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Chapter 5

Chapter 5. Torques and Moments of Force Maintaining Equilibrium or Changing Angular Motion. To this juncture. Analysis focused on linear motion (translation) kinematics : position, change of position, rate of change of position, acceleration kinetics : Newton’s 3 laws of linear motion

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Chapter 5

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  1. Chapter 5 Torques and Moments of Force Maintaining Equilibrium or Changing Angular Motion

  2. To this juncture • Analysis focused on linear motion (translation) • kinematics: position, change of position, rate of change of position, acceleration • kinetics: Newton’s 3 laws of linear motion • F = ma • Ft =  mv • Fd =  KE +  GPE

  3. What about Rotation? • All points on a body or object move in circles (or parts of circles) about the same fixed central line or axis • body spins around an axis (real or imaginary) • Force ==> linear motion • ????? ==> angular motion

  4. Line of actionrelative to axis of rotation • Centric force • line of action passes through the axis of rotation • tends to cause translation

  5. Line of actionrelative to axis of rotation • Centric force • Eccentric force • line of action does not pass through the axis of rotation

  6. Line of actionrelative to axis of rotation • Centric force • Eccentric force • Force couple • two eccentric forces

  7. Torque • Definition • the turning effect of a force • the tendency of a force to cause rotation What factors affect the tendency of the force to cause rotation???

  8. Torque • Definition • the turning effect of a force • the tendency of a force to cause rotation

  9. Moment Arm(lever arm) Perpendicular distance from line of action of a force to a specified axis of rotation

  10. Effect ofchangingthe line of action of force

  11. To describe a torque • Specified axis of rotation • Torque magnitude (F x r) • units: Newtons x meters (Nm) • foot x pound (ft-lb) • Direction (sense) of the torque • clockwise (-) • counterclockwise (+)

  12. Examples of Torque Click on the picture to go to a good website

  13. Examples of Torque Show the moment arm and identify the sense for each force on each figure

  14. Sculling: offsetting torques to create translation Are all three designs equal in net torque created at the stern??

  15. ADLs must consider torque for safety and convenience Medicine caps must be removable by the disadvantaged, but inoperable by kids.

  16. Torque • Rotary force that produces angular acceleration. • An increase in the magnitude of the applied force, or in the perpendicular distance of the force's line of action to the axis of rotation, results in an increase in the acting torque.

  17. Torque • The greater the amount of torque acting at the axis of rotation, the greater the tendency for rotation to occur and the greater the angular acceleration of a given body.

  18. Muscle Torque • Muscle applies force by?

  19. Muscle Torque • Muscle applies force by creating tension on bones • Muscle crosses a joint or joints • brachialis and biceps brachii • soleus and gastrocnemius • other examples????

  20. Muscle Torque • Moment arm - shortest (perpendicular) distance between a force's line of action and an axis of rotation. • The moment arm for a muscle with respect to a joint center is the perpendicular distance between the muscle's line of action and the joint center.

  21. Muscle Torque • Muscle applies force by creating tension on bones • Muscle crosses a joint or joints • Muscle has a moment arm

  22. Borelli

  23. Muscle Torque • As a joint moves through a range of motion, there are changes in the moment arms of the muscles crossing the joints. • For any given muscle, the moment arm is largest when the angle of pull on the bone is closest to 90 degrees.

  24. Elbow at 90o of flexion

  25. Elbow at 135o of flexion

  26. Elbow at 45o of flexion

  27. Muscle Torque • Changes in a moment arm directly affect the joint torque that a muscle generates. • For a muscle to generate a constant joint torque during an exercise, it must produce more force as its moment arm decreases.

  28. Couple • A pair of equal, oppositely directed forces that act on opposite sides of an axis of rotation to produce torque.

  29. Muscle Torque • Concentric torque - when net torque and joint movement occur in the same direction. • Eccentric torque - torque in the direction opposite joint motion.

  30. Torque • Movement speed - when other factors remain constant, increased movement speed is associated with increased resultant joint torque during exercise such as the squat.

  31. Torque • However, increased movement speed during weight training is generally undesirable because increased speed increases not only the muscle tension required, but also the likelihood of incorrect technique and subsequent injury.

  32. Torque • Acceleration of the load early in the performance of a resistance exercise also generates momentum, which means that the involved muscles need not work as hard throughout the range of motion as would otherwise be the case.

  33. Torque • For these reasons it is both safer and more effective to perform resistive exercises at slow controlled movement speeds.

  34. Torque • During eccentric contractions, muscle and bone function as a second class lever.

  35. Strength training and torqueBiceps Curl • Muscle as torque generator • moment arm changes through ROM • muscle’s ability to create force changes through ROM (Read Chapter 12 in McGinnis) • External forces as torque generators • segment weight • handheld weight (dumbbell)

  36. Strength training and torque

  37. Calculate Extensor Muscle Force Forces: P (hand held load) = 90 N W (HAT weight) = 80 N Moment Arms Lw = 25 cm Lp = 60 cm Lm = 5 cm

  38. Calculate Extensor Muscle Force Clockwise torque: -90 N * 60 cm = ? -90 N * 0.60 m = -54 Nm -80 N * 25 cm = ? -80 N * .25 m = -20 Nm (-54 Nm) + (-20Nm) = -74 Nm

  39. Calculate Extensor Muscle Force T = 0 -74 Nm = F * 5 cm -74 Nm = F * 0.05 m F = -74Nm ÷ 0.05 m F = 1500 N

  40. Additional calculations

  41. Measuring torque to assess the effects of Lifetime Fitness

  42. Torque Decline with Age

  43. Athletes vs Sedentary

  44. Power Decline

  45. Torque & Baseball Pitching

  46. Forces and Torquesin Equilibrium • Static Equilibrium • sum of forces on the body = 0 •  F = 0 • sum of torques on the body = 0 •  T = 0

  47. Human Machines • Torques and Moments of Force

  48. Human Machines • The human body has many structures that function in a machine-like fashion

  49. Human Machines • When analyzing human machines, it is important to recall Newton’s 3 Laws of motion. • Law of Inertia • Law of Acceleration • Law of Action-reaction

  50. Functions of a Machine • Provide mechanical force advantage. • Provide speed of motion advantage. • Provide range of motion advantage. • Change the direction of the resistive force. • Balance two or more forces.

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