Free Fall in Gravity Field
Purpose:
To investigate the acceleration of freely falling objects
Required Equipment and Supplies:
5 large washers
kite string
masking tape
meter stick
pie pan
Discussion:
Consider a long piece of string with washers tied at equally spaced intervals – say 10 centimeters
apart. With the help of a balcony, hold one end of the high up, with the rest of the string hanging
vertically over the edge, directly above a pie pan on the floor below. If you drop the string, the
washers will make a clanging sound as they hit the pan below. Under the influence of gravity
will the equally spaced washers hit at equally spaced time intervals? A little thought reveals that
because the washers accelerate as they fall, the time interval between clangs decreases as the
washers strike the pan.
The goal of this activity is to tie washers onto a piece of string in such a way that when the string
is held vertically and dropped, the washers will hit the pie pan at equal time intervals. This
experiment does not require the use of standard units of time, such as seconds or minutes. We
will simplify calculations by using the elapsed time between washers hitting a pan. We call this
unit a “beat” because the sense of rhythm is used to judge whether or not the washers hit the pan
in equally spaced time intervals.
Once you finish the set-up, you will be performing this experiment outside of the classroom in
the nearby courtyard.
PART 1: Equal Spacing
Objective: To observe the behavior of equally spaced objects in free fall.
Procedure:
1. Set Up: Use a string as long as the highest available location from which the string can be
dropped. Tie the first nut to one end of the string. Tie the remaining 5 washers to the string
such that they are evenly spaced.
2. Take your string and pie pan out to the courtyard. Have many people carry the string to be
sure it does not tangle. This may seem simple, but you will spend about 10 minutes untangling
the string if you’re not careful!
3. Invert the pie pan on the floor so the washers will make a clang upon impact. Hold the string
so the first nut just rests on the pan and the second is at least 10 cm above the pan. Let go and
listen to the “clangs” carefully!
PART 2: Equal Timing
Objective: to repeat the drop from part 1, but this time manipulate your string such that you will
hear the clang of the washers hitting the pie pan at equal time intervals.
Procedure:
1. Set Up: Use the same materials from part 1. Calculate the appropriate positions for The 6
other washers so they strike the pan in equal time intervals.
Hint 1: The falling washers speed up (accelerate) as they fall due to the force of gravity. Thus
as time goes on, the washers fall a greater distance in a given time because they are
going faster. How will you compensate for this so the clangs occur at equal time
intervals?
Hint 2: d = ½gt2 (for an object in free fall).
The distance d an object falls in time interval t is given by the equation ½gt2. The
distance is equal to a constant (½ g) multiplied by t2. The constant in your set-up is the
distance between the first two washers. In other words, to calculate your new spacing
intervals, consider the following. Use the equation to calculate your distances.
d1 = [constant] (1 s)2
d2 = [constant] (2 s)2
d3 = [constant] (3 s)2
d4 = [constant] (4 s)2
…and so on, where your constant (call it x) is the distance between the 1 st and 2nd
washers. Use this pattern to figure out what the distances between your 5 washers
should be. Remember that d is the distance from the end of the string. Be sure to use a
constant such that all 4 intervals (between 5 washers) will fit on your string.
Be sure to include these calculations in your lab report.
2. Repeat steps 2 & 3 from part 1