Rui Mao
Contact: [email protected]
Biography: I am an electrical engineering student at USC. I was born in China and
raised in Spain. I started studying English when I was 10. I started practicing
breakdancing when I was in England when I was 18. I realized that applying
analytical thinking could help improve my dancing skills, so I developed an interest
in the relation between physic and breakdancing.
Abstract: Engineering thinking can be applied to all aspects of life. Breakdancing is
an example of how it can be incorporated into a seemingly unrelated field.
Breakdancing moves can actually be translated into physical principles. Once the
principles are understood, one can improve his dancing skills by applying this
knowledge. For example, some power moves in breakdancing can be translated into
basic rotations principles. Knowing the properties of angular momentum and
moment of inertia would help the dancer make a more efficient and appealing
performance.
Key words: breakdancing, power moves, physics, rotations, statics.
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Rui Mao
Dr. Harly Ramsey
Illumin
November 2, 2011
Breakdance: the Engineering of power moves
The term engineer is mostly applied to the scholars who sit at their desks and
work on projects with pens and pencils. However, this is not the only engineering
that exists, and anyone can use knowledge, instruments and material available in
order to accomplish an objective as an engineer. This is the case of breakdancers,
who are, without even knowing, engineers. They use their bodies as the material
and their immediate surroundings as their instruments to achieve their goals.
Breakdacers, or Bboys, find intuitive ways to improve their dancing. However, there
is value in examining a more analytical approach, from the physics point of view, to
understand how dance ‘works’.
Dancers desire to minimize the amount of effort they need to use. This is
where physics knowledge comes into play. Although a minimal amount of strength
is required, the performance depends mainly on the ability to use that strength
efficiently. A normal performance of breakdance would include power moves and
freezes, which are the most interesting from a physics standpoint because of their
difficulty.
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Power moves include a large portion of rotations. One of them is the
backspin, which is basically spinning on the back (see Fig.1) [1]. In the backsping,
the bboy wants the maximum amount of rotations and at the fastest rate, in other
words, the highest angular speed. There are two main ways to obtain a faster
rotation, reducing the amount of friction and/or lowering his moment of inertia [2].
By reducing friction, the bboy minimizes the amount of force that is dragging him to
become slower. To decrease friction the bboy can reduce the area of contact and/or
reduce the coefficient of friction. Using a smaller supporting area when rotating
would decrease the fricion, i.e. not laying on his whole back when spinning but in
just a concentrated area of the back. Also, the bboy can wear certain clothing that is
more slippery, that way the coefficient of friction is reduced.
Furthermore, the position in which the bboy spins is very important because
that determines the moment of inertia. The closer the mass is to the axis of rotation,
the less the moment of inertia, so if the bboy make the further parts of his body
closer (such as extremities) he can decrease his moment of inertia and therefore
increase the angular speed of his body rotation. For example, in figure 1, the bboy
has his legs extended, if he were to bend them into a fetus position he would obtain
a faster rotation speed. Someone can try the following experiment and compare it to
the first example. He can hold some weights in his hands and sit in a rotating chair,
extend his arms while holding the weights and begin spinning. While rotating, he
can move his arms towards his chest. He will realize that he spins slower when he
has his arms extended. As a result of putting the weights towards his chest, he
reduced his moment of inertia as can be observed in figure 2. It is the same concept
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in the backspin. If the bboy manages to make parts of his body closer to the axis of
rotation he can spin faster.
Figure 1: A bboy performing a backspin (adapted)
http://home.swipnet.se/b-boyz/moves.htm
Figure 2: rotating in a chair (adapted)
http://physics.ucsc.edu/~josh/6A/book/torque/node15.html
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On the other hand, there are many other types of spinning in breakdancing
apart from backspins. Some of the most vertiginous are the 90s and airflares.
Although they involve basically the same principles as described in backspins, there
are some variations. Doing a 90, is to spin upside down over one hand as it can be
observed in figure 3. In this situation the moment of inertia is much lower compared
to the backspin because the mass is concentrated closer to the axis of rotation, and
therefore it is easier to have a higher angular velocity [3]. Also, the bboy spins faster
doing 90s because the friction has been reduced since the surface of contact is
decreased (now it is just his one hand). Often the breaker uses gloves or even puts tshirts in his hands to reduce the coefficient of friction as well as protecting their
hand so that he can spin quickly and smoothly. In the picture if the bboy used gloves
and were positioned closer to the axis of rotation, he would obtain a faster rotation
that would be visually more appealing.
