Kinetics part 4 + 5

inertia

property of an object that resists change of motion

athropometrics

mass and moment of inertia

what is inertia proportional to

proportional to the mass of an object → the greater the mass, the greater the inertia and the greater the force needed to overcome the inertia

moment of inertia

aka mass of inertia

• rotational counterpart of mass

• resistance to change in angular velocity; resistance to rotational forces

Newton’s first law

= inertial law

bodies at rest tend to stay at rest; bodies in motion tend to stay in motion unless either is acted upon by an external force

Newton’s second law

= force-acceleration relationship

the acceleration for a body of constant mass is proportional of the resultant forces causing it and the change takes place in the direction in which the force acts

• acceleration is inversely proportional to the mass of the body

Newton’s second law equation

Force = mass x acceleration

Newton’s third law

= action-reaction

for every action, there is an equal and opposite reaction

momentum

quantity of motion possessed by a body

• represented by p

• p = mass x linear velocity

impulse

change in the momentum of the body

= FΔt

FΔt = mΔv

mechnical work

= energy expenditure

• forces or moments applied over a distance in the direction of the force applied

• no mechanical work is done if there is no movement

work energy relationship

linear: W = F x d

angular: W = M x θ

power

work done over time

power: linear movement

P = F x d/t = F x v

power: angular movement

P = M x θ/t = M x ω

• power developed through angular motion tends to be related to muscle performance

positive power

rate of work done by muscles during concentric contractions (working against gravity)

• internal muscle forces over external force

• power generated or acceleration

negative power

rate of work accomplished by muscles undergoing eccentric contractions (muscles controlling with gravity)

• external load over internal muscle force

• power absorption or deceleration

pressure

ratio of force and area on which the force is applied → units: N/m²