Transcript ESS 303 -- Biomechanics

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) 5N + 7N – 10N = 2N
Coplanar forces: forces in a plane
(connect the vectors and calculate the
displacement
+
=
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 force
Joint reaction force
Friction
Fluid resistance (concept of laminar flow)
Inertial force
Muscular force
Elastic force
Contact Forces (GRF)
Free Body Diagram
Muscle Force
(Line of Application)
Joint Reaction
Force
Point of Application
Angle of Pull
Force of gravity
(weight of segment)
Force of Friction
Direction of Motion
Ffriction
Instant of Motion
Friction Force
Coefficient of friction (μ)
Static friction
Dynamic friction
Fpull
Static Dynamic
Applied Force
Fweight
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