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Electric circuit demonstrates weightlessness
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2011 Phys. Educ. 46 250
(http://iopscience.iop.org/0031-9120/46/3/F05)
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Frontline
Weight
Electric circuit demonstrates weightlessness
The effects of apparent weightlessness have been
shown by various demonstrations [1–6]. However,
to engage and to benefit students in large classes we
set up a demonstration of the apparent weightlessness effect through a freely falling object using a
simple electric circuit.
The apparatus consists of two L-shaped cop250
P h ysic s E ducat ion
per strips (2.5 cm in width and 10 cm in length).
The vertical ends of both strips are placed opposite inside a transparent cylindrical container (an
adapted CD container). They are glued to the inside
wall of the container and are connected with wires
(6 m in length), which link to a 12V battery, in series
with a mounted lamp. The lamp is attached to the
May 2011
Frontline
Figure 1. Apparatus for the demonstration.
front of a cap worn by a demonstrator. The horizontal ends of the copper strips are overlapped with a
0.5 cm gap between each, and the lower one is glued
to the bottom surface of the container (figure 1). A
plasticine bar (mass 50 g) is placed on top of the
upper strip. This keeps the strips in contact so that
the circuit is complete and the lamp switches on.
Without an introduction to the concept of
apparent weightlessness, we asked the students
to predict what would happen to the lamp as the
container fell freely after being dropped. We
found that most students thought that the lamp
would be dimmer because of the movement of the
system. In contrast, some thought that the lamp
would be brighter. Other responses were that the
lamp would blink, its brightness would remain
the same and the lamp would go off. After collecting the students’ predictions, our container
was dropped at the front of the classroom, from a
height of about 2.7 m, onto sponge padding placed
on the floor (figure 2). To the surprise of most of
the students, during the container’s freefall the
lamp went off.
We then discussed the concept of apparent
weightlessness, which explains why the lamp
goes off. The lamp is only switched on when the
plasticine mass presses on the upper copper strip.
This happens before the container is dropped.
However, when the container is falling the situation is different. When the container is initially
dropped, the plasticine is still in contact with the
upper strip.
Besides the gravitational force (Wcopper strip) that
acts on the upper copper strip, there is also the
contact force from the plasticine pressing on the
May 2011
Figure 2. The container is dropped at the front of
the class, from a height of about 2.7 m, onto
sponge padding placed on the floor.
upper copper strip (Ncopper strip by plasticine; figure 3).
Therefore, the initial acceleration of the upper copper strip is greater than the gravitational acceleration (acopper strip > g). In contrast, because there is
the contact force of the upper copper strip acting
upwards on the plasticine (Nplasticine by copper strip), the
initial acceleration of the plasticine is then less than
g (aplasticine < g). The difference in accelerations
makes the velocity of the upper strip greater than
that of the plasticine. Indeed, a little gap between
them is generated during freefall. When the plasticine is no longer in contact with the upper strip,
there is no contact force that presses on the upper
strip, the two copper strips are then separated and
therefore the circuit is open. This causes the lamp
to go off.
P h ysic s E ducat ion
2 51
Frontline
g
N copper strip by plasticine
Wcopper strip
a copper strip > g
N plasticine by copper strip
Wplasticine
a plasticine < g
Figure 3. Free-body diagrams show the different initial accelerations of the plasticine and the upper
copper strip at the start of freefall.
After that, during the fall, each object in the
container moves vertically downward with g and
the apparent weightlessness takes place—no contact force appears between each object. The lamp
stays off until the container hits the floor when it is
switched on again.
This demonstration apparatus is inexpensive and
easy to construct using a simple electric circuit.
Observers can witness the changing state of the
lamp. The demonstration, along with a discussion
of free-body diagrams, is a useful tool for teaching
the topic of apparent weightlessness, especially for
large classes.
Educ. 45 292
[2] Kwok P 2010 A simple demonstration to
visualize weightlessness Phys. Educ. 45 19
[3] Corona A, Sliško J and Planinšic G
2006 Freely rising bottle of water also
demonstrates weightlessness Phys. Educ. 41
208
[4] Marshall R 2003 Freefall and weightlessness
Phys. Educ. 38 108
[5] Shiells R 1981 Weightlessness Phys. Educ. 16
52
[6] Kruglak H 1962 Demonstrations of
weightlessness Am. J. Phys. 30 929
Acknowledgements
The authors would like to thank all of the students
who participated in this study. Many thanks go
to members of the Physics Education Network of
Thailand (PENThai) at Mahidol University, Thailand Center of Excellence in Physics (ThEP) and
the Faculty of Science, Prince of Songkla University, Thailand.
K Arayathanitkul Department of Physics,
Faculty of Science, Mahidol University, Rama
6 Rd, Ratjathevi, Bangkok, Thailand, 10400,
S Rakkapao Department of Physics, Faculty
of Science, Prince of Songkla University, Hat
Yai, Songkhla, Thailand, 90112, S Prasitpong
Institute for Innovative Learning, Mahidol
University, Phuttamonthon 4 Rd, Salaya,
Phuttamonthon, Nakhon Pathom, Thailand,
73170 and N Emarat Department of Physics,
Faculty of Science, Mahidol University, Rama 6
Rd, Ratjathevi, Bangkok, Thailand, 10400
References
[1] Sliško J and Planinšic G 2010 Hands-on
experiences with buoyant-less water Phys.
2 52
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