ESS 303 – Biomechanics

Linear Kinetics

Kinetics

The study of the forces that act on or

influence movement

Force = Mass * Acceleration: F = M * a

Force is a vector (magnitude & direction)

SI units for force are Newtons; (kg * a)

Newton’s 3 Laws of Motion

Inertia: the resistance of an object to changes in motion; an object at rest tends to stay at rest unless acted upon; an object in motion tends to stay in motion unless acted upon

Acceleration: change in motion is proportional to the force impressed; the direction of the change in motion is in the direction of the force impressed

Action/Reaction: Objects “push back” with equal force and in the opposite direction as the force impressed (equal and opposite)

Linear Kinetics

Concurrent forces: forces that act on the same point at the same time

Colinear forces: forces in a straight line (calculate the sum)

Coplanar forces: forces in a plane (connect the vectors and calculate the displacement

5N + 7N – 10N = 2N

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More Terms

Linear Momentum: Quantity of motion, a product of an object’s mass and velocity

Impulse: Product of force and time, changes the momentum of a system

Impact: Collision, typically a large force exchange over a small time period Perfectly elastic impact: All velocity is conserved Perfectly plastic impact: All velocity is lost Coefficient of restitution: Elasticity reference for

colliding bodies

Impacts

Elastic Plastic

Contact Forces

Ground reaction forceJoint reaction forceFrictionFluid resistance (concept of laminar flow)Inertial forceMuscular forceElastic force

Contact Forces (GRF)

Free Body Diagram

Force of gravity(weight of segment)

Muscle Force(Line of Application)

Joint ReactionForce

Angle of Pull

Point of Application

Force of Friction

Coefficient of friction (μ)Static frictionDynamic friction

Direction of Motion

Ffriction Fpull

FweightF

rictio

n F

orce

Applied Force

Static Dynamic

Instant of Motion

Linear Equations

Pressure: p = F / A; 1 Pascal = 1N/m2

Work: W = F * Distance, Joules (J)

Power: P = (W / t) = (F * Velocity); Watts

Weight = Mass * Acceleration

Linear Kinetics Problems

Calculate weight for a mass of 72kg? F = (M * a) = (72kg * 9.81m/s2) = 706.32 N How much force is needed to accelerate 500N

to 6.5 m/s2? 500N / 9.81m/s2 = 50.97kg F = (M * a) = (50.97kg * 6.5 m/s2) = 331.29N Calculate pressure if 500N are applied over an

11m2 area? p = (F / a) = (500N / 11m2) = 45.45 Pascals