] |
Hoberman Sphere, 1Q40.22
Topic and Concept:
Rotational Dynamics, [:RotationalDynamics#ConsrvationAngMomentum: 1Q40. Conservation of Angular Momentum]
Location:
Cabinet: [:MechanicsCabinet:Mechanic (ME)]
Bay: [:MechanicsCabinetBayA8:(A8)]
Shelf: #4
attachment:Hoberman16-400.jpg
Abstract:
A collapsible sphere suspended from the ceiling is given a spin. A weight attached to the collapse mechanism is dropped causing the sphere to reduce its moment of inertia. Upon collapsing, the rotational speed of the sphere increase showing that angular momentum is conserved.
Equipment |
Location |
ID Number |
|
|
|
Hoberman Sphere |
[:MechanicsCabinetBayA8: ME, Bay A8, Shelf #4] |
|
String |
[:MechanicsCabinetBayA8: ME, Bay A8, Shelf #2] |
|
5 kg mass |
[:MechanicsCabinetBayB1: ME, Bay B1, Shelf #1] |
|
Important Setup Notes:
- The Hoberman Sphere is fragile! Handle with care.
Setup and Procedure:
- List steps for setup then procedure.
- ...
Cautions, Warnings, or Safety Concerns:
- Be careful not to drop the 5 kg mass on your foot...ouch!
Discussion:
For the purposes of this discussion we will consider the open Hoberman Sphere as a thin-walled, hollow sphere of radius Ropen and the closed Hoberman Sphere a solid sphere of radius Rclosed. The moment of inertia of each of these configurations is then
Iopen = (2/3)*M*Ropen2
Iclosed = (2/5)*M*Rclosed2
Clearly, since M is constant and Ropen > Rclosed, Iopen > Iclosed. The magnitude of the angular momentum of the rotating sphere is given by L = I*ω where I is the moment of inertia and ω is the angular speed. Since angular momentum is conserved, when the weight drops causing Iopen -> Iclosed, ω must increase which is what we observe.
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Videos:
[https://www.youtube.com/user/LectureDemostrations/videos?view=1 Lecture Demonstration's Youtube Channel]
References:
[https://en.wikipedia.org/wiki/Hoberman_sphere Wikipedia - Hoberman Sphere]
[https://en.wikipedia.org/wiki/Angular_momentum Wikipedia - Angular Momentum]
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