How does moving your legs propel you forward in swimming?

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Discussion Overview

The discussion revolves around the mechanics of leg movement in swimming, specifically how vertical leg kicks contribute to forward propulsion. Participants explore the physics behind swimming techniques, the role of fluid dynamics, and the relationship between leg movement and forward motion in water.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant expresses confusion about how a downward kick can propel a swimmer forward, questioning the mechanics of vertical leg movement.
  • Another participant suggests that the upward motion of the leg may not contribute to backward movement due to the dynamics of fluid resistance, proposing that the downward kick encounters greater resistance than the upward retraction.
  • A different viewpoint highlights that the effective propulsion from leg kicks is influenced by the angle of the leg and the arc of movement, with the foot's position playing a significant role in water displacement.
  • Some participants discuss the importance of maintaining a horizontal body position to maximize propulsion and minimize resistance during swimming.
  • There is mention of the role of buoyancy in swimming, indicating that additional force is required beyond buoyancy to achieve forward motion.
  • One participant introduces the concept of fluid friction and its impact on swimming efficiency, noting that faster movements encounter greater resistance.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the mechanics of leg propulsion in swimming, with multiple competing views and uncertainties expressed regarding the effectiveness of different kicking techniques and their contributions to forward motion.

Contextual Notes

Some discussions involve assumptions about fluid dynamics and the biomechanics of swimming that may not be universally agreed upon. The relationship between leg movement and propulsion is complex and influenced by various factors, including body position and water resistance.

  • #31
marcusl said:
I'm speaking of vortices in the fluid, not rotational motion of a flagellum. At the low Reynolds numbers that Purcell considers, inertia effects are negligible compared to viscous effects (he points out that inertial forces die away in distance of order 0.1 Angstrom!). How are vortex rings sustained in that regime?

As I mentioned, Purcells' paper is a *starting* point, kind of like how frictionless surfaces and massless pulleys are used in introductory mechanics. Since you (apparently) don't have access to Lighthill:

http://maeresearch.ucsd.edu/~elauga/research/references/LaugaPowers09_RPP.pdf

and
http://www.pnas.org/content/early/2011/07/28/1106904108.full.pdf

Figure 8 of #2 is instructive, as vortices are clearly present. In the fluid. More than 0.1 Angstrom away.
 
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  • #32
Thank you, I will read these.
 

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