Chan Shu Kui Memorial School
AL-PHYSICS
EXPERIMENT 1. Investigation of the factors determining the
period of oscillation of a Bifilar Suspension
Name
:________________
Class No.
:____
Date :________
Apparatus A wooden bar of rectangular cross section and about 40 cm long,
cotton, stopwatch, metre scale, two stands and clamps, spirit level,
set-square.
Setting
Tie a loop of the cotton round the bar near each end. Tie lengths
of cotton to these loops and pass the free ends of the cotton
through split corks held by separate clamps at the same horizontal
level.
Start with each loop on the bar 1 cm from the end and the
suspension threads about 30 cm long.
Adjust the height and position of the clamps so that
(a) the suspended bar is horizontal (use spirit level),
(b) the suspension thread are vertical (use set-square against
vertical thread and the suspended bar),
(c) the suspension threads measured from bar to cork are equal
in length.
2002-2003 AL-PHYSICS PRACTICAL WORKSHEET 1
P.1
Chan Shu Kui Memorial School
AL-PHYSICS
Task 1. To investigate how the period of oscillation varies with
the distance d between the vertical suspension threads.
Set the bar oscillating through a small angle about a vertical axis and
with a stopwatch take two determinations of the time for 30
complete oscillations.
Measure the horizontal distance d between the vertical threads.
Move each loop 2 cm farther from the end and repeat the timings,
and then continue to move the loops 2 cm at a time.
Task 2. To investigate how the period of oscillation varies with
the length l of the suspension threads.
Re-set the loops and hence the suspension threads to 2 cm from the
ends of the bar. Keeping this distance between the suspension
threads constant throughout the experiment, vary the equal lengths l
of the suspension threads between 20 and 40 cm and obtain two
readings of the time for 30 oscillations for each value of l.
Results
Task 1
:
Constant length of vertical threads l /m =
Distance
between
vertical
threads d /m
Time for 30 oscillations
t1 /s
t2 /s
Time for 1
oscillation
T /s
Mean t /s
2002-2003 AL-PHYSICS PRACTICAL WORKSHEET 1
Log T
Log d
P.2
Chan Shu Kui Memorial School
AL-PHYSICS
Task 2
Constant distance apart of vertical threads d /m =
Length of
vertical
thread l /m
Time for 30 oscillations
t1 /s
t2 /s
Time for 1
oscillation
T /s
Mean t /s
Log T
Log l
Things to do
1.
2.
3.
4.
5.
6.
7.
In experiment 1, plot a graph with the values of log T
against log d.
Measure the slope and interpret your result of the
graph.
Find the relation between T and d.
In experiment 2, plot a graph of log T against log l.
Measure the slope and interpret your result of the
graph.
Find the relation between T and l.
State the possible source(s) of errors of the
experiments. Compare the results obtained in (2) and
(4) with the theoretical values shown in Theory (*).
Suggest some methods to improve the accuracy of the
experiment.
2002-2003 AL-PHYSICS PRACTICAL WORKSHEET 1
P.3
Chan Shu Kui Memorial School
Theory
AL-PHYSICS
Assuming that the relation between the periodic time T and the
distance d between the vertical threads is of the form T  d a ,
then
T
k
da
where k is a constant
Taking logs to the base 10 and rearranging,
log t  a log d  log k
Thus the graph of log T against log d is a straight line whose
slope,
PN
, obtained from two well-separated points on the line,
QN
is the value of a, which may be either positive T  d a or negative
T  d  a orT 
1
. In the case of experiment 2, assuming the
da
relation between T and l to be T  l a .
Theory shows that the full relationship between the periodic time
T, the length l of the vertical suspension threads and their
distance d apart is
T  2
4Il
*
Mgd 2
where M and l are the mass & the M.I. about the axis of
oscillation respectively.
2002-2003 AL-PHYSICS PRACTICAL WORKSHEET 1
P.4