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)

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

Friction

Acts at the interface of surfaces in the opposite direction of the motion

Unit: N Magnitude determines ease/difficulty

of motion Example:

R

Fa

Fs

Friction

Maximum static friction (Fm)

Kinetic friction (Fk) – constant magnitude friction generated between two surfaces in contact during motion

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?

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???

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

Friction

Effect of fluid? Synovial fluid Speed skating Bowling ball

Collisions

Momentum: the quantity of motion that an object/body possesses Product of mass and velocity

M = mv

m = 80 kgv = 7 m/s

m = 90 kgv =6 m/s

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)

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

Collisions

Impulse examples