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Analyzing free fall with a smartphone acceleration sensor

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182 The Physics Teacher Vol. 50, March 2012 DOI: 10.1119/1.3685123
used as an accelerometer in the context of laws governing
free fall.1 The app SPARKvue2 (see Fig. 1) was used together
with an iPhone or an iPod touch, or the Accelogger3 app if an
Android device was used. The values measured by the smart-
phone were then exported to a spreadsheet application for
analysis (e.g., MS Excel).
Mode of operation of acceleration sensors
in smartphones
It makes sense to fundamentally understand how ac-
celeration sensors work before using them in the classroom.
Smartphone acceleration sensors are microsystems that
process mechanical and electrical information, so-called
micro-electro-mechanical systems (MEMS). In the simplest
case, an acceleration sensor consists of a seismic mass that is
mounted on spiral springs and can therefore move freely in
one direction. If an acceleration a takes effect in this direction,
it causes the mass m to move by the distance x. This change in
position can be measured with piezoresistive, piezoelectric, or
capacitive methods and is a measurement of the current ac-
celeration.4 In most cases, however, the measurement is made
capacitively. Figure 2 shows a simplified design of a sensor of
this kind:5 Three silicon sheets, which are placed parallel to
each other and connected with spiral springs, make up a series
connection of two capacitors. The two outer sheets are fixed;
the middle sheet, which forms the seismic mass, is mobile.
Acceleration causes the distance between the sheets to shift,
leading to changes in capacity. These are measured and con-
verted into an acceleration value. Strictly speaking, they are
therefore not acceleration sensors but force sensors.
To measure acceleration three-dimensionally, three sen-
sors have to be included in a smartphone. These sensors have
iPhysicsLabs
Jochen Kuhn and Patrik Vogt, Column Editors,
Department of Physics/Didactics of Physics, University of Kaiserslautern,
Erwin-Schrödinger-Str., 67663 Kaiserslautern, Germany; kuhn@physik.uni-kl.de
Analyzing free fall with a
smartphone acceleration
sensor
Patrik Vogt and Jochen Kuhn, Department of Physics/
Didactics of Physics, University of Kaiserslautern, Erwin-Schrödinger-Str.,
67663 Kaiserslautern, Germany; vogt@physik.uni-kl.de
The column features short papers (generally less than
1000 words) describing experiments that make use of
the sophisticated capabilities of mobile media devices
produced by various manufacturers. Each month, in
this space, we will present examples of how students
can use (often their own) devices to investigate inter-
esting and important physical phenomena. We invite
readers to submit manuscripts to the column editors.
The contributions should include some theoretical
background, a description of the experimental setup
and procedure, and a discussion of typical results.
Submissions should be sent to the email address of the
column editors given above. We look forward to hear-
ing from you.
This paper provides a first example of experiments in this
column using smartphones as experimental tools. More
examples concerning this special tool will follow in the next is-
sues. The differences between a smartphone and a “regular” cell
phone are that smartphones offer more advanced computing
ability and connectivity. Smartphones combine the functions of
personal digital assistants (PDAs) and cell phones.
Smartphones are usually equipped with a microphone
alongside a number of other sensors: acceleration and field
strength sensors, a density of light sensor, and a GPS receiver.
As all the sensors can be read by appropriate software (applica-
tions, or “apps”), a large number of quantitative school experi-
ments can be performed with smartphones. This article focuses
on this subject, providing suggestions on how a smartphone
can be used to improve mechanics lessons, in particular when
Fig. 1. Screenshot from the app SPARKvue, show-
ing the setting of the experiment.
Fig. 2. Design and mode of operation of acceleration sensors.5
Fig. 3. The orientation of the three independent acceleration-
sensors of an iPhone or iPod touch; the sensors measure
the acceleration in the direction of the three plotted axes.
The Physics Teacher Vol. 50, March 2012 183
.
.
delivering a sufficient degree of accuracy for school instruc-
tion.
References
1. Corresponding ideas were previously published in P. Vogt, J.
Kuhn, and S. Gareis, “Beschleunigungssensoren von Smart-
phones: Möglichkeiten und Beispielexperimente zum Einsatz
im Physikunterricht” (translated as “Acceleration sensors of
smartphones: Possibilities and examples of experiments with
smartphones in physics lessons”) Praxis der Naturwissen-
schaften - Physik in der Schule (translated as Practices of Sciences
– Physics in Schools) 7/60, 15–23 (Oct. 2011).
2. itunes.apple.com/de/app/sparkvue/id361907181 (temporary
web address).
3. de.androidzoom.com/android_applications/tools/accelog-
ger_bgq_download.html (temporary web address).
