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: R FaFa FsFs

5. Friction  Maximum static friction (F m )  Kinetic friction (F k ) – 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 = 80 kg v = 7 m/s m = 90 kg v =6 m/s

11. Collisions  Momentum Principle of conservation of momentum …..in the absence of external forces, the total momentum of a system remains constant… M 1 = M 2 m 1 v 1 + m 2 v 2 = (m 1 + m 2 )(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(v 2 -v 1 /t) Ft = (mv) 2 – (mv) 1 Ft = M

13. Collisions  Impulse examples