Physics of Physical Activity

1. The study of the structure and functions of biological systems by means of the methods of mechanics (Hatze, 1974) Physics of Physical Activity

2. skill analysis • correction • pinpointing errors • developing a new technique • adapting to new equipment • understanding complex movement behavior Why study biomechanics?

3. 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

4. Such analysis allow us to describe motion in terms of: • Kinematics • Kinetics

5. Kinematics describing movements with respect to time and space most often using: • High speed cinematography or videography • Stroboscopy • Optoelectric • electrogoniometry • accelerometry

6. Kinetics examines the forces that produce the movement and result from the movement • pressure and force transducers • force platform • isokinetic dynamometer

7. Kinematics

8. Measuring Kinematics • Setup • Calibration • Record • Analysis

9. 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)

10. Kinematic Data

11. Kinematic Data

12. 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)

13. 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

14. Equilibrium Static equilibrium • System is at rest Dynamic equilibrium • System is moving with constant velocity

15. 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

16. 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

17. 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

18. 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

19. Angular Kinetics Torque • turning effect on a body measured as the product of force and moment arm length (e.g., changing tires)

20. 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)

21. Moment of Inertia: Examples

22. Sir Isaac Newton (1643-1727) Laws of Motion • Law of Inertia • Law of Acceleration • Law of Reciprocal Actions

23. 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

24. 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)

25. Force Acc For Constant Mass

26. Mass Acc Exert Constant Force

27. If Same Acc is needed Mass Force

28. Action-Reaction Third Law • every action has an equal and opposite reaction (basis for using force plates in directly measuring forces)

29. Weight (N)

30. Ground Reaction Force and Example

31. 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

32. 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

33. Drag Will depend on: • fluid density • frontal area of body (e.g., rowing shells) • drag coefficient (dependent on shape) • movement velocity

34. 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

35. 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%

36. Lift Resultant Lift Drag Component of air resistance that is directed at right angles to the drag force Air Flow

37. 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

38. 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.

39. “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'.

40. 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

41. Magnus Force

42. 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

43. And wrapping things up for this section