Torque, Moment Arm, MA, and Composition of Forces PDF

PT PAP 101- Torque/Moment Arm, Composition of Forces & Mechanical Advantage of Levers October 22, 2019 www.gmu.ac.ae COLLEGE OF ALLIED HEALTH SEIENCES Objectives At the end...

PT PAP 101- Torque/Moment Arm, Composition of Forces & Mechanical Advantage of Levers October 22, 2019 www.gmu.ac.ae COLLEGE OF ALLIED HEALTH SEIENCES Objectives At the end of this Lecture, the student should be able to Define Torque and Moment arm Analyze the composition of forces acting in a human body Define Mechanical advantage Explain the Mechanical Advantage of all the force systems.  Torque ▪ Torque is the amount of force needed by a muscle contraction to cause rotatory joint motion ▪ Torque (T) is a product of the magnitude of applied force (F) and the distance (r) that force lies from the axis of rotation, θ (moment arm) is the angle between the force vector and lever arm. Typically, it is equal to 90° ▪ T= rFsin(θ)  Moment Arm ▪ The [MA] Movement arm of any force vector will always be the length of a line that is perpendicular to the force vector and intersects the joint axis. ▪ The length of the Moment arm always relates to the angle of application of force. ▪ Lever Arm[LA]: It is the distance from the axis to the point at which a force is applied to the lever Moment Arm of a Muscle Force  Moment arm of Gravity ✔ The gravity acts vertically downward, the angle of application of gravity changes as the segments moves in space ✔ The force of gravity will be applied perpendicular to a segment whenever the segment is parallel to the ground ✔ When a body lever is parallel to the ground, gravity acting on that segment exerts its maximum torque Fms of Biceps & Gravity 1.The MA of any force is greatest when the action line is applied at 90ᶿ to its lever or when the action line is as close to 90ᶿ as possible 2. Resistance arm created by gravity on COG at the point of application Composition of forces Torque = Force X perpendicular distance or =Force X Moment Arm Net force acting while flexing the elbow If biceps is contracting (EF) with a force of 120lb applied at a distance (EA) 1 inch from the axis. Weight of forearm/hand segment(R) is 10lb and COG lies 10inch(RA) from the axis TEF = (F) (EA) TR = (F) (RA) TEF =(120lb)(1in) TR =(10lb)(10in) TEF =120inlb TR =100inlb ▪ Biceps exerts a torque of 120 in-lb on the forearm in a counterclockwise or positive direction TEF = +120in-lb ▪ Torque exerted by gravity is in clockwise or negative direction TR = –100in-lb ▪ Net rotation of a lever can be determined by finding the sum of all the torques acting on the lever ▪ If the sum of all torque is zero, the torques are balanced and the lever will not rotate ▪ When the torque of biceps is +120in-lb and the torque of gravity is – 100in-lb, resultant torque is 20 in-lb in a counter clockwise direction-flexion of forearm & hand Analysis of forces while extending elbow with weight in the hand There are three forces acting i.muscle force of biceps on forearm at its attachment (Fms=120lb) ii.force of gravity on forearm at the COG (G=10lb) iii.weight of the ball (WH=5lb)  Calculation Lever arms for these forces are 1,10,15 in We can find the net torque acting on the lever by finding the sum of the torques created by each force Torque exerted by biceps on the forearm in a counterclockwise/positive direction T Fms= (+ 120lb)×(1in) = +120in-lb Torque exerted by gravity on fore arm is in clockwise/negative direction TG = (− 10lb)×(10in) = −100in-lb Torque exerted by weight is clockwise /negative direction TWH = (− 5lb)×(15in) = −75in-lb  Net rotation of a lever can be determined by finding the sum of all the torques acting on the lever Net torque T= TFms + TG + TWH (+120in-lb)+(−100in-lb) +(−75in-lb) = −55in-lb  Types of muscle contraction and Levers ✔When an active muscle shortens -concentric contraction, it must be moving the segment under consideration in the direction of its pull so that the muscle will be the effort force ✔When an active muscle lengthens-eccentric contraction it must be pulling in a direction opposite to motion of the segment under consideration so that the muscle will be acting as resistance force ✔When a lever is in rotational equilibrium muscles acting on the segment under consideration are neither shortening nor lengthening- isometric contraction.  Terminologies to know ✓ Effort force force that is causing the rotation of the lever ✓ Resistance force force that is opposing the rotation of the lever ✓ Fulcrum Axis or the point where the rotation occurs E F R ✓ Effort Arm [EA] it refers specifically the distance of the lever EA RA arm from the effort force to the axis ✓ Resistance Arm[RA] it refers specifically the distance of the lever arm from the resistance force to the axis ✓ Torque the ability of any force to cause rotation of the lever  Lever principle It is the ratio of the length of the lever on the applied effort side of the fulcrum to the length of the lever on the load force side of the fulcrum. MA= DE : DL Mechanical advantage refers to how much a simple machine multiplies an applied force. The location of the effort, load, and fulcrum will determine the type of lever and the amount of mechanical advantage the machine has. Mechanical Advantage= Effort Arm [1.2] Resistance Arm [Effort arm >Load arm] Mechanical Advantage Levers are used to multiply force, In other words, using a lever gives you greater force or power than the effort you put in. In a lever, if the distance from the effort to the fulcrum is longer than the distance from the load to the fulcrum, this gives a greater mechanical advantage  Levers 1st order Lever: MA>1 2nd Order lever : Effort moves far than Load MA>1(Easy to lift) 3rd Order lever: Load moves far than Effort , MA< 1(Hard to lift, but the object moves more)- Uses more force to lift  First Class Levers This exists whenever 2 parallel forces are applied on either side of the axis at some distance from the axis, creating rotation of the lever in opposite direction  Second class levers This exists whenever two parallel forces are applied at some distance from the axis, with the resistance force applied closer to the axis than the effort force Third class levers This exists whenever 2 parallel forces on a lever are applied so that the effort force lies closer to the axis of the lever than does the resistance 23