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How Do I Move? Chapter 7 The Science of Biomechanics Biomechanics Examines the internal and external forces acting on the human body and the effects produced by these forces Aids in technique analysis and the development of innovative equipment designs Draws on knowledge from sports medicine, physical therapy, kinesiology, and biomechanical engineering Sport Books Publisher 2 Centre of Mass Centre of mass outside of body Located at the balance point of a body; a point found in or about a body where the mass could be concentrated Generally, 15 cm above the crotch, or approximately 55% of standing height in females and 57% in males Sport Books Publisher 3 1 The Laws of Motion Newton’s Three Laws of Motion: Describe the relationship between all the external forces acting on the human body at any time and the resulting motion of the total body Sir Isaac Newton developed these laws to explain why things move the way they do Sport Books Publisher 4 Newton’s First Law: Inertia An object will not change its state of motion (it will continue to be at rest or moving with constant velocity), unless acted upon by a net , external force For example: because of their large mass, football linemen are difficult to move out of the way Sport Books Publisher 5 Newton’s Second Law: Acceleration For linear movements, the acceleration (a) a body experiences is proportional to the force (F) causing it, and takes place in the same direction as the force F = m.a where m is the mass of the body For angular movements, the angular acceleration of a body is proportional to the moment of force causing it, and takes place in the same direction as the moment of force Sport Books Publisher 6 2 Impulse, Impact, and Momentum Momentum is the amount of motion a body possesses Impulse is the application of an external force over a short time period Impact is the application of an external force Momentum is created by an impulse and is lost through impact Impulse and impact are both associated with bodies that are changing their state of motion by experiencing large accelerations over relative short time periods Collision or impact skills can sometimes manipulate the time of contact and reduce the magnitude of the external force To increase impulse, a sprinter must increase the net external force per step Sport Books Publisher 7 Newton’s Third Law: Action-Reaction Every action has an equal and opposite reaction The two acting forces are equal in magnitude, but opposite in direction Example: -the sprinter exerts a force onto the blocks, and simultaneously the blocks exert an equal force back onto the sprinter Sport Books Publisher 8 Maximizing Height and Range To maximize vertical distance (height), one must maximize the takeoff velocity and take off vertically To maximize horizontal distance (range), one must maximize the takeoff velocity and take off at an angle of 45 degrees to the horizontal Sport Books Publisher 9 3 Taking off and Landing at Different Heights If the human body is the projectile, a higher centre of mass can be achieved by elongating the body at takeoff and by raising as many body parts as possible The range a projectile will travel increases if the takeoff height is greater than the landing height, and decrease if the takeoff height is less than the landing height Sport Books Publisher 10 Air Resistance Another force acting on a projectile Will change the state of motion of the projectile and its path Sport Books Publisher 11 Effects of air resistance on a discus Discus with direction of travel and relative flow velocity vectors superimposed Free body diagram of a discus indicating lift, drag, and gravity vectors Angle of attack (theta) defined for a discus with direction of travel and longitudinal axis drawn. Sport Books Publisher 12 4 Equilibrium The state of a system that is not experiencing any change in its direction or speed Static equilibrium System is at rest Dynamic equilibrium System is moving with constant velocity Sport Books Publisher 13 Balance Process whereby the body’s state of equilibrium is controlled for a given purpose The base of support and location of the centre of mass is manipulated If the line of gravity passes through some part of the body’s base of support, the body will be balanced Sport Books Publisher 14 Stability A measure of the difficulty with which equilibrium can be disturbed A net external force is required to overcome the static equilibrium of a sprint start There is a trade-off between maximizing stability and acquiring speed off a mark Sport Books Publisher 15 5 To increase stability in static equilibrium Increase the base of support Increase the inertia of the body Decrease the vertical distance between the centre of mass and the base of support Increase the distance between the point where a vertical line from the centre of mass intersects the base of support Sport Books Publisher 16 Qualitative Analysis of Human Motion The study of human motion, in the absence of measuring, by observing a movement and applying biomechanical principles in assessing performance It is subjective and yet systematic evaluation of a movement or skill It is based on a direct visual observation of a movement or its video recording It is dependent on the constraints or limitations of an event or performer Sport Books Publisher 17 6