Biomechanics

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Biomechanics

• May 5, 2005
• Dr. Kelsey Jordahl

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Size
Factor of 108 in size Factor of 1024 in mass!
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Scaling Surface Area to Volume ratio
V ? l3 Volume to length m ? l3 mass to
length S ? l2 Surface to volume so S/V ? l ?
m1/3
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Example for cube
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Consequences of falling for organisms of
difference sizes

• gt100 kg serious injury possible even at ones
  own height (cows, horses, elephants, very large
  humans)
• 100 g 100 kg may be injured if fall from
  greater than own height (dogs, cats, squirrels,
  most humans)
• 100 mg 100 g no injury from any height (mice,
  insects, baby birds)
• lt100 mg never really fall at all airborne
  (pollen, spores, very small animals)

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You can drop a mouse down a 1000 yard mine shaft
and, arriving at the bottom, it gets a slight
shock and walks away.
A rat is killed, a man is broken, and a horse
splashes.
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Scaling
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Land Mammals
Factor of 106 in mass (only 100 in size)
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Scaling factors for mammals
y ? la lm1/3
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Example of isometry and its consequences
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Example of allometry bone proportions in
pelycosaurs
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Example of allometry shape change in human
ontogeny
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Dimensionless numbers

• Mechanical advantage
• MAFo/Fi
• Flatness index
• FIS1.5/V
• Strain
• ?x/x0
• Froude number
• Frv2/gl
• Walking on water
• ?/?l2g

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Final ExamThursday, May 14(same time location)

• Chapters 22, 25, 29 (sections 26.7, 26.8,
  26.9 also helpful)
• S. Vogel, Lifes Devices, chapters 3 4, on
  reserve in library
• Handout from today