| Mechanics
> Gravitation |
DCS#
1L10.30 |
CAVENDISH
EXPERIMENT
-
APPARATUS
Cavendish
apparatus
|
201A |
laser
|
202-21-B6
|
stopwatch
|
101-05-D4
|
masking
tape
|
202-02-E3
|
meter
stick
|
101-05-F
|
DESCRIPTION
- A torsion balance with an optical
lever is used to determine the universal
gravitational constant. Start with the large lead balls at
one extreme and note the position of the laser spot. Move the
balls to the other extreme, allow the system to settle (which takes 1-2
hours), and note the new position of the
spot. Use the distance between the two to calculate G.
Meiners suggests a quicker version of this in which the initial
acceleration of one of the small balls is calculated from the distance
the laser spot travels in the
first 60 seconds. At equilibrium the gravitational torque on the
torsion pendulum part is equal and opposite to the torque due to the
support wire. Immediately after the large
masses are swung to their new positions, these two torques have the
same
magnitude and direction - the torsion wire remains twisted for a short
time since the pendulum has a large rotational
inertia and its period of oscillation is long
( ≅ 10 minutes).
- So Fnet
= 2GMm/r2 = ma where a = 2sball/t2
and can be determined using the optical lever.
M = 1.5 kg
m = 0.015 kg
distance between large and small masses, r = 0.039 m
distance between small mass and pivot point = 5.0 cm
-
NOTES
- The two
screws that release
the masses should be loosened gently and at the same time.
Make sure the apparatus is level and the wire is free to twist.
Allow plenty of time for the system to reach equilibrium prior to class.
A time lapse video is available on the Physics Cinema Classics video
disk.
REFERENCES
Meiners 8-8.7