lecture 3:forces and torques&body...
TRANSCRIPT
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Lecture 3:Forces and Torques&BodyMechanics
Laura Wilkinson
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Forces and the Body
• Electro-chemical forces– Chemical binding– Nerve conduction– Muscle fibers
• Body Forces– Gravity (mg)– Friction (Electromagnetic)– Air resistance (friction)
• Other Forces: _ Magnetic (show brain activity -- NMR) _ Strong Nuclear Force (Binds Nucleus) _ Weak Nuclear Force (Radioactivity)
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Friction
• Materials in contact make and break weakbonds
• Sliding friction and static friction aredifferent
• Static Friction (Ff < mN)
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Reduced Friction in Joints
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WALKING
Friction Friction
Coefficients of Friction: Rubber Heel on Road 1.00 (dry)Steel on ice (skating) 0.03Steel on steel (ouch) 0.15Cartilage (joint) 0.003Tendon and sheath 0.013
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Joints: Where Muscle Meets Bone
• Muscles Contract and Pull -- TENSION
• Bones Compress and Push -- COMPRESSION
Elbow
Rowing
tensioncompression
Muscle and Bone Work as Levers
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Muscles Contract
• Skeletal Muscle– Striated– Fast (0.1 sec)– Move bones– Connect to tendons– Fibers/cells fuse
• 1-2 cm long• 100 mm diameter
– Myofibril units– 15-20% contraction
• Cardiac muscle– Striated– Slower (1 sec)– Involuntary– Single nucleus cells– Synchronized contractions
• Protein Filaments– Actin (5 nm x 1.5 mm) forms filaments– Myosin II (10 nm x 2 mm) ATP energy source– Ca++ activated: negative myosin heads -> positive actin end
• Smooth muscle– Non-striated– Slower (few sec)– 100% contraction
• Sphincters• Paristolic channels• Arterial walls
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Muscle Contraction:Actin and Myosin II Fibers Slide
(sacromere)
- Band
M-line
disk
(thin) actin filament
(thick) myosin II
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Myosin Head “Walks” to + endof Actin
Myosin with Globular Head
Myosin II head hydrolizes ATPproducing structural (allosteric)changes - a kind of lever.
Actin
Myosinattached
released
motion
cocked
ATP
attached
hydrolosis
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Skeletal Levers
• First Class Lever
• Second Class– TippyToes
• Third Class:– Forearm Curl
F1 F2
F1
F2
L1
L2
F1
F2
q
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LeversFirst Class Lever
F1 F2
Torque: t=FL sin q
F1L1 - F2L2 = 0torques balance
F1+F2 = Ffulcrum (up)forces balance
L1 L2
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Tippy ToesSecond Class Lever
Torque: t=FL sin qF1L1 - F2L2 = 0F1-F2 = Fgravity (weight)
F1
F2
q Weight(opposite gravity)
Calf Muscles
Ball of Foot
q
?Shin Bone Compression
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Skeletal LeversThird Class:Forearm Curl
Torque: t=FL sin q
F1L1 - F2L2 = 0
F2-F1 = Fhumerus (down)
F1
F2
L1
L2Gravity
Biceps (two headed)Fhumerus (compression)
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HEAVY LIFITING
18°
30° W
F
T
WL sin 60° - 3/4L T sin 12° = 0T = 5.6 WW= Fsin 30° - T sin 18° = 0.5 F - 1.7 WF = 5.4 W
muscle tension (T)
weight of upper body WForce
of
vertEb
ra
(F)
Muscle tension (T)
Vertabrae
Compressio
n (F)
L
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Don’t do it!!!muscle tension (T)
weight of upper body WForce
of
vertEb
ra
(F)
Proton Spin Density MRI T2 weighted MRI
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Lecture 3: Physics of the Skeleton