Name:
Class:
Experiment: Making a grandfather clock
In this experiment you will construct a simple timekeeping
device that forms the basis of the grandfather clock.
Check that you have: a retort stand, a piece of string, a
metal cube, a cork, a protractor, a crocodile clip, and a
stopwatch.
Section 1: Exploration
i. Set up the string, the metal cube, the cork, the
protractor and the retort stand as shown in Figure 1.1.
Use the clip to keep the string at a set length. Get the
cube to swing over a range of angles and string lengths.
You do not need to make accurate measurements here –
just observe the motion in a qualitative way. Describe
the motion below.
Figure 1.1: Experimental set-up.
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ii. In this experiment you will investigate the time it takes for a block to swing a set number of
times for different lengths of the string. You will need to decide on values for the starting
angle and number of swings. (One swing counts as the bob swinging from where it’s
released at one side, over to the other side, and back across to the start again.)
Quantity and symbol
Value
Number of swings for your experiment (between 1 and 15)
Fixed starting angle
Developed by the Physics Education Group, CASTeL, Adapted for Leaving Certificate Classes.
Spring 2011
2
Section 2: The effect of length on the swing of a pendulum
In this section you will investigate how the length of the pendulum affects the time taken for a
set number of swings. The period of a pendulum (denoted by T) is defined as the time it takes
to complete one swing. In this section you will investigate the effect of length on the period.
You are asked to form a hypothesis; this is simply a prediction of what you think will happen.
i. Put forward a hypothesis that predicts how length affects the period of a swing. It is not
important whether your hypothesis turns out to be correct; what matters is the process of
checking your hypothesis. You should therefore not change your hypothesis during or after
the experiment.
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Plan an experiment to test your hypothesis. Discuss your control variables and the values
you choose for them.
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Make sure you discuss your plans with the teacher before you continue.
Developed by the Physics Education Group, CASTeL, Adapted for Leaving Certificate Classes..
Spring 2011
3
Experiment: Making a grandfather clock
ii. Make the pendulum length equal to 10 cm. All lengths should be
measured from the centre of the cube to the pivot (i.e., the bottom
of the cork). Set the pendulum swinging, and record the time (tN)
it takes to complete N swings in Table 2.1. Make two measurements
of the time for N swings and calculate the average of these. Obtain a
value for the time for one swing (T), which is the average time
divided by number of swings: T  tN / N .
Increase the length in steps of 10 cm and repeat the experiment for as
many different lengths as you can.
Figure 2.1: Measuring the length of a pendulum.
Table 2.1: Pendulum data taken at fixed mass and starting angle.
l (cm)
l (m)
tN (___)
measurement 1
tN (___)
measurement 2
tN (___)
average
T (___)
Do your results support or falsify your hypothesis? Explain.
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iii. Two students performed a similar experiment to yours and obtained results like yours,
shown in Table 2.2. They also calculated by how much the period changed each time they
changed the length of the pendulum, and called this quantity T.
Developed by the Physics Education Group, CASTeL, Adapted for Leaving Certificate Classes.
Spring 2011
4
Table 2.2: Changes in period with changing length.
l (m)
0.10
0.20
0.30
0.40
0.50
0.60
0.70
T (s)
0.63
0.90
1.10
1.27
1.42
1.56
1.68
l1 (m) to l2 (m)
Δl (m)
ΔT (s)
0.10 to 0.20
0.20 to 0.30
0.30 to 0.40
0.40 to 0.50
0.50 to 0.60
0.60 to 0.70
0.10
0.10
0.10
0.10
0.10
0.10
0.27
0.20
0.17
0.15
0.14
0.12
Does the period change by the same amount for each 10 cm increase in the length of the
pendulum? If not, do you think the differences are significant? Explain. (Hint: How does
a change in these values compare to the difference in time that you measured?)
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iv. In an experiment similar to
yours, a student only took
four data points. She changed
the length of the pendulum by
equal amounts each time and
completed a table similar to
Table 2.2.
Figure 2.2 at right shows four
possible graphs for her data.
Each graph shows how the
period of the pendulum
changes when the length is
changed.
Assuming her data follows a
similar pattern to yours,
which graph best represents
her data? Explain how the
arrows help you obtain an
answer.
Figure 2.2: Possible best fit lines to show how the
period of a pendulum varies with its length.
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Developed by the Physics Education Group, CASTeL, Adapted for Leaving Certificate Classes..
Spring 2011
Experiment: Making a grandfather clock
5
v. Plot your data from Table 2.1 in Figure 2.3. Use your graph to estimate how long a
pendulum with a period of 1.0 s would be, and verify your estimate experimentally.
Predicted length:
_________
Experimental length:
_________
Figure 2.3: Variation of the period of your pendulum with length.
You have then put together some of the essentials of a grandfather clock. The grandfather clock
pendulum you have just made shows most of the principles involved in making a pendulumbased clock, but it is clearly not a very good one. If you wish to find out more about the clocks
the internet is a great source, on sites like HowStuffWorks, YouTube and Wikipedia. A book
called Longitude by Dava Sobel describes how the development of pendulum clocks allowed
for huge improvements in navigation in the 18th century.
Developed by the Physics Education Group, CASTeL, Adapted for Leaving Certificate Classes.
Spring 2011