How to speed up a slow Grandfather - The Pendulum

Arthur Hermann Fiske Middle School
6145 S. Ingleside Avenue
Chicago IL 60637
(312) 535-0991


To introduce the concept of periodic motion and relate it to its use in a
Grandfather type clock. To discover that the period of a pendulum is dependent
on the length of the pendulum and independent of the weight of the bob and the
amplitude. Students will discover that a slow running Grandfather clock can be
corrected by changing the length of the pendulum.


Each group needs a stop watch, pendulum and metal washers.
Materials needed for a pendulum are string and paper clip.
To make a play Grandfather clock you will need a ringstand and a rod and a
clockface made from paper.


Begin the class by saying "I'm sure your all wondering why I called you here at
this time. In my hand I hold a memo from our CEO that threatens our jobs. We
have been given one hour to solve a problem that a has caused a hugh drop in
sales." Point to the play clock. "As you can see our clock is five minutes
slow. In a moment we will break into our work groups and start working on
possible solutions to our problem: What can be done to the pendulum to have the
clock run on time?"

At this time you may want a student to come to the front to point out the pivot
point, length, and bob. Be sure to demonstrate what a cycle is and how to count
cycles. Next pass out a pretend memo from the OLD TYME CLOCK CORP. The memo
should state the problem and indicate they may be fired if they do not solve the
problem by the end of the period. Each group is given a different length of
string to create a pendulum. Lengths may vary from 25cm to 2m. Each group will
find the time for their pendulum for 20 cycles. On the board draw a graph with
numbers across the top indicating time in seconds. Start with zero using 5
second periods, up to 50 seconds.

When the students have found the number of seconds per 20 cycles ask the
students to come forward and tape their pendulum at the appropriate time. Once all
the groups have taped their pendulum to the graph, discuss the results.
Students will hopefully see the direct relationship between the length and the
period of time. Next, each group is given a string of the same length, but this
time students may add different numbers of washers. As before the groups will
find the period of their pendulum and then come to the front to tape their
pendulum on another graph. Hopefully the students will understand the idea that
the weight of the bob does not influence the period of the pendulum. Lastly,
you should demonstrate the effect of amplitude on a pendulum. You could
probably do this in front with help of a student. At this point you should
compare the three variables; length, weight and amplitude. Using the collected
data ask the students to write their solution to correcting the problem of the
slow grandfather clock.

Performance Assessment:

This activity will take more than one period, a good place to break would be
after finding the effect of length on a pendulum. For assessment, students
should be able to describe what should done to the pendulum and explain how this
would solve the problem. The student's understanding of this material can be
evaluated by having them predict what the correct length would be for the clock
to kept the correct time. Hopefully by experimenting students will discover the
length at 20 seconds is the correct length.

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