How to speed up a slow Grandfather - The Pendulum

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

Objectives:

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.

Materials:

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. 

Strategy:                                    

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.