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Newton’s Laws of Motion

Newton’s Laws of Motion. Linear Kinetics. Aim. The aim of these slides is to introduce Newton’s Laws of Motion These slides include an introduction to: Newton’s Law of Gravitation Newton’s 1 st , 2 nd and 3 rd Laws of Motion

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Newton’s Laws of Motion

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  1. Newton’s Laws of Motion Linear Kinetics

  2. Aim • The aim of these slides is to introduce Newton’s Laws of Motion • These slides include an introduction to: • Newton’s Law of Gravitation • Newton’s 1st, 2nd and 3rd Laws of Motion • The relationship between forces applied to bodies and the motion that those bodies experience

  3. Newton’s Law of Gravitation • All bodies are attracted to one another with a force which is proportional to the product of their masses (m), and inversely proportional to the square of the distance (d) between them

  4. Mass Greater mass =greater gravitational force Smaller mass =lower gravitational force Distance Greater distance =smaller gravitational force Smaller distance =greater gravitational force Most bodies in sport have relatively small mass Attractive force between them can be considered negligible Implications of Newton’s Law of Gravitation

  5. Weight • Weight (W) is the attractive force between the earth and any body in contact with it or close to its surface • Product of the mass (m) of the body and the acceleration caused by the attractive force between it and the earth(g = 9.81 m·s-2) i.e. W = m×g • Gravity is based on: • Mass of bodies • Distance between bodies rpoles requator • r = radius of earth • requator > rpoles • gequator< gpoles • Wequator< Wpoles

  6. 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

  7. Examples of Newton’s First Law? Air Resistance Friction & Air Resistance

  8. 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 × acceleration • F = m× a

  9. F = 500 N a = ? F = m× a  a = 333 m·s-2 Applications of Newton’s 2nd Law • Assuming mass remains constant, the greater the force the greater the acceleration • Acceleration is inversely proportional to mass • if force remains the same and mass is halved, then acceleration is doubled • if force remains the same and mass is doubled, then acceleration is halved

  10. 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

  11. 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

  12. Weight (W) vector Vertical GRF (Fz ) vector C Fz (N) A D B E Time (s) Explaining motion using Newton’s Laws - SVJ A B C D E D

  13. Effects of Forces • If only one force acting: F = m× a • If two (or more) forces acting: ∑F = m× a • In SVJ: Fz - W = m× az 

  14. C Fz (N) A D B E Time (s) Effect of Forces Fz < W az = negative Fz = W az = 0 Fz > W az = positive Fz = W az = 0 Fz < W az = g E D B A C

  15. Weight (W) vector Vertical GRF (Fz ) vector C Acceleration (m·s-2) A D B E Time (s) Explaining motion using Newton’s Laws - SVJ A B C D E D

  16. 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

  17. Recommended Reading • Enoka, R.M. (2002). Neuromechanics of Human Movement (3rd edition). Champaign, IL.: Human Kinetics. Pages 57-59 & 64-66. • Grimshaw, P., Lees, A., Fowler, N. & Burden, A. (2006). Sport and Exercise Biomechanics. New York: Taylor & Francis. Pages 69-80 & 97-101. • Hamill, J. & Knutzen, K.M. (2003). Biomechanical Basis of Human Movement (2nd edition). Philadelphia: Lippincott Williams & Wilkins. Pages 341 & 351-356. • McGinnis, P.M. (2005). Biomechanics of Sport and Exercise (2nd edition). Champaign, IL.: Human Kinetics. Pages 77-99.

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