Film Canister Pendulums

Essential Question
How does a pendulum work? How do you find the period of a swing?

Objectives

 * Collect data and construct a graph to find a relationship between the length of the period and another variable.
 * Explain the relationship between the physical characteristics of the pendulum and its period.


 * Quantify independent and dependent variables

Introduction & Student Background
Almost anyone asked about a pendulum knows the type of motion. Good examples include old cuckoo clocks, a grandfather clock, swings, and anything dangling on a length of string. Pendulums also come into play every time you walk or run. Your arms and legs swing at a natural frequency depending on their lengths and weight distributions.

Ask the students what determines the period of the pendulum. You will get several answers including mass and length.

So long as the mass is concentrated at the end of the pendulum, the length is the only variable of importance. This is called the simple pendulum or point-mass limit. Once the mass is spread along the length then it becomes more complicated and is called a physical pendulum.

Vocabulary

 * period: the complete motion from a starting point at rest back to the same rest point on the same side

Materials

 * string - preferably braided rather than twisted
 * film canisters OR black and gray plastic cans
 * washers that fit into the film canisters
 * duct tape
 * 1/4 inch wooden dowels about 1 foot in length
 * straws to fit over the dowels
 * timers - stopwatches
 * rulers or metersticks to measure the length of the pendulum
 * wire to pull the string through the film canister lids
 * small nail to punch hole in film canister lids
 * work sheet copies
 * calculators for use w/ older kids

PREP (Building the Pendulum)

 * 1) Remove the lid from the canister
 * 2) Using a small nail, push a hole through the center of the lid. Bend the wire in half and push the loop through the lid.
 * 3) Catch the long string in the loop and pull it through the lid.
 * 4) Disengage the wire and set the wire aside.
 * 5) In the free end of the string (the end that would be outside the canister) tie a loop that is just large enough to go around the dowel.
 * 6) Measure the length of your construct from the end of the loop to the bottom of the canister. Tie a knot in the end of the string to make the length the desired length. If you have groups, assign each group a different length.
 * 7) Make the platform to swing your pendulum from by duct taping a dowel off the edge of the table.
 * 8) Place the straw over the projecting end of the dowel and put the string around the straw.

ACTIVITY

 * 1) Measure the period of the pendulum swings by timing 10 complete swings and dividing the time by 10. It may be helpful to keep the swings small.
 * 2) Repeat 3 times for better accuracy.
 * 3) Repeat with washers in the film canister, which raises the mass of the pendulum. The largest variability in measured periods between successive trials will come with an empty canister. The added weight of the washers will make this pendulum more nearly a simple pendulum with negligible weight anywhere except in the canister. Repeat the measurement with different weights.
 * 4) Collect/Record data of the mass (number of washers) and the period.

How it Fits to Standard

 * Forces on objects make them move. Changes in forces will cause changes in the motion.
 * Forces and motions can be measured.

Follow Up
The students could find small variations in their period, but nothing significant or with a trend. Is weight of the pendulum the determining factor of the length of the period? Try varying the length of the string and see what happens.