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Newton’s Laws

Newton’s Laws. Developed by Sir Isaac Newton (1642 – 1727) Define the mechanical basis of linear kinematics. Newton’s First Law The Law of Inertia. “A body will maintain a state of rest or constant velocity unless acted on by an external force that changes that state.”.

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Newton’s Laws

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  1. Newton’s Laws Developed by Sir Isaac Newton (1642 – 1727) Define the mechanical basis of linear kinematics

  2. Newton’s First Law The Law of Inertia “A body will maintain a state of rest or constant velocity unless acted on by an external force that changes that state.”

  3. Newton’s First Law The Law of Inertia • EXAMPLES: • If an object is sitting still it won’t move unless a force is applied to it. • A moving object won’t stop on its own without some external force acting on it. This external force may be friction or air drag.

  4. Newton’s Second LawThe Law of Acceleration “A force applied to a body causes an acceleration of that body of a magnitude proportional to the force, in the direction of the force, and inversely proportional to the body’s mass.”

  5. Newton’s Second LawThe Law of Acceleration • EXAMPLE: • A 1 kg ball is kicked with a force of 10 N. The resulting acceleration uses the basic formula for force: F = ma therefore, a = F/m = 10 N / 1 kg = 10 m/s2 That is, the ball accelerates at a rate of 10 m/s2 in the direction in which it was kicked.

  6. Newton’s Third LawThe Law of Reaction “For every action there is an equal and opposite reaction.” OR “When one body exerts a force an another, the second body exerts a reaction force on the first that is equal in magnitude and opposite in direction.”

  7. Newton’s Third LawThe Law of Reaction • EXAMPLES: • While standing or sitting on a swiveling chair, if you rotate your upper body in one direction, the chair and your lower body tend to rotate in the opposite direction. • If you push against a rigid surface (like a wall) with a force of 50 N, it pushes back with a reaction force of 50 N.

  8. Ground Reaction Forces (GRF) • See text pp. 385-387 • The reaction force produced by individuals when walking, running, etc. • As each part of the foot contacts the ground it produces a reaction force.

  9. Ground Reaction Forces (GRF) • In runners, the magnitude of the vertical component of GRF is generally two to three times the runners body weight. • Runners with a longer stride tend to generate a GRF with a larger horizontal component which can retard forward motion and increase shock to the knees. (This assumes that the increase in stride length isn’t accompanied by an increase in horizontal velocity.)

  10. FH FH R FV R FV As stride length increases, the horizontal component (FH) of the Ground Reaction Force (R) will tend to increase, retarding forward motion.

  11. Center of Gravity Center of Gravity FH FH R dH FV R dH FV This increase is actually more closely related to the horizontal distance (dH in the illustration) from the position of foot plant to the center of gravity.

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