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Linear Kinetics Reaction Forces Chapter 7 KINE 3301 Biomechanics of Human Movement

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Definitions Kinetics: the study of forces and the motion they cause. Force: a force is a push or pull. Forces can be divided into: – Contact forces – Non-Contact force such a gravity. Mass is a scalar with units of kg. Force is a vector ( direction & magnitude ). The units of force are Newton’s (N). Inertia is the tendency of an object to remain at rest or in motion at a constant speed in a straight line.

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Definition of a Newton A Newton is the amount of force necessary to accelerate a 1 kg mass with an acceleration of 1 m/s 2. m = 1 kg a = 1 m/s 2 1 N force F = m a N = (kg) ( m/s 2 ) When you multiply mass in kg by acceleration in m/s 2 the result is N.

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I – Law of Inertia I – Law of Inertia: a body at rest stays at rest, a body in motion stays in motion along a straight line unless acted upon by a net force. This law is also called the Law of Motion as it refers to constant velocity and zero velocity. Net Force The net force acting on the box to the right is zero.

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II – Law of Acceleration

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III – Law of Reaction III – Law of Reaction: for every action there is an opposite and equal reaction. The force applied accelerates the ground in the direction of the force. The reaction force accelerates the performers center of mass in the direction of the reaction force.

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A free-body diagram (FBD) is a picture of all of the external forces acting on a performer or object. Begin by drawing the force of gravity (mg) at the center of mass of the object, the add x, y forces at any points of contact.

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Σ F = ma Σ Fx = ma x Σ Fy = ma y Σ Fz = ma z Force & Acceleration are vectors. Reaction Forces in Running

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A 60 kg runner experiences a horizontal acceleration ax of −4.8 m/s 2 and a vertical acceleration (ay) of 16 m/s 2. Complete the FBD, solve for the horizontal (Rx) and vertical (Ry) reaction forces that caused these accelerations.

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A 70 kg diver experiences a horizontal acceleration a x of 5.1 m/s 2 and a vertical acceleration (a y ) of 15 m/s 2. Complete the FBD, solve for the horizontal (Rx) and vertical (Ry) reaction forces that caused these accelerations.

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An 80 kg basketball player experiences a horizontal reaction force (Rx) of −300 N and a vertical reaction force (Ry) of 1700 N. Complete the FBD, solve for the horizontal (a x ) and vertical (a y ) acceleration caused these forces.

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A 75 kg runner experiences a horizontal reaction force (Rx) of 420 N and a vertical reaction force (Ry) of 1980 N. Complete the FBD, solve for the horizontal (a x ) and vertical (a y ) acceleration caused these forces.

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The 76 kg triple jumper shown below experiences a horizontal acceleration (a x ) of −11 m/s 2 and a vertical acceleration (a y ) of 18 m/s 2, complete the FBD and compute the horizontal (Rx) and vertical (Ry) reaction forces that caused these accelerations.

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The 54 kg gymnastic vaulter shown below experiences a horizontal acceleration (a x ) of −14 m/s 2 and a vertical acceleration (a y ) of 20 m/s 2, complete the FBD and compute the horizontal (Rx) and vertical (Ry) reaction forces that caused these accelerations.

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Draw a free-body diagram of a pole vaulter after her feet have left the ground, both the right and left hands are in contact with the pole.

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LHx = −180 N, LHy = −20 N, RHx = 30 N, RHy = 840 N, m = 60 kg Solve for a x, a y of the center of mass. The forces at each hand were assumed to be positive.

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