Although backspins and 90s involve rotations, they are different. Comparing
a 90 to a backspin would be like comparing spinning a pen in its vertical axis and its
horizontal axis. Lets say the vertical axis of the pen is the imaginary line that goes
from one end to the other passing through its center of mass, and the horizontal axis
is the one that is perpendicular to the vertical and goes through the center of mass.
If someone takes a pen, puts it perpendicular to the ground and spins it around the
vertical axis, he will observe that the number of rotations per second is much higher
that if then take the same pen and spin it laying on the ground using the horizontal
axis, given that approximately the same amount of force is used (or torque in this
case). It is noticeable that the vertical rotation has less surface of contact as well as
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less inertia, which is the reason the rotation has a higher angular speed. This
comparison would the exaggeration of comparing the 90s and the backspin, the 90s
being like the pen rotating around the vertical axis.
Figure 3: A bboy performing 90s (adapted)
http://www.thejakartapost.com/news/2009/06/30/boys-and-girls-bounce-breakdancingbattle.html
On the other hand, there are airflares, which are a more complex type of
spinning. An airflare is like a 90 but the bboy have his legs opened with a V shape
and he spins at an angle, and the axis of rotation changes (see figure 4). What is
special about airflares is that the axis of rotation constantly changes, so instead of
rotating with the axis of rotation perpendicular to the ground, there would be an
angle between axis of rotation axis and the vertical. It is like the precession of a
spinning top when is about to fall, before it stops, the rotation axis starts to change
describing circles as observed in figure 5. A curious fact about precession is that,
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despite existing torque acting on it, the magnitude of the angular momentum is
conserved [4]. The torque only changes direction of the angular momentum, not
how fast its magnitude. In other words, it does not affect how fast it spins. An
analogy between precession and centripetal forces can be made. For example, recall
the situation in which someone spins around with his hands extended holding some
weights. Assume that there is no friction, so theoretically the weights will keep
spinning forever. The weights are moving at a constant speed, despite the fact that
there is force acting on it. This centripetal force is exerted by the person and makes
the weights rotate. So it is observed that the force doesn’t affect the speed (the
weights do not go faster or slower), but it affects the direction [5]. This is similar to
what the torque is doing in the precession, it is not making it spin faster or slower, it
is just changing the direction of the angular momentum, which is the same as the
axis of rotation.
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Figure 4: A bboy performing an airflare
http://www.phys.ufl.edu/~upnews/cgi-bin/article.cgi?y=07&m=12&a=1
Figure 5: Example of a spinning top doing precession
http://en.wikipedia.org/wiki/File:PrecessionOfATop.svg
Backspins, 90s and airflares are some of the power moves, which are the
dynamics of breakdancing. On the other hand, the freezes are the statics. A freeze is
any static position in which the dancer finishes or stops before performing a new
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move. There are many types of freezes, they range from simply standing with arms
crossed to complicate upside down positions. The breakers desire to minimize the
amount of force that their body exerts so that they can make the difficult freezes
possible. The basic requirement to perform a freeze is to obtain static equilibrium,
which means that net force and net torque is equal to zero. The gravity does not
help; it acts on the bboy generating forces and torques that oppose equilibrium. So
the bboy will need to compensate those forces in order to obtain balance. It can be
done either by using the force of the muscles to oppose gravity or displaying the
body in a shape that is easy to equilibrate [6]. Free body diagrams help to
understand what forces and torques are acting and what needs to be compensated.
However they can become quite complicated because the body cannot be
considered as a whole, but as the sum of little pieces of mass where each piece
generates weight and torque. There are almost infinitely many pieces so the
calculations are complex. However, there are ways to decrease the effort without
those computations. For example, obtaining a shape where the center of mass is
right above the supporting point.
The bboy wants to minimize the force that his muscles do, to achieve that he
has to find the ‘right’ position in which he doesn’t have to do almost any force at all.
