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MVS330_lect06_Friction+Drag

# MVS330_lect06_Friction+Drag - Movement Science 330 Lecture...

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Movement Science 330 Lecture 6 (Enoka pp. 61-68) Friction Fluid Resistance

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Outline Kinetics: Forces in human motion Free Body Diagrams Ground Reaction Forces Friction Fluid Resistance
Friction Parallel to the surface, it is the mechanism by which the surface pushes back on us

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Static Friction force (F s ) Friction force depends on: properties of the surfaces force acting perpendicular to the surfaces e.g. ground-foot friction: F g,y is the perpendicular force F g,y mg F s,max = F g,y • μ s
F s = • F g,y F g,y is vertical ground reaction force is coefficient of friction dimensionless depends on the properties of the surfaces static ( s ) dynamic ( d ) ( s > d ) F a,x F g,y F s F a,x = applied force F s = friction force mg

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Static friction F s,max = largest static friction force possible F s,max = s • F g,y Block will not move if F a,x F s,max s running shoe - loose gravel : 0.3 running shoe - grass: 1.5 bicycle racing tire concrete: 0.8 F a,x F g,y F s F a,x = applied force F s = friction force mg
Friction vs. Traction Friction : 2 fairly smooth surfaces < 1.0 Traction : interdigitation/penetration of one surface into another, appears > 1.0 e.g. spikes, cleats, claws, hooves

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Effect of Surface Area How is friction impacted by surface area? IT IS NOT How is traction affected by surface are? surface area = traction
Dynamic friction Block moves if F a,x > F s,max (max static friction) Moving block experiences dynamic friction F d = d F g,y d < s F a,x F g,y F s F a,x = applied force F s = friction force mg

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Understanding Friction