012–06802B
Gravitational Torsion Balance
11
3.
Carefully rotate the swivel support so that the large
masses are moved to Position II. The spheres should be
just touching the case, but take care to avoid knocking the
case and disturbing the system.
Note:
You can reduce the amount of time the pendulum
requires to move to equilibrium by moving the large
masses in a two-step process: first move the large masses
and support to an intermediate position that is in the
midpoint of the total arc (Figure 16), and wait until the
light beam has moved as far as it will go in the period;
then move the sphere across the second half of the arc
until the large mass support just touches the case. Use a
slow, smooth motion, and avoid hitting the case when
moving the mass support.
4.
Immediately after rotating the swivel support, observe the
light spot and record its position (S
1
).
5.
Use a stop watch to determine the time required for one
period of oscillation (
T
). For greater accuracy, include
several periods, and then find the average time required
for one period of oscillation.
Note:
The accuracy of this period value (
T
) is very
important, since the
T
is squared in the calculation of G.
6.
Wait until the oscillations stop, and record the resting
equilibrium point (S
2
).
Analysis
1.
Use your results and equation 1.9 to determine the value
of
G
.
2.
The value calculated in step 2 is subject to the following
systematic error. The small sphere is attracted not only to
its neighboring large sphere, but also to the more distant
large sphere, though with a much smaller force. The
S
1
S
2
L
Figure 16
Two-step process of moving the large
masses to reduce the time required to stop
oscillating
Position I
Position II
intermediate
position
Summary of Contents for AP-8215
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