Return to Physics IndexHow to speed up a slow Grandfather - The Pendulum

Arthur Hermann Fiske Middle School

6145 S. Ingleside Avenue

Chicago IL 60637

(312) 535-0991Objectives:

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.