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Linear Kinetics - continued

Linear Kinetics - continued. Ground reaction force in activities. Runner … Increases stride over slow-moderate speed range Longer stride produces GRF’s w/larger counteractive horizontal components Longer stride lengths increases shock transmission up the extremity ( stress on the knees).

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Linear Kinetics - continued

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  1. Linear Kinetics - continued

  2. Ground reaction force in activities • Runner … • Increases stride over slow-moderate speed range • Longer stride produces GRF’s w/larger counteractive horizontal components • Longer stride lengths increases shock transmission up the extremity ( stress on the knees)

  3. Ground reaction force in activities • High jumper • Move with large horizontal velocity with slight downward directed vertical velocity at stride before take-off • GRF reduces horizontal velocity & creates upward vertical velocity

  4. Friction • Acts at the interface of surfaces in the opposite direction of the motion • Unit: N • Magnitude determines ease/difficulty of motion • Example: Fa Fs R

  5. Friction • Maximum static friction (Fm) • Kinetic friction (Fk) – constant magnitude friction generated between two surfaces in contact during motion

  6. Friction • What factors determine the amount of force needed to move an object? • Coefficient of friction ()  interaction between two surfaces • Unit-less • Relative ease of gliding • Influencing factors?

  7. Friction • Normal reaction force • Greater the force, greater the friction • Can we decrease the reaction force? • Can we reduce or increase the friction? • Alter the coefficient of friction? • Examples???

  8. Friction in everyday life • Does greater contact area generate more friction? • Shoes/feet & surface interface • Rolling friction: deformation during contact • Influenced by weight, radius & deformity • Bike tires

  9. Friction • Effect of fluid? • Synovial fluid • Speed skating • Bowling ball

  10. Collisions • Momentum: the quantity of motion that an object/body possesses • Product of mass and velocity M = mv m = 90 kg v =6 m/s m = 80 kg v = 7 m/s

  11. Collisions • Momentum • Principle of conservation of momentum …..in the absence of external forces, the total momentum of a system remains constant… M1 = M2 m1v1 + m2v2 = (m1 + m2)(v)

  12. Collisions • Impulse: product of a force and the time interval over which the force acts Impulse = Ft - from Newton #2 F = ma  F = m(v2-v1/t) Ft = (mv)2 – (mv)1 Ft = M

  13. Collisions • Impulse examples

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