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ACTION/REACTION FORCES

Understand how forces and reactions impact motion and shape of bodies, including internal and external forces, friction, centripetal force, elastic force, buoyancy force, power, and types of energy in the context of Newton’s Third Law.

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ACTION/REACTION FORCES

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  1. ACTION/REACTION FORCES • body segments exert a force • environment delivers reaction force • Newton’s Third Law • for every force applied on a body • that body applies an equal & opposite force

  2. T = F x FA • F is board reaction • FA is d  • t is board recoil time

  3. within defined body body free of support muscles contractions change body shape c of g positionNOT changed outside defined body body in contact muscle contractions change body position c of g position CHANGED Internal Forces External Forces

  4. cause Motion muscle, pull of gravity buoyancy concentric contraction friction fluid lift oppose/resist Motion muscle, connective tissue eccentric contraction friction fluid drag Motive Forces Resistive Forces

  5. FORCE and PRESSURE • Force - push, pull, blow, rub that changes a body’s motion or shape • pressure - amount of force acting over area • p & F direct relationship, p & A indirect

  6. FRICTION FORCE • Force created between 2 contacting surfaces • can be a Motive or a Resistive Force • texture of surfaces [rough = more friction] • force pressing together the 2 surfaces • size of surface area NOT a factor • high coefficient of friction = low slippage

  7. CENTRIPETAL FORCE • Center seeking • forces rotating body to stay on a circular path • if Fc is insufficient, body will leave path on a tangent

  8. ELASTIC FORCE • Recoil ability • measure of how quickly reformation follows deformation • coefficient of elasticity = square root of rebound ht/dropped ht • high e = faster reforming

  9. BUOYANCY FORCE • upward lift Force on a body immersed in a fluid • Archimedes Principle • upward buoyant F = wt of volume of water displaced • Negative Buoyancy - body wt. greater than the wt. of the volume of water displaced

  10. POWER • P = Force x velocity • P = Force x distance/time

  11. Kinds of Energy 1. chemical 2. electrical 3. heat 4. mechanical 3 types of Mechanical E 1. Elastic 2. Gravity - Potential 3. Kinetic ENERGY: the ability to do work

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