In physics, swing is defined as a free movement around a fixed point (X).
To exemplify this deﬁnition, a particular type of periodic motion called Pendulum Motion can be considered.
At points A and B in Figure 2.a below, the kinetic energy (velocity) is equal to zero and the potential energy is at its maximum.
The potential energy can be calculated using the formula m･g･h where g is the acceleration due to gravity,
h is the height differential between the point A or B compared to C, and m is the mass.
Figure 2.a 省略
Starting from point A, the velocity will increase up to point C and then B, where the velocity is equal to zero (V = 0).
The motion will then be reversed, thus the velocity will increase up to point C and then decrease up to point A, where the velocity again will be 0.
The velocity is at its maximum at point C.
This pendulum motion is emulated by the dancer, both from a kinematic and visual point of view (figure 2.b).
The dancer's A barycentre represents the moving point in a Pendulum.
In figure 2.b below, the dancer will start at point A (velocity 0).
The speed will gradually increase, starting with a small movement (low velocity) up to point C (maximum velocity) and then decelerates up
to point B by means of a "control action" as described in the Drive Action above.
Figure 2.b 省略
Qualitative analysis of the pelvic trajectory.
In order to better understand the principle of swing applied to dancing, a scientific study has been made into the movement of the barycentre of both partners in speciﬁc figures (in this case the Feather Step).
This analysis was accomplished in a scientific laboratory with precise scientific equipment focusing specifically on the pelvic trajectory.
The first part of the analysis was a qualitative evaluation of video clips, made by both the dancers and their teachers
immediately after the dance steps were performed.
After only a few minutes, a part of the kinematic analysis was conducted thanks to the power and utility of the software.
Figures 2.c shows the stroboscopic image of the Feather Step, 2.d shows the trajectories of the pelvis of both Man and Lady during the Feather Step, and Figure 2.e shows a graph of the velocity of horizontal translation over time of the markers placed on the pelvis of the Man and Lady.
Note: the stroboscopic image (2.c) was obtained taking a frame every two tenths of a second (5 images per second) in order to produce the maximum precision.
The variations in the velocity during the execution of the ﬁgure were deduced through a qualitative analysis of the figures below. In the stroboscopic image (figure 2.c), the time difference between each frame is constant.
Where two images appear further from one to another, a higher velocity (of translation) can be deduced.
Furthermore the relationship between pelvis and leg movements in the various stages of each step, adaptive phenomena, and the presence and/or correction of any technical errors can also be understood.
In figure 2.d, the tracks left by the markers on the pelvis make it quick and easy to visualise the homogeneity (smoothness and togetherness) of the performance.
All the evaluations carried out using this method have very important practical implications in terms of teaching, technical evaluation, and medicine, both in relation to posture and traumatology.
Hypothetically, the evaluation method used here could also be useful as an additional tool in competition analysis.
It would be possible to conduct similar analyses on competing couples, based on simple video clip.