Kinesiology Test 2 ( linear kinetics introduction, frictional forces)

what is kinetics

• the study of causes of human motion

• study of forces, moments and masses, but without detailed knowledge of position and orientation of objects involves (“push or pull”)

what is newtons 3rd law

for every action there is an equal opposite reaction

example of action reaction forces int running

• action: concentric action of plantar flexors

• reaction: ground pushes back on the runner helping then to move forward

what is Newtons 1st law of motion (law of inertia)

every body continues in its state of rest, or of uniform motion in a straight line, unless it is compelled to change that state of forces impressed upon it

what is inertia

• resistance to action or change (resistance to acceleration)

what is newtons 2nd law of motion (law of acceleration)

change of motion of an object is proportional to force impressed and is made in the direction of the straight Line in which the force is impressed

non contact forces

weight

contact forces

• GRF

• Joint reaction

• muscle

• friction

internal force classification (pull)

• act inside body; recall that forces come in pairs (action-reaction)

  • tensile > pulling

  • compressive > pushing

external force classification (push)

GRF forces during movement can vastly exceed one body weight. a force plate is required to measure it.

common measurement devices in kinetics

• force plates

• pressure platforms

• handheld dynamometers

force plates

• measure ground reaction forces

• typically used in gait and sporting analysis. they measure the force between the body and the ground but give no information on the position of the limb or joint angles (kinematics)

pressure formula

pressure = force/area

what is friction

acts at the interface of surfaces in contact

frictional types

• static ( limiting friction)

  • maximum force just before two forces slide

• dynamic (sliding or kinetic friction)

  • friction between two moving surfaces

• rotational

  • friction that opposes rotation/twisting movements (cutting on an athletic surface)

factors influencing friction

• frictional forces act parallel to contact area and opposes the motion

• frictional force depends on two things

  • 1. normal force

  • 2. nature of the surfaces involved (μ)

Friction = μ ⋅ Rn

limiting value of static friction

• as F increases the magnitude of opposing friction also increases until it reaches a MAX static friction

• Three Scenarios:

  • (1) F < Fm NO MOTION

  • (2) F = Fm MOTION IMPENDING

  • (3) F > Fm MOTION

coefficient of friction

• fundamental force acting tangentially to motion

• unites number

• indicated “relative ease of sliding” or “how sticky two surfaces are”

• influencing factors:

  • 1. surface roughness

  • 2. surface hardness

  • 3. molecular interaction between surfaces

• μK < μS for two bodies in contact

  • k= kinetic (moving)

  • s= static

what is the coefficient of friction within synovial joints

0.001 or even Lower

what is traction

practical application of friction, refers to the grip of adhesion of one object moving or trying to move on another object. environmental conditions, design of contacting elements and softness of ground will influence traction