This activity can be done as a teacher demonstration with the apparatus made ahead of time or as a mini-lab. Instructions for using the activity as a mini-lab are included at the end of the demonstration section.
Description
The accelerated motion of free fall is demonstrated with washers tied to a string and dropped onto a metal pan. One string has washers tied in 5 equal distances and the other string has the washers tied in equal time increments.
Discussion
Students often have difficulty understanding the accelerated motion of free fall. With this demonstration they can clearly hear the difference in the beat between the equal distance string (irregular, accelerating beat) and the equal time string
(regular beat). The students should then be encouraged to analyze the difference between the two strings. For the equal time string, it is obvious that the washers travel farther in a given period of time at the top of the string than they did at the bottom of the string. A more detailed analysis would measure and analyze the distance traveled during each time increment. The calculations all center around the kinematics equation:
.99 m
 v i t
.44 m d

1
2 at
2
.1 m where the initial velocity is zero.
Hints for this demonstration
The strings and washers tend to tangle very easily. Store them wrapped on a piece of sturdy cardboard and handle them one at a time.
Adapted from: Hobbs, Winn, and Senior, AAPT PTRA Manual Role of Demonstrations in Teaching
Physics, ©1999 AAPT

How to present this demonstration
Place a metal pan upside down on the floor so it makes the maximum amount of noise. Have a student stand on a ladder or sturdy lab table and drop the washers onto the pizza pan. It is best to start with a washer resting on the pizza pan Drop the string of equally spaced washers first. The students can clearly hear that the time intervals are different. Then drop “equal time” string so that students can hear that the time intervals are the same. To increase the class involvement, repeat this several times and ask the class to clap the rhythm of the washers hitting the pan for both situations.
Equipment needed for this demonstration

A metal pan or a metal waste basket

Equal distance string – a 2.8 meter string with washers attached at one end and in five equal increments

Equal time string – washers tied at 0 m, 0.11 m, 0.44 m, 0.99 m, 1.76 m, 2.75 m from one end. (this is based on a 0.15 second time interval).
Other ideas

Make really long string/washer combinations and drop them off of the football stadium or band stand onto a pan. An option that the students will love is to replace the washers with water balloons. The sound is much more dramatic.

This activity can be done as a minilab by having the students calculate and create their own equal time, equal distance drop. A possible write-up for this lab is given below.
Adapted from: Hobbs, Winn, and Senior, AAPT PTRA Manual Role of Demonstrations in Teaching
Physics, ©1999 AAPT

Problem:
How would you place four lead sinkers on a string so that they hit a pie pan at equal time intervals?
Materials:
3 m of string, 4 sinkers, pie pan, meterstick
Procedure:
1.
Tape one end of the string to the pie pan.
2.
Attach a sinker on the string 2.7 meters from the pan.
3.
Use the kinematic equations to determine other 3 sinker locations.
4.
Show all calculations in a lab book.
5.
Describe each step of your calculation.
6.
Attach remaining sinkers.
7.
Drop the string and see if you succeeded.
Adapted from: Hobbs, Winn, and Senior, AAPT PTRA Manual Role of Demonstrations in Teaching
Physics, ©1999 AAPT

Teacher Notes
Here are some additional suggestions on how to present the demo:
Drop the string of equally spaced washers from as high as possible onto a metal wastebasket (or the pizza pan or metal pie pan). If the metal wastebasket is turned upside down, the sound will be enhanced.
Have the students describe the sound.
The intervals should be unequal, becoming shorter.
Then drop the second string with washers closer together on the bottom from the same height as the first.
Have the students describe the sound.
The interval between the clicks should now be equal.
Look at both strings again and explain these intervals.
The further a washer falls the more it is accelerated. Since the washers at the top of the string have fallen further they are moving faster as they land.
The rate of acceleration is a constant rate found to be 9.8 meters (about 32 feet) per second for each second the object is falling. This means that a falling object is moving
9.8 m/s faster at the end of each second than it was moving just one second earlier.
If the object is dropped from rest so that the initial velocity is zero, the equation to determine velocity is: v = at where a = (acceleration of gravity = 9.8 m/s
2
) and t = (time is in seconds)
Acceleration due to gravity is symbolized by g .
What is the final velocity of an acorn that takes 2 seconds to fall from the top of tree?
HINT: Remember acceleration due to gravity means that every second the velocity changes by 9.8 m/s. (Assume no loss due to air friction.) v f
= gt v f
= 9.8 m/s 2 (2s) v f
= 19.6 m/s
Adapted from: Hobbs, Winn, and Senior, AAPT PTRA Manual Role of Demonstrations in Teaching
Physics, ©1999 AAPT