Spiros Prassas California State University Κινητικές αρχές και η εφαρμογή τους στην Γυμναστική

Spiros Prassas California State University

Spiros Prassas California State University Important performance variables from a biomechanical perspective …ability to gain height …ability to rotate …ability to swing …ability to land

Spiros Prassas California State University Force …is the quantity that produces or tend to produce a change in the state of motion of an object or body, i.e.…produces acceleration

Spiros Prassas California State University …force The relationship between force an motion is addressed by Newton’s laws of motion:

Spiros Prassas California State University First Law—Law of Inertia Every body persists in its state of rest or uniform motion in a straight line, unless it is compelled to change its state by external forces, i.e  An (external) force is required to stop, start, or alter motion, i.e, an (external) force is required to change the velocity of of an object, but not to maintain it

Spiros Prassas California State University Second Law—Law of acceleration The acceleration of a body is proportional to the force causing it, it is in the direction of that force, and it is inversely proportional to its mass:

Spiros Prassas California State University Third Law—Law of action/reaction To every action there is always an equal and opposite reaction.  This law implies that: Forces occur in pairs, and Action/reaction pairs never act on the same object… Although the magnitude of the action/reaction forces is the same, their effect on the respective objects are not… why?

Spiros Prassas California State University Friction

Spiros Prassas California State University …friction Frictional forces arise between objects in contact. They are parallel to the contact surface and always oppose, or tend to oppose the relative motion of the objects involved

Spiros Prassas California State University …friction Frictional forces are equal to: Where:  is the coefficient of friction, and N is the perpendicular (Normal) force between the two objects

Spiros Prassas California State University …friction …therefore, frictional forces can be altered by altering either  : how?  Different floors  Different shoes  Different tires  Different lubricants Or N: how?  Alter weight (mass)  Alter position,

Spiros Prassas California State University Static friction Arises between surfaces at rest in relation to each other… …is variable in magnitude…

Spiros Prassas California State University Kinetic friction Arises between surfaces in relative motion…

Spiros Prassas California State University

Spiros Prassas California State University Torque

Spiros Prassas California State University …torque Torque represents the rotational effect of force When muscles contract, the force produced is applied to bones, which rotate about a joint The magnitude of the torque produced by a force depends on magnitude of the force, the direction of the force,and the distance from the point of (force) application to the axis of rotation

Spiros Prassas California State University  d F  = F d (1)  =F d (2)

Spiros Prassas California State University Torques that produce or tend to produce counterclockwise (CCW) rotations are positive Torques that produce or tend to produce clockwise (CW) rotations are negative Positive (CCW) or negative (CW) torques should not be associated with a particular joint movement (flexion, extension, etc.)  

Spiros Prassas California State University

Spiros Prassas California State University

Spiros Prassas California State University …torque The rotational equivalent to F=ma (Newton’s 2 nd Law) is:

Spiros Prassas California State University

Spiros Prassas California State University

Spiros Prassas California State University

Spiros Prassas California State University

Spiros Prassas California State University Work Energy Power Momentum

Spiros Prassas California State University Work Mechanical work is defined as : d F

Spiros Prassas California State University Work …for rotary motion, mechanical work is defined as :

Spiros Prassas California State University …Mechanical Energy… …is represented by the ability of objects to do Work because of…  Their motion (kinetic…)  Position (gravitational potential…)  Configuration (elastic…)

Spiros Prassas California State University

Spiros Prassas California State University Work-Energy relationship Under special circumstances, the sum of the kinetic and (gravitational) potential energy of a system is constant, i.e. it is “conserved”. …if PE is negligible, the Work-Energy relationship is expressed as follows: Practical Implications

Spiros Prassas California State University Work-Energy relationship …practical applications

Spiros Prassas California State University W= F d F d The amount of work that the H 2 O does on the diver is set… By diving deep into the pool, the force doing the work is small…

Spiros Prassas California State University d F Again, the amount of work that the H 2 O is doing on the diver is set—the same as in the previous dive. If, however, the diver belly flaps (or back flaps—as he/she did) into the pool, he/she pays the price… W= F d

Spiros Prassas California State University

Spiros Prassas California State University FB HB

Spiros Prassas California State University Linear Momentum

Spiros Prassas California State University …linear momentum… …is the “quantity of (linear) motion” possessed by an object/body …is proportional to the product of the mass and the velocity possessed by an object/body…

Spiros Prassas California State University …in the absence of external forces, the linear momentum of a system is constant… (equation)

Spiros Prassas California State University Impulse/impulse-momentum relationship The product of Force and time—left side of the equation above—is known as Impulse (J) and equation (1) describes the Impulse-momentum relationship (1) Since…

Spiros Prassas California State University J= F t F t The Impulse that the H 2 O is doing on the diver is set… By diving deep into the pool, the force of the Impulse will be small…

Spiros Prassas California State University t F Again, the Impulse that the H 2 O is doing on the diver is set—the same as in the previous dive. If, however, the diver belly flaps (or back flaps!) into the pool, he/she pays the price… J= F t

Spiros Prassas California State University ForceForce Negative Impulse Positive Impulse T i m e Slowing down * Speeding up * Speed changes if there is a difference between the positive and negative impulses—in the illustrated case, the subject will “speed-up”* (why?) *in this example, forward is the positive direction Locomotion

Spiros Prassas California State University

Spiros Prassas California State University

Spiros Prassas California State University …angular momentum… …is the “quantity of (angular) motion” possessed by an object/body …is proportional to the product of the moment of inertia and the angular velocity possessed by an object/body…

Spiros Prassas California State University Conservation of angular momentum …Angular momentum is constant, i.e. it is “conserved” in the absence of external torques…

Spiros Prassas California State University Angular impulse/angular momentum relationship …therefore it changes only when external torques act for a time period Practical applications

Spiros Prassas California State University …angular momentum… …The total angular momentum of a multi- segment system is made up of the sum of the angular momenta of its parts, i.e. Practical implications

Spiros Prassas California State University …”transfer” of angular momentum… The conservation of L principle, plus the fact that total L is made up of the sum of the angular momenta of its parts, is utilized in order to “transfer” momentum…  Among the parts, and  Among different axes of rotation Practical implications

Spiros Prassas California State University

Spiros Prassas California State University From Kreighbaum, E…(modified)

Spiros Prassas California State University

Spiros Prassas California State University …and so… what?

Spiros Prassas California State University

Spiros Prassas California State University

Spiros Prassas California State University

Spiros Prassas California State University

Spiros Prassas California State University

Spiros Prassas California State University

Spiros Prassas California State University

Spiros Prassas California State University

Spiros Prassas California State University

Spiros Prassas California State University

Spiros Prassas California State University