4. M. Glück, MEMS in der Mikrosystemtechnik: Aufbau, Wirk-
prinzipien, Herstellung und Praxiseinsatz Mikroelektromecha-
nischer Schaltungen und Sensorsysteme (translated as MEMS in
Microsystem Technology: Structure, Principles of Effects, Produc-
tion and Practical Insert of Micro-Electromechanical Circuits and
Sensor Systems) (Vieweg+Teubner, Wiesbaden, 2005).
5. P. Schnabel, “Elektronik-Kompendium(translated as “Elec-
tronic Compendium”) (Keyword: MEMS- Micro-Electro-
Mechanical Systems), www.elektronik-kompendium.de/sites/
bau/1503041.htm (temporary web address).
6. This is difficult to understand for pupils because they perceive
the exact opposite: At first, the device suspends motionless
from a string and then falls, accelerating to the floor. This
is why they can only understand the measured acceleration
process if they have previously been instructed on the way ac-
celeration sensors function. In addition, the learners’ previous
experience of being pressed to the floor in a lift accelerating
downwards, and the resulting conclusion that one is weightless
in a free-falling lift, can also help them understand the process.
to be positioned orthogonally to each other and determine the
acceleration parts ax, ay, and az of each spatial direction (x-, y,
and z-axis) independently (see Fig. 3).
Study of free fall by a smartphone
A suitable way of examining free fall is to suspend the
smartphone from a piece of string, which is burnt through to
start the fall [see Fig. 4(a)]. In order to avoid damaging the
device, we place a soft object under the cell telephone (e.g., a
cushion) for it to land on. After having started the measure-
ment of acceleration with a measuring frequency of 100 Hz,
we burn the string through and the free fall commences. The
acceleration value measured can be seen in Fig. 4(b).
At first, the smartphone is suspended from the string and
the acceleration of gravity of 9.81 ms-2 takes effect [left part of
Fig. 4(b)]. After approx. 0.6 s, the free fall begins and the sen-
sors cannot register any acceleration, because they are being
accelerated with 1 g themselves.6 This state is maintained until
the cell phone’s fall is stopped by landing on the soft object. As
can be seen in Fig. 4(b), the sensor continues to move slightly
and returns to complete immobility after a period of 1.5 s.
The measurement can then be terminated and exported to a
spreadsheet program (e.g., MS Excel) in order to determine
the time it takes to fall Δt.
It is obvious that the smartphone has a dual function in this
experiment. It serves both as falling body and as electronic
gauge, making it possible to determine the free-fall time with
a good degree of accuracy. For the measurement example
described, the falling time was calculated to be Δt = 0.56 s for
a falling distance of s = 1.575 m. If these values are applied to
the distance-time equation for uniform acceleration (without
initial distance and initial speed and with the influence of the
gravitational field for acceleration)
(1)
the acceleration of gravity g is calculated with the formula
Fig. 4. Free fall: (a) Experimental setup and acceleration process. (b) Presentation
of measurements after the export of data from the smartphone into MS Excel.
(a) (b)
t
... The use of various sensors on smartphones for measuring physical quantities is quite interesting to study, and it becomes an ongoing research in recent years [8,9]. Research that uses one sensor to find real physical quantities is a magnetometer sensor to find gravitational acceleration value [10], an accelerometer sensor to measure free-fall motion [11], a light sensor that can be used to study oscillations [12], and magnetic sensor to measure the magnetic field of a small magnet [13]. While research that uses two sensors simultaneously is the gyroscope and accelerometer sensors to study the oscillation material [14], rotation and accelerometer sensors to examine the relationship between angular velocity and centripetal acceleration [15], rotation and accelerometer sensors to find the rotation angle and the acceleration of physical pendulum [16], and two sensors (gyroscope and accelerometer) combination to get accurate orientation estimates [17]. ...
... The combination of accelerometer and gyroscope sensor data provides more comprehensive data in circular motion learning. The accelerometer sensor measures the motion or acceleration from one position to another with 3 axis points, namely the X, Y, Z axes [11]. While the gyroscope sensor is used to track the rotation or the rotation of object motion [8,9]. ...
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... The oscillations of a spring-mass oscillator is one classic example of simple harmonic motion in which the period of oscillations T is expressed as 6 (1) where m is the suspended mass, k the spring constant, and m eff(s) is the effective mass of the spring and is equal to onethird of the mass of the spring for the case in which the ratio between the suspended mass and the mass of the spring is substantially greater than one. 6 Equation (1) can be written as (2) where m T = m + m eff(s) is the total suspended mass. Equation (2) presents a linear relationship between the square of the period T 2 and the total suspended mass m T with slope S: ...
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Electronic Compendium") (Keyword: MEMS-Micro-Electro-Mechanical Systems)
  • P Schnabel
P. Schnabel, "Elektronik-Kompendium" (translated as "Electronic Compendium") (Keyword: MEMS-Micro-Electro-Mechanical Systems), www.elektronik-kompendium.de/sites/ bau/1503041.htm (temporary web address).