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INTRODUCTION

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After completing this chapter, you will be able to:

  • Discuss the interrelationships among kinematic variables.

  • Correctly associate linear kinematic quantities with their units of measure.

  • Identify and describe the effects of factors governing projectile trajectory.

  