430 likes | 620 Views
The study of the structure and functions of biological systems by means of the methods of mechanics (Hatze, 1974). Physics of Physical Activity. skill analysis correction pinpointing errors developing a new technique adapting to new equipment understanding complex movement behavior.
E N D
The study of the structure and functions of biological systems by means of the methods of mechanics (Hatze, 1974) Physics of Physical Activity
skill analysis • correction • pinpointing errors • developing a new technique • adapting to new equipment • understanding complex movement behavior Why study biomechanics?
Quantitative • Involves measurement of variables that are thought to optimize or maximize performance • Qualitative • Involves obtaining visual or aural information to assess performance Analyses for physical activity
Such analysis allow us to describe motion in terms of: • Kinematics • Kinetics
Kinematics describing movements with respect to time and space most often using: • High speed cinematography or videography • Stroboscopy • Optoelectric • electrogoniometry • accelerometry
Kinetics examines the forces that produce the movement and result from the movement • pressure and force transducers • force platform • isokinetic dynamometer
Measuring Kinematics • Setup • Calibration • Record • Analysis
What is measured? • Time: temporal characteristics of a performance, either of the total skill or its phases (seconds, milliseconds, etc) • Displacement: length and direction of the path an athlete takes from start to finish (inches, meters, cm, etc) • Velocity: displacement per unit of time (m/s) • Acceleration: rate of change of velocity (m/s2)
Angular Motion • Time: temporal characteristics of a performance, either of the total skill or its phases • Angular Displacement: direction of, and smallest angular change between, the rotating body’s initial and final position (revolutions, degrees, radians) • Angular Velocity: angular displacement per unit of time (e.g., degrees/sec) • Angular Acceleration: angular velocity per unit of time (e.g., degrees/sec2)
Center of Mass • Located at the balance point of a body; a point found in or about a body where the mass could be concentrated • Generally, 15 cm above the crotch, or approximately 55% of standing height in females and 57% in males Dance improvisation for ~1.5 mins recorded at 200 Hz (>20000 frames) with 61 markers
Equilibrium Static equilibrium • System is at rest Dynamic equilibrium • System is moving with constant velocity
Stability A measure of the difficulty with which equilibrium can be disturbed. Stability can be increased by: • Increasing the base of support • Increasing the inertia of the body • Decreasing the vertical distance between the center of mass and the base of support
The only cause of motion of the human body is the application of an external force • Force is any action, a push or pull, which tends to cause an object to change its state of motion by experiencing an acceleration Kinetics
Types of Motion • Linear Motionis caused by forces which act through a body’s center of mass • Angular motionis caused by forces that do not go through the center of mass
Kinetics • Massis the measure of how much matter an object has • Inertia is the reluctance of an object to change its state of motion from rest to moving, to moving faster, or to slowing down back to rest
Angular Kinetics Torque • turning effect on a body measured as the product of force and moment arm length (e.g., changing tires)
Angular Kinetics Moment of inertia • resistance to rotary motion that results from combination of mass and distribution of the mass of an object • minimize resistance to angular rotation must move mass closer to axis of rotation (e.g., choking-up in baseball, spinning in skating or gymnastics)
Sir Isaac Newton (1643-1727) Laws of Motion • Law of Inertia • Law of Acceleration • Law of Reciprocal Actions
First Law • a body continues in a state of rest or uniform motion until acted upon by an external force of sufficient magnitude to disturb its current state Inertia
Second Law the acceleration of the body is proportional to the force exerted on it and inversely proportional to its mass (Acceleration or F=ma)
Force Acc For Constant Mass
Mass Acc Exert Constant Force
If Same Acc is needed Mass Force
Action-Reaction Third Law • every action has an equal and opposite reaction (basis for using force plates in directly measuring forces)
Weight (N)
Weight versus Mass W = m x g Mass • Measure of inertia • Measured in kilograms (kg) Weight (a force, F = ma) • Measure of the force of gravity (g) • Measured in Newtons (N) • Varies directly with the magnitude of the acceleration due to gravity (9.8 m/s2) A person with a weight of 980 N would have what mass in lbs? 2.2 lbs = 1 Kg
Fluid Mechanics Drag • Fluid force that opposes the forward motion of the body and reduced the body’s velocity. Lift • Component of air resistance that is directed at right angles to the drag force
Drag Will depend on: • fluid density • frontal area of body (e.g., rowing shells) • drag coefficient (dependent on shape) • movement velocity
Types of Drag • Surface (hydrodynamic drag) • referring to interaction between body surface and the water • Profile (Form) • refers to resistive forces resulting from poor body position • Wave
Profile Drag • Low pressure pocket forms and “holds back” the cyclist. As velocity doubles this • resistive force quadruples!!!! • Important factors: • Shape • smoothness • orientation (crouch can lower • resistance ~30%
Lift Resultant Lift Drag Component of air resistance that is directed at right angles to the drag force Air Flow
Angle of Attack • Refers to the tilt of an object relative to the flow velocity • A function of the shape of an object and the flow velocity • If the angle of attack increases too much, it approaches a critical maximum angle (stall angle), beyond which the lift force decreases as the drag force becomes dominant
High velocity/Low Pressure Low velocity/High Pressure Lift According to Bernoulli's Law (inverse relationship between velocity and pressure), faster air has lower air pressure, and thus the high pressure beneath the wing pushes up to cause lift.
“Reverse Lift” The desire to further increase the tire adhesion led the major revolution in racing car design, the introduction of inverted wings, which produce negative lift or 'downforce'.
The changes in flight path are always perpendicular to the flow velocity of the projectile The pressure difference across opposite sides of an object (which spins about an axis that is not aligned with the flow velocity vector) can generate a change in its flight path through a type of lift force known as Magnus force. Magnus Force
Summary • Measuring Biomechanics in physical activity • Kinematics and Kinetics of physical activity • Linear and Angular Motion of physical activity • Laws of Motion apply to physical activity • Fluid dynamics can impact physical activity