This usually happens when the center of mass is well supported. The bboy wants to
dispose his body in a pose where it looks fancy enough, and those impressive poses
have different center of mass, because as one changes the position the center of
mass changes. For example, when someone is standing up, his center of mass should
be around the stomach (see figure 6). However, if he tries to bend his upper body
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towards 90 degrees with respect of his legs, his center of mass will be moving from
the stomach towards the front (see figure 7). Having the center of mass in the
stomach, right above our legs (supports) is when the position is easiest. So when
someone is standing up normally the weight is evenly distributed through the legs,
and the muscles barely have to make an effort to stay in that position, but when he
bends there is no longer an even distribution of the support so the muscles have to
make more and more effort to avoid falling. When reached 90 degrees it is almost
impossible to not as observed in figure 8. In the picture, the woman needs to hold to
the chair to prevent her from falling. It is the same idea with the freeze, the more
even the weight is distributed and the better disposition of the center of mass the
easier is to stay in equilibrium.
Figure 6: center of mass inside body (adapted)
http://primalhealthyfit.com/wp-content/uploads/2011/09/squat1.gif
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Figure 7: center of mass shifted (adapted)
http://media.mercola.com/imageserver/public/2011/May/Standing-Forward-Bend.jpg
Figure 8: bent 90 degrees (adapted)
http://primalhealthyfit.com/wp-content/uploads/2011/09/squat1.gif
This is the case of the airchair. The airchair is a sophisticated freeze where
the bboy stands just in one of his hands and ‘sits’ in a imaginary chair perpendicular
to a normal position [7]. It is a difficult position to obtain because it is hard to reach
equilibrium, as it can be observe in figure 9. To maintain equilibrium the breaker
wants have his center of mass right above the support, which will be on his hand in
this case. To achieve this, bboy has to try shifting his the center of mass from the
lower part of the stomach to somewhere right above the support. The bboy can
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bend his feet, as in the image, to shift his center of mass. This would be how one
could make much easier obtaining equilibrium.
Figure 9: A bgirl performing a airchair (adapted)
http://www.bboy.org/
Furthermore, there are other ways to distribute the weight. For instance,
instead of using just one hand, the bboy can try using two hands and the head to
support himself. This is the case of the baby freeze [8]. In the baby freeze, the bboy
has three supporting points and it has the center of mass above and in between
those there points, so that the weight is evenly distributed. This way the effort is
significantly reduced as it can be seen in figures 10 and 11.
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Figure 10: A bboy performing a baby freeze
http://www.break-dancing.net/baby-freeze.html
Figure 11: A bboy performing a baby freeze (adapted)
http://www.break-dancing.net/baby-freeze.html
There are many ways breakdancers can improve their moves, and using
physics is one of them. There are limitations in the analyses that have been done
with the power moves and freezes because there were many approximations. More
variables have to be considered in order to be more precise. However the basics are
the same. On the other hand, Bboys do not necessarily have to know the physics;
they do need a lot of ‘knowledge’ to achieve good performances. But most of it is
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based on intuition and the ‘feeling’ of approximately what is needed in order to
perform the move. The best bboys are the ones that can apply their knowledge and
use their environment to perform their moves. Therefore breakdancing is not just a
matter of strength; it is also an exercise in analytical thinking.
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Bibliography
[1] B. Ninja. (2009, March 19). Breakdancing Backspin Tutorial [online]. Available:
http://dancejam.com/videos/1054598879-breakdancing-backspin-tutorial
[2] M. Dang. (2009, Jul 2009). Physics of Break Dancing [online]. Available:
http://www.falconium.org/node/328
[3] H. D. Young, R. A. Freedman, A. L. Ford, and F. W. Sears. Sears and Zemansky's
University Physics: with Modern Physics. 12th edition. San Francisco: Pearson Addison
Wesley, 2008.
[4] D. Kleppner and R. J. Kolenkow, “Rigid body motion and the conservation of
angular momentum” in An Introduction to Mechanics. New York: Cambridge
University Press, 2010, pp. 304-315.
[5] A. P. Saint and J. A. Garcia. “Fuerzas Centrales” in Física : 2 Bachillerato. Madrid:
McGraw-Hill, 2007, pp. 138-159.
[6] F. P. Beer. Vector Mechanics for Engineers: Statics. Boston: McGraw-Hill/Higher
Education, 2010.
[7] B. Excid. (2008, Jun 04). The Air chair stall guide! [online]. Available:
http://www.bboy.org/forums/freezes/96112--air-chair-stall-guide-.html
[8] V. Horiuchi. (2011, Oct 05). baby freeze [online]. Available:
http://www.youps3tube.com/video/0MBKvjfZ8pY/How%2Bto%2BBreakdance%2
BI%2BPilot%2B%252F%2BBaby%2BFreeze%2BI%2BFreeze%2BBasics/Entertain
ment
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