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Transcript
BioLab - Biomechanics Teaching & Learning Tool Box Newton’s Laws of Motion Linear Kinetics Newton’s First Law of Motion • Law of Inertia Every body will remain in a state of rest or constant motion (velocity) in a straight line unless acted on by an external force that changes that state • A body cannot be made to change its speed or direction unless acted upon by a force(s) • Difficult to prove on earth due to the presence of friction and air resistance BioLab - Biomechanics Teaching & Learning Tool Box Newton’s Second Law of Motion • Law of Acceleration A force (F) applied to a body causes an acceleration (a) of that body which has a magnitude proportional to the force, and takes place in the direction in which the force acts • Vitally important in sport as it forms the link between force and motion: • Force = mass • F=m acceleration a BioLab - Biomechanics Teaching & Learning Tool Box Applications of Newton’s 2nd Law • Assuming mass remains constant, the greater the force the greater the acceleration • What is the acceleration of the ball (ball mass = 1.5 kg)? F = 500 N F=m a=? a F 500 = ∴a = m 1.5 a = 333 m·s-2 BioLab - Biomechanics Teaching & Learning Tool Box Newton’s Third Law of Motion • Law of Reaction For every force that is exerted by one body on a second body there is an equal (magnitude) and opposite (direction) simultaneous force exerted by the second body on the first • Therefore every force which is applied by a body is accompanied by a reaction force on that body • Difficult to visualise but can be felt: – e.g. In boxing the force applied by a punch is experienced by the opponent’s chin and the puncher’s hand BioLab - Biomechanics Teaching & Learning Tool Box Examples of Newton’s 3rd Law • Ground Reaction Force (GRF) is a special type of force explained by Newton’s 3rd Law of Motion • Equal in magnitude and opposite in direction to the force applied to the ground by the body • Needs to be considered separately in horizontal (friction) and vertical (normal) directions BioLab - Biomechanics Teaching & Learning Tool Box Summary • Newton’s Law of Gravitation – Attractive forces exist between bodies (e.g. a body and the Earth) that are proportional to the product of their masses and inversely proportional to the distance between them • Newton’s First Law (Inertia) – A force is required to accelerate (i.e. change the velocity of) a body • Newton’s Second Law (Acceleration) – The acceleration of a body is proportional to the sum of the forces acting on it • Newton’s Third Law (Reaction) – Any body that applies a force to another body experiences a simultaneous reaction force that is equal in magnitude and opposite in direction to the applied force BioLab - Biomechanics Teaching & Learning Tool Box