SECTION 2.1 FUNDAMENTAL MECHANICAL CONCEPTS

1. SECTION 2.1FUNDAMENTAL MECHANICAL CONCEPTS

2. Review • Anatomical Reference planes • Joint Movement Terminology • Movements at the Joints

9. FUNDAMENTAL MECHANICAL CONCEPTS • VECTORS VS SCALARS • FORCE • MASS VS WEIGHT • CENTRE OF MASS(C/M) • INERTIA • SPEED • VELOCITY • ACCELERATION • PRESSURE • FRICTION

10. VECTORS VS SCALARS • VECTOR - A math expression which represents quantities which possess both magnitude and direction

13. SCALARS • Scalar quantities only have magnitude • Graphically scalar quantities are represented by a dot or a point .

14. Vector websites • http://id.mind.net/~zona/mstm/physics/mechanics/mechanics.html • - visualizing components • http://www.hazelwood.k12.mo.us/~grichert/sciweb/mechanic.htm- intro info • http://id.mind.net/~zona/mstm/physics/mechanics/vectors/vectors.html

15. FORCE(N) • Vector quantity • A physical quantity which causes or tends to cause a change in the motion or shape of a body • - a push or pull

16. 4 PROPERTIES OF FORCE • 1. Magnitude – How much force is applied? • 2. Direction – In what direction is the force applied? • 3. Point of application – Each force is considered to act at one point. • 4. Line of Action – The line is a straight line which extends infinitely along the direction of the force.

19. Mechanical loads on the Human Body • The skeletal system is subjected to a variety of different forces so that bone is loaded in various directions. These are loads produced by gravity, weight bearing, muscle, and external forces • Compression • Tension • Shear • Mechanical Stress

20. A compressive force presses the ends of the bones together and is produced by muscles, weight bearing, gravity, or some external loading down the length of the bone • Tensile force • is a pulling force • that creates tension in the object/body • tends to lengthen and narrow • source is usually muscle

21. Shear Forces • A 3rd category of force • While compressive and tensile forces act along the longitudinal axis of the bone or other body, shear forces act parallel or tangent to the surface • Tend to cause one portion to slide or displace

22. Squat or Knee Bend Shear Force This position places a large amount of stress on the ligaments and tendons that prevent the femur from sliding off the tibial plateau

23. Stress and Strain • Another way of evaluating the behavior of bone or any other material when subjected to loading is to measure the stress, or load per cross-sectional area, or the strain or deformation with respect to the original length of the material.

24. Stress = Load per cross sectional area . • resulting force distribution as a result of an external force Strain = deformation with respect to the original length Normal Strain – change in length Shear Strain – change in angle Angle Length  In length

27. REPRESENTATION OF FORCESACTING ON A SYSTEM • Free-body diagram is a stick figure drawing of the system being analyzed on which the vector representations of the external forces acting on the system are drawn AR W Fr GR FORCE

28. MASS , WEIGHT & GRAVITY • MASS is a measure of the quantity of matter (KG). The same any where in the universe • WEIGHT is a measure of force(pull of gravity on a mass). Varies depending on gravitational pull • W = m. g (where m =mass, g = gravity) • g = downward acceleration of an object due to gravity • = 9.8m/sec2

29. CENTRE OF MASS • An imaginary point where the total mass of the body is considered to be centered • Balance point

30. In non-symmetrical bodies the position of the centre of mass is dependent upon the mass of each of the segments and how the segments are arranged

32. CENTRE OF MASS • The centre of mass of an individual is an imaginary point at which all the mass of the athlete is thought to be concentrated. It is the balance point of the athlete - the point at which the coach would support if able to balance the athlete on one finger. This point will change in the athlete's body as the athlete moves their arms and legs.

33. The force of gravity exerts a force on each of the particles forming the body. All these little forces can be represented by a single force W which equals the weight of the body and acts through the position of the centre of mass • In the anatomical position it is approx 56% of standing height for females and 57% for males

34. Segmental Method used to determine the total Body centre of mass Total body Centre of mass

35. When a collection of objects act together as a single entity, then the motion of that entity can be described by the position of the centre of mass of the system of objects. For example, when describing the motion of a car travelling down the highway, it is not necessary to worry about the location of each wheel, or the engine, or the passengers, etc. Instead a single point is sufficient, the point located at the centre of mass of all the components which make up the car.

37. http://www.phy.ntnu.edu.tw/java/block/block.html The location of the centre of mass can be calculated if the mass mi and location (xi,yi) of each component is known. By definition the centre of mass is located at (xcm,ycm) where xcm = (m1x1 + m2x2 + ......) / (m1 + m2 + ......)ycm = (m1y1 + m2y2 + ......) / (m1 + m2 + ......) plabpc.csustan.edu/general/tutorials/CenterOfMass/CenterOfMass.htm

38. CENTRE OF MASS & STABILITY • 4 FACTORS AFFECT STABILITY • 1. Height of C/M • 2. Area of base of support • 3. Position of the C/M in relation to the base of support • 4. Mass

39. INERTIA • Inertia is defined as a body or objects resistance to a change in motion. A change in motion may refer to trying to start something moving, stop it moving, or change it from moving at constant speed.

40. In linear motion an objects inertia is dependent solely on the mass of the object.

41. POSITION, DISTANCE & DISPLACEMENT • http://www.hazelwood.k12.mo.us/~grichert/sciweb/mechanic.htm • For a Position vs time graph for cyclist -> http://id.mind.net/~zona/mstm/physics/mechanics/kinematics/xvaVsTime/xVsTime.html • Another graphing demo • http://www.physics.gatech.edu/academics/classes/2211/main/demos/displacement/DDis.html

42. Distance vs displacement • Distance(l) = scalar, the length of a straight line path from one point to another followed by an object or body 4m B C Total Distance (l) = 9m 2m 3m A D

43. Displacement (d) • Vector, the length and direction of a line joining the initial position with the final position A 1m B  2m 2.2m C Displacement = 2.2m at 

44. SPEED & VELOCITY(m/sec) • SPEED & VELOCITY: Speed is the term used to refer to how fast something is travelling. • - When we are interested in not only how fast something is moving but also in the direction in which it is moving then it is the VELOCITY that is defined. Velocity possesses both magnitude and direction.

45. ACCELERATION(m/sec2) • Acceleration defines how much an object is speeding up or slowing down. If an object or body is in a state of constant velocity, or a state of equilibrium then the acceleration is equal to zero. • Also a vector quantity

46. PRESSURE • Pressure is defined as the distribution of force over a given area. The greater the area over which the force is distributed the smaller the pressure. Snowshoes are an excellent example of how the amount of area is increased to spread out the force and thus decrease the pressure , making it possible to walk on the snow.

47. PRESSURE (Pascals) • P = F/A • Where P = pressure • F = force(N) • A = area(m2) • 1 pascal = 1 N/ m2

48. PROBLEM • If a person who weighs 400N is standing on one foot which has a contact area of 100cm2. What is the pressure exerted? • F = 400N • A = 100cm2 • P=F/A = 400/100 = 4N/cm2

49. GRAVITY

50. TEXT BOOK • Page 62-70