A Level Physics Notes: Experimental Physics – The Bifilar Suspension
Apparatus:
Two retort stands/clamps/bosses, two metre rulers, stop watch, two lengths of cotton thread.
Procedure:
1. Set up the above apparatus with two lengths of thread of equal lengths (approx. 30cm).
Do not tie the threads too tightly as you will need to vary the
distance
during
the experiment.
2. Record distancein
cm, which should initially be about 60cm..
3. Set the lower ruler into oscillation in such a way that it oscillates about a VERTICAL axis at O.
4. Use your stop-watch to record the time,
in
seconds, it takes the lower ruler to complete one oscillation. You
should do this by measuring the time for at least 10 complete
oscillations. However, if the total time is less than 10 seconds you should use more than 10 oscillations.
5. Repeat the above for five more different values of
6. Tabulate your results and include two extra columns showing the
logarithms of bothand
You should label these columns as
and
Note it does not matter which 'log' button you use ('ln' or 'lg') as long as you always use the same one. Do not worry if some of the log values are negative.
7. Plot a graph ofagainst
8. Measure the gradient,
and
calculate the
-
axis intercept,
of
your graph.
9. The period of a bifilar suspension is expected to be given by
an equation of form
Where
that
depends on the other physical properties of the set up (e.g. the mass
of the lower ruler). The values of
and
the power
can
be found by the method you have used (see full theory below).
Read the theory below and state what values you have found for&
Theory:
If:
Taking logs on both sides of the equation yields:
This has the form of
where
the gradient
and
the intercept