HESS 510 Chapter 3 Basic Biomechanical Factors & Concepts PPT Series 3B Laws of Motion and Physical Activity Terminology Human motion is produced or started by some action of the muscular system(source of force production), and motion cannot occur without a force. Motion may be categorized as either linear motion or angular motion. Linear motion (translatory motion) is motion (movement) along a line
Rectilinear motion - Motion along a straight line (recti = straight) Curvilinear motion - Motion along a curved line Laws of Motion and Physical Activity Terminology Angular motion (rotary motion) - Rotation around an axis In the body, the axis of rotation is provided by the various joints Linear and angular motion are related Examples Angular motion of the knee and hip joints produce the linear motion of walking
In sports, cumulative angular motion of the joints produces linear motion to a thrown object (ball, shot) or to an object struck with an instrument (bat, racket) Laws of Motion and Physical Activity Terminology Center of rotation - The point or line around which all other points in body move When the axis of rotation is fixed, such as in a door hinge, all points of the door have equal arcs of rotation around the center of the hinge In joints, the axis is not usually fixed due to accessory motion As a result, the location of exact center of rotation changes as the
joint angle changes; The instantaneous center of rotation is the center of rotation at a specific instant in time during movement. Laws of Motion and Physical Activity Terminology Quantity measurements for motion - Scalars versus vectors Scalar quantities - Described by a magnitude (or numerical value) alone, such as speed as in miles per hour or meters per second Other quantities are length, area, volume, mass, time, density,
temperature, pressure, energy, work, and power Vector quantities - Described by both a magnitude and direction such as velocity as in miles per hour in an eastward direction Other quantities are acceleration, direction, displacement, force, drag, momentum, lift, weight, and thrust Laws of Motion and Physical Activity Terminology Displacement - Change in the position or location of an object from its original point of reference. A displacement is a vector that is the shortest distance from the initial to the final position
Distance - Actual sum length of measurement traveled The difference between distance and displacement is that the distance is the actual physical length between two points while displacement is the length of the shortest route between these two points Laws of Motion and Physical Activity Terminology Angular displacement (Change in location of a rotating body), or the angle in radians (degrees, revolutions) through which a point or line has been rotated in a specified sense about a specified axis Linear displacement Movement in a straight line through
a certain distance and direction Speed - How fast an object is moving or distance that an object moves in a specific amount of time Velocity - Includes the direction and describes the rate of displacement Laws of Motion and Physical Activity Newtons Laws of Motion Newton's laws of motion describes the relationship between a body, the forces acting upon it, and the motion of the body in response to those forces. These three laws form the basis of classical mechanics and explain the interrelationships between the
concepts of force, mass, and inertia Newton's laws of motion have many applications to physical education activities and sports. Laws of Motion and Physical Activity Newtons Laws Law of Inertia 1. A body in motion tends to remain in motion at the same speed in a straight line unless acted on by a force . A body at rest tends to remain at rest unless acted on by a force. Inertia - Resistance to action or change Tendency for the current state of motion to be maintained, whether the body segment is moving at a particular velocity or is motionless In human movement, inertia refers to resistance to
acceleration or deceleration. Inertia is the reluctance to change status and status may only be changed by an outside force. Laws of Motion and Physical Activity Newtons Laws Law of Inertia Inertia The greater an objects mass, the greater its inertia The greater the mass, the more force needed to significantly change an objects inertia Example: Runner on an indoor track must apply considerable force to overcome moving inertia and stop before hitting the wall. 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 directed activity will be very costly to energy reserves Example - Handball and basketball are so much more fatiguing than jogging and dancing (tackle dodge) Laws of Motion and Physical Activity Newtons Laws Law of Acceleration 2. 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. Acceleration - The 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 a body (The quantity of matter which a body contains, as measured by its acceleration under a given force or by the force exerted on it by a gravitational field) Affects the speed and acceleration in physical movements Laws of Motion and Physical Activity Newtons Laws Law of Acceleration Examples A much greater force is required from the muscles to accelerate a 230-pound man than to accelerate a 130-pound man to the same running speed A baseball maybe accelerated faster than a shot because of the difference in mass The force required to run at half speed is less than the force required to run at top speed
To impart speed to a ball or an object, the body part holding the object must be rapidly accelerated Laws of Motion and Physical Activity Newtons Laws Law of Reaction 3. For every action there is an opposite and equal reaction. Examples 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 and back, while the surface pushes up and forward Force of the surface reacting to the force we place on
it is the ground reaction force. Laws of Motion and Physical Activity Newtons Laws Law of Reaction Force of the surface reacting to the force we place on it is the ground reaction force. We provide the action force while the surface provides the reaction force It is easier to run on a hard track than on a sandy beach owing to the difference in the ground reaction forces of the two surfaces The track resists the runner's propulsion force, and the reaction drives the runner ahead
Laws of Motion and Physical Activity Terminology and Concepts : Friction Friction - Force that results from the resistance between surfaces of two objects from moving on one another Depending on the activity, increased or decreased friction may be desired: To run, we depend on friction forces between our feet and the ground so that we may exert force against the ground and propel ourselves forward When friction is reduced to slick ground or shoe surface, we are more likely to slip In skating, we desire decreased friction so that we may slide across the ice with less resistance
Laws of Motion and Physical Activity Terminology and Concepts : Friction Static friction or kinetic friction Static friction - The amount of friction between two objects that have no movement. Kinetic friction - Friction occurring between two objects that are sliding upon one another. Static friction is always greater than kinetic friction It is always more difficult to
initiate dragging an object across a surface than to continue dragging Static friction may be increased by increasing the normal or perpendicular forces pressing the two objects together, as by adding more weight to one object sitting on the other object Laws of Motion and Physical Activity Terminology and Concepts : Friction To determine the amount of friction forces, consider both forces pressing the two objects together and
the coefficient of friction Coefficient of friction - Ratio between force needed to overcome the friction over the force holding the surfaces together Depends upon the hardness and roughness of the surface textures. Reducing the coefficient of friction promotes ease of movement between the two adjoining surfaces. Laws of Motion and Physical Activity Terminology and Concepts : Friction 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 friction is always much less than static or kinetic friction
Laws of Motion and Physical Activity Terminology and Concepts : Balance, Equilibrium, and Stability Balance - Ability to control equilibrium, either static or dynamic Equilibrium - State of zero acceleration where there is no change in the speed or direction of the body. Equilibrium is both static and dynamic. Static equilibrium - Body is at rest or completely motionless Dynamic equilibrium - All applied and inertial forces acting on the moving body are in balance, resulting in movement with unchanging speed or direction.
