Cavendish Balance, 1L10.30

Location:

• Cabinet: Mechanics (Will be permanently installed in lecture halls eventually)

• Bay: (B12)

• Shelf: #1

attachment:CavendishBalance.jpg (photo from Pasco online)

Description:

Two small lead balls are attached to a cross-beam which is suspended by a fine wire to form a torsion pendulum. Two large lead spheres are placed opposite the small balls to exert a force onto the suspended system.

Setup:

1. Setup requires 48 hour advanced notice before lecture. (Once the Cavendishes are installed in the room this condition may be lifted.)
2. Level the Cavendish Torsion Balance on a vibrational free platform.
3. The two large lead spheres are then placed opposite the smaller balls
4. The system must then be allow to reach equilibrium, this takes a few hours.

Cautions, Warnings, or Safety Concerns:

1. The large masses are made of lead and weight about 1.5Kg
2. The torsion balance inside the Cavendish is very delicate and must be handled carefully. Also carry by its base.

Demonstration:

The objective of this experiment is to determine a numerical value for Newton's constant, G. G = {2 π^{2 L r}2}/{M T^{2}] θ, F = (GmM)(r}-2). The two large lead spheres are then moved to the opposite position from the initial setup position. This exerts a torque which moves the cross-beam torsion wire system. Now let the system come to a new equilibrium position which is shown by a laser beam reflected off a small mirror that is attached to the cross-beam torsion wire system. The laser dot is reflected to the opposite wall and hence the motion of the cross-beam can be observed and measured. The finial equilibrium position will take several hours as the simple harmonic motion of the torsion pendulum damps out.

• The two smaller masses have a mass: m=38g
• The center-to center distance between of the smaller ball: d=50.0mm.
• The two larger masses have a mass: M=1.5Kg

References:

1. [http://store.pasco.com/pascostore/showdetl.cfm?&DID=9&Product_ID=1655&groupID=306&Detail=1 Pasco]

2. [attachment:CavendishManual.pdf Cavendish Manual (pdf)]

3. [http://en.wikipedia.org/wiki/Cavendish_experiment wikipedia]

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