Chapter 3 - Basic Mechanical Concepts PDF

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Anatomical Kinesiology with professor Sabitini....

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Basic mechanical concepts Mechanics- study of physical actions of forces Mechanics is divided into ○ Statics- standing still, touching toes, having 0 acceleration ■ Study of systems in state of constant motion. At rest, constant velocity (no acceleration) ○ Dynamics- moving ■ Study of systems not in steady-state motion, acceleration is non-zero Biomechanics ○ Kinematics ■ Description of motion and includes consideration of time, displacement, velocity, acceleration, and space factors of a systems motion ■ How fast, how high, how far an object travels ■ Linear kinematics ● Movement along a line, with all points moving in same direction at same speed ■ Angular kinematics ● Involves rotation about an axis of point ○ Kinetics ■ Study of forces associated with the motion of a body ■ Interested in what causes or tends to cause motion ■ Also includes linear kinetics and angular kinetics Laws of motion and physical activities ○ Displacement- actual distance that the object has been displaced from its original point of reference ■ Go in a circle 6 steps but 0 displacement because it ends in the same spot ○ Distance- actual sum length of measurement traveled ■ Object may have traveled a distance of 10 meters along a linear path in 2 or more directions but on be displaced from its original reference point by 6 meters ○ Body motion is produced or started by some actions of muscular system ○ Motion cannot occur without a force ○ Muscular system is source of force in humans

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○ Torque- rotational force ○ Linear motion (translatory motion)- motion along a line ■ Rectilinear motion- motion along a straight line ● No movement just straight (train with no hills) ■ Curvilinear motion- motion along a curved line ● Straight line but moves up and down (walking) ● Athlete, throwing a ball, gymnast flipping Angular rotational motion ○ Rotation about a point (axis) ○ Any axis, external axis of rotation ○ In the body, the axis of rotation is provided by the various joints Linear and angular motion are related ○ Angular motion of the joints produces the linear motion of walking General motion ○ Combination of linear and angular motion ■ Translation and rotation ○ Rotation of wheels results in linear motion of the car ○ Coordinated rotation of the hip, knee, and ankle joints eventually results in linear motion of the ball (kicking soccer ball) Newton’s 1st law: inertia ○ A body will stay at rest ○ A body will continue to move with the same velocity unless… ■ A net force acts on the body ● To speed up or slow down an object's motion, and/or to change an object's direction of motion, and object's inertia must be overcome ● Resistance to action or change, mass, weight, friction, gravity, momentum all contribute to inertia ● Net force- produce 1000 newtons of force, the weight in hand is 600, net is 400 ■ In human movement, inertia refers to resistance to acceleration or deceleration ○ Force is required to change inertia ■ Any activity carried out at a steady pace in a consistent direction will conserve energy ■ Any irregularly paced or directly activity will be very costly to energy reserves

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■ Running because easy and can do it for a long time (less inertia) vs. basketball bc it takes more force and (more inertia) Newton's 2nd law: F=ma ○ ∑ F=ma ○ Sum of all forces acting on object objects mass objects acceleration Force ○ Effect of one body on another ○ A push or pull applied to an object that is needed to change the state of motion of object accelerate object ○ Example: tom brady vs. linebacker - feel same force but tom gets hurt more ○ Muscles are the main source of force that produces or changes movement of a body segment, the entire body, or some object thrown, struck, or stopped ○ Strong muscle are able to produce more force than weak muscles ■ Both maximum and sustained exertion over a period of time ○ Many activities, particularly upper extremity, require a summation of forces from the beginning of movement in the lower segment of the body to the twisting of the trunk and movement at the shoulder, elbow and wrist joint ■ Ex. opening a doorknob or using a screwdriver Law of acceleration ○ Acceleration- Rate of change in velocity ■ To attain speed in moving the body, a strong muscular force is generally necessary ○ Mass- the amount of matter in the body ■ Affects the speed and acceleration in physical movements ○ A change in the acceleration of a body occurs in the same direction as the force that caused it. ○ The change in acceleration is directly proportional to the force causing it and inversely proportional to the mass of the body ■ Higher force, greater acceleration ■ Greater mass, more force required to accelerate Mass ○ Quality of matter of which a body is composed ○ Direct measure of a body’s resistance to change on linear motion

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■ Ex. an object's inertia wrt linear motion ■ Ex. it is more difficult to change the motion of a large object (large mass) than a small object ■ Large mass (shot) vs small mass (tennis ball) Centric and eccentric forces ○ Centric forces result in linear (translational) motion only ○ Eccentric (off - center) forces always result in rotational motion (sometimes linear motion too) Eccentric forces: torque (moment of force) ○ Turning effect of an eccentric force ○ Equal to the product of force and the perpendicular distance from the force’s line of action ○ T = F*d┴ ○ Force: any eccentric force will cause a torque ○ Moment arm: special name given to the perpendicular distance from forces line of action and the axis of rotation ■ Shovel example ○ Most important Newton's 3rd law: action reaction ○ For every action there is an opposite and equal reaction ○ Action - reaction- describes how objects interact with one another ■ As we place force on a surface by walking over it, the surface provides an equal resistance back in the opposite direction to the soles of our feet ■ Our feet push down & back, while the surface pushes up & forward ■ Force of the surface reacting to the force we place on it is ground reaction force ■ Has to be instant ○ Ex. pushing a lawn mower is action-reaction because as soon as you push into it, it pushes into you. ○ Ex. bouncing on a trampoline is not because there is a delay Law of reaction ○ We provide the action force while the surfacr provides the reaction force ■ Ex. it is easier to run on a hard track than on a sandy beach due to the difference in the ground reaction forces of the two

surfaces ■ Friction because surface you put force into dictates how much force comes back to you ● Friction ○ The force that results from the resistance between surfaces of two objects from moving upon one another ■ Depending increased or decreased friction may be desired ■ To run, we depend upon friction forces between our feet and the ground so that we may exert force against the ground and propel ourselves forward ■ With slick ground or shoes, friction is reduced and we are more likely to slip ■ In skating, we desire decreased friction so that we may slide across the ice with less resistance ○ Static friction- the amount of friction between two objects that have not yet begun to move. ■ Always greater than kinetic friction ■ May be increased by increasing the normal or perpendicular forces pressing the 2 objects together such as in adding more weight to one object sitting on the other object ○ Kinetic friction- friction occurring between two objects that are sliding upon one another ○ Rolling friction- resistance to an object rolling across a surface such as a ball rolling across a court or a tire rolling across the ground ■ Rolling is always much less than static or kinetic ● Center of gravity ○ Balance, stability, equilibrium ○ Levers ○ Torque ○ Theoretical point as which all of the body’s weight is considered to be concentrated ■ Point about which a body will balance ● CG location is dependent on weight distribution of body ○ All 3 planes intersect ● Center of gravity of the human body ○ In anatomical position, GC is near the waist

○ Females- 53-56% of standing height ○ Males- 54-57% of standing height ● Balance, equilibrium, and stability ○ Equilibrium- states of zero acceleration where there is no change in the speed or direction of the body ■ Static or dynamic ○ Static equilibrium- body is at rest or completely motionless ○ Balance- ability to control equilibrium, either static or dynamic ■ Controlling the forces that may cause this (no acceleration) ○ Dynamic equilibrium- all applied and inertial forces acting on the moving body are in balance, resulting in movement with unchanging speed or direction ○ To control equilibrium and achieve balance, stability needs needs to be maximum ○ Stability is the resistance to a ■ Disturbance of the body’s equilibrium ● Controlling the forces that may cause this (no acceleration) ■ Stability is enhanced by determining body’s center of gravity & appropriately changing it ○ 9 general factors ■ Applicable to enhancing equilibrium, maximizing stability & ultimately achieve balance ● 1. A person has balance when the center of gravity falls within the base of support ● 2. A person has balance in the direct proportion to the size of the base ○ The larger the base of support, the more balance ● 3. A person has balance depending on the weight (mass) ○ The greater the weight, the more balance ● 4. A person has balance, depending on the height of the center of gravity ○ The lower the center of gravity, the more balance ● 5. A person has balance, depending on where the center of gravity is in relation to the base of support ○ Balance is less if the center of gravity is near the

edge of the base ○ When anticipating an oncoming force, stability may be improved by placing the center of gravity nearer to the side of the base of support expected to receive the force ● 6. In anticipation on an oncoming force, stability may be increased by enlarging the size of the base of support in the direction of the anticipated force ● 7. Equilibrium may be enhanced by increasing the friction between the body and the surfaces it contacts ● 8. Rotation about an axis aids balance ○ A moving bike is easier to balance than a stationary bike ● 9. Kinesthetic physiological functions contribute to balance ○ Semicircular canals of the inner ear, vision, touch (pressure) and kinesthetic sense all provide balance info to the performer (eyes closed vs. open) ○ Balance and its components of equilibrium and stability are essential in all movements and are all affected by the constant force of gravity as well as by inertia ● Types of machines found in the body ○ Musculoskeletal system arrangements provides for 3 types of machines in producing movement ■ 1. Levels (most commons) ■ 2. Wheel-axles ■ 3. Pulleys ○ Machines function in four ways: ■ 1. Balance multiple forces ■ 2. Enhance force (ex. Reduce total force needed) ■ 3. Enhance range of motion & speed of movement (ex. Resistance moved further and faster than applied force) ■ 4. Alter direction of applied force ● Levers ○ Humans move through a system of levers

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■ Lever- a rigid bar that turns about an axis of rotation or a fulcrum Three points determine type of lever and for which kind of motion it is best suited ■ Axis(A)- fulcrum- the point of rotation ■ Point (F)- of force application (usually muscle insertion)- effort ■ Point ®- of resistance application (center of gravity of lever) or (location of an external resistance) Levers rotate about an axis as a result of force being applied to cause its movements against a resistance or weight ■ Bones represent the bars ■ Joints are the axes muscles contract to apply forces First class lever ■ 1st- force ■ 2nd - axis ■ 3rd- resistance ■ Produce balanced movements when axis is midway between force & resistance (seesaw) ■ Produce speed and range of motion when axis is close to force (triceps in elbow extension) ■ Produce force motion when axis is close to resistance (crowbar) ● Make it easier by ○ Increase weight ○ Axis is closer to resistance ■ Force is applied where muscle inserts in bone, not in belly of muscle ● Ex: in elbow extension with shoulder fully flexed and arm beside ear, the triceps applies force to the olecranon of ulna behind the axis of elbow joint ● As the applied force exceeds the amount of forearm resistance, the elbow extends Second class lever ■ 1st- axis ■ 2nd- resistance ■ 3rd- force ■ Resistance is between axis and force

■ Produces force movements since a large resistance can be moved by a relatively small force ■ Raising body up on the toes ● Ex: Plantar flexion of foot to raise the body up on the toes where the ball of the foot serves as the axis as ankle plantar flexors apply force to the calcaneus to left the resistance of the body at the tibial articulation with the foot ■ Ex: Wheelbarrow ● Dirt is resistance ● Wheel is axis ● Holding it is force ■ Few second class levers in the body ○ Third class lever ■ 1st- axis ■ 2nd- force ■ Third- resistance ■ Force is between axis and resistance force ■ Produces speed and range of motion movements ■ Most common in human body ■ Requires a great deal of force to move even a small resistance ● Paddling a boat ● Shoveling- application of lifting force to a shovel handle with lower hand while upper hand on shovel handle serves as axis of rotation ■ Biceps brachii in elbow flexion- using the elbow joint as the axis, the biceps brachii applies force at its insertion on radial tuberosity to rotate forearm up, with the center of gravity serving as the point of resistance application ■ Brachialis- true 3rd class leverage ● Pulls on ulna just below elbow ● Pull is direct and true since ulna cannot rotate ■ Other examples ● Hamstrings contracting to flex leg at knee while in a standing position ● Using iliopsoas to flex thigh at hip

● Torque and length of lever arms ○ Torque- (movement of force) the turning effect of an eccentric force ○ Eccentric force- force applied off center or in a direction not in line with the center of rotation of an object with a fixed axis ■ In objects without a fixed axis it is an applied force that is not in line with the object’s center of gravity ○ For rotation to occur an eccentric force must be applied ○ Force arm- perpendicular distance between location of force application & axis ■ Aka moment arm or torque arm ■ Shortest distance from axis of rotation to the line of action of the force ■ The greater the distance of force arm, the more torque produced by the force ○ Resistance arm- distance between the axis and the point of resistance application ○ Often we purposely increase force arm length in order to increase torque so that we can more easily move a relatively large resistance (increasing our leverage) ○ Human leverage for sport skills requires several levers ■ Throwing a ball involves levers at shoulder, elbow, and wrist joints ○ The longer the lever, the more effective it is in imparting velocity ■ A tennis player can hit a tennis ball harder with a straight arm drive than with a bent elbow because the lever (including the racket) is longer and moves at a faster speed ○ Longer levers produce more linear force and thus better performance in some sports such as baseball, hockey, golf, field hockey ○ For quickness, it is desirable to have a short lever arm ■ Baseball catcher brings his hand back to his ear to secure a quick throw ■ Sprinter shortens his knee lever through flexion that he almost catches his spikes in his gluteal muscles ○ Proportional relationship between force components and resistance components ■ Greater resistance or resistance arm requires greater force or

longer force arm ■ Greater force or force arm allows a greater amount of resistance to be moved ● Musculoskeletal lever system ○ Generally favors speed over strength ■ Resistance moves ● 1. A greater distance than muscle insertion point ● 2. At a greater velocity than muscle insertion point ■ V=D/T ■ Cover more ground at same time period is because of velocity ● Factors in use of anatomical levers ○ Anatomical leverage system can be used to gain a mechanical advantage ○ Improve simple or complex physical movements ○ Some habitually use human levers properly ■ Some develop habits of improperly using human levers ● Wheels and axles ○ Used primarily to enhance range of motion and speed of movment in the musculoskeletal system ■ Function essentially as a form of a lever ○ When either the wheel or axle turn, the other must turn as well ■ Both complete one turn at the same time ○ Center of the wheel and the axle both correspond to the fulcrum ○ Radii of the wheel and the axle correspond to the force arms ○ If the wheel radius > axle radius: ■ due to the longer force arm, wheel has a mechanical advantage over the axle ■ a relatively smaller force may be applied to the wheel to move a relatively greater resistance applied to the axle ■ if the radius of the wheel is 3 times the radius of the axle, then the wheel has a 3 to 1 mechanical advantage over the axle ○ If application of force is applied to the axle ■ mechanical advantage results from the wheel turning a greater distance & speed ■ if the radius of the wheel is 3 times the radius of the axle, then outside of the wheel will turn at a speed 3 times that of the axle ■ the distance that the outside of the wheel turns will be 3 times that of the outside of the axle ○ Ex: shoulder internal rotators and humerus ■ Humerus acts as the axle

■ Hand and wrist are located at the outside of the wheel when elbow is flexed 90 degrees ■ With minimal humerus rotation, the hand and wrist travel a great distance ■ Allows us significantly increase the speed at which we can throw objects ● Pulleys ○ Single pulleys: ■ Change direction of force ■ MA= 1 ○ Compound pulleys ■ Each additional rope increases MA by 1 ○ Ex. lateral malleolus acting as a pulley for peroneus longus tendon ■ As peroneus longus contracts, force generated towards its belly ■ Force transmitted to plantar aspect of foot via the lateral malleolus resulting in eversion plantar flexion ○ Quadriceps tendon and patella...