Laws of Motion and Physical Activity Terminology and Concepts : Balance, Equilibrium, and Stability To control equilibrium and achieve balance, stability needs to be maximized. Stability - is the resistance to: 1) a change in acceleration and 2) a disturbance in equilibrium. Stability is enhanced by determining the body's center of gravity and appropriately changing it. Center of gravity - Point at which all of the body's mass and weight are equally balanced or equally distributed in all directions
Laws of Motion and Physical Activity Terminology and Concepts : Balance, Equilibrium, and Stability Generally, balance is desired; however, some circumstances exist where movement is improved when the body tends to be unbalanced General factors to enhancing equilibrium, maximizing stability, and achieving balance A person has balance when the center of gravity falls within the base of support. A person has balance in direct proportion to the size of the base. The larger the base of support, the more balance. A person has balance, depending on the weight (mass). The greater the weight, the more balance.
A person has balance, depending on the height of the center of gravity. The lower the center of gravity, the more balance. Laws of Motion and Physical Activity Terminology and Concepts : Balance, Equilibrium, and Stability General factors to enhancing equilibrium, maximizing stability, and achieving balance (continued) A person has balance depending on where the center of gravity is in relation to the base of support. Balance is reduced 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 the side of the base of support expected to receive the force
Rotation about an axis aids balance; a moving bike is easier to balance than a stationary bike 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 Laws of Motion and Physical Activity Terminology and Concepts : Force Skeletal Muscles are the main source of force that produces or changes movement of a body segment, the entire body, objects that are thrown, struck, or stopped. Without forces acting on an object, there would be no motion. Force is calculated as a product of mass times acceleration (MA). The mass of a body segment or the entire body
multiplied by the speed of acceleration determines the force. Example: In throwing a ball, the force applied to the ball is equal to the mass of the arm times the arm's speed of acceleration. (Summation of forces) Laws of Motion and Physical Activity Terminology and Concepts : Summation of Forces and Momentum Many activities 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 joints. (Baseball Pitcher throwing a fastball) Momentum (quantity of motion) - Equal to mass
times velocity. The greater the momentum, the greater the resistance to change in the inertia or state of motion. A larger person with greater mass moving at the same velocity as a smaller person will have more momentum A person with less mass moving at a higher velocity may have more momentum than a person with greater mass moving at a lower velocity Laws of Motion and Physical Activity Terminology and Concepts : Mechanical Loading Significant mechanical loads are generated and absorbed by the tissues of the body. Both internal or
external forces may present mechanical loading. Only muscles can actively generate internal force, but tension in tendons, connective tissues, ligaments, and joint capsules may passively generate internal forces External forces are produced from outside the body and originate from gravity, inertia, or direct contact All tissues, in varying degrees, resist change in shape but tissue deformation may result from external forces or internally generated forces. Laws of Motion and Physical Activity Terminology and Concepts : Mechanical Loading To prevent injury or damage from tissue deformation, the body must be used to absorb energy from both internal
and external forces. It is advantageous to absorb force over larger aspects of our body (larger surface area) and to spread the absorption rate over a greater period of time Stronger and healthier tissues are more likely to withstand excessive mechanical loading and the resultant excessive tissue deformation Laws of Motion and Physical Activity Terminology and Concepts : Mechanical Loading Excessive tissue deformation due to mechanical loading may result from: Tension (stretching or strain) Compression Shear
Bending Torsion (twisting) Laws of Motion and Physical Activity Functional Application : Laws of Motion and Mechanics In the performance of various sport skills, such as throwing, many applications of the laws of leverage, motion, and balance may be found: In throwing, the angular motion of the levers (bones) of the body (trunk, shoulder, elbow, and wrist) is used to give linear motion to the ball when it is released In throwing, the individual's inertia and the ball's inertia must be overcome by the application of force (Law of inertia). Muscles of the body provide the force to move the body parts and the ball
Law of acceleration is in effect with the muscular force necessary to accelerate the arm, wrist, and hand Laws of Motion and Physical Activity Functional Application : Laws of Motion and Mechanics The greater the force (mass times acceleration) that a person can produce, the faster the arm will move, and thus the greater the speed that will be imparted to the ball The longer the lever, the greater the speed that can be imparted to the ball. The body from the feet to the fingers can be considered as one long lever. The longer the lever, from natural body length or the body movements to the extended backward position, the greater will be the arc through which it accelerates and thus the greater the speed imparted to the thrown object
The reaction of the feet against the surface on which the subject stands is illustrative of the law of reaction Laws of Motion and Physical Activity Functional Application : Laws of Motion and Mechanics Balance, or equilibrium, is a factor in throwing when the body is rotated posteriorly in the beginning of the throw The body is moved nearly out of balance to the rear Balance changes again with the forward movement Balance is reestablished with the follow-through when the feet are spread and the knees and trunk are flexed to lower the center of gravity. End of PPT Series 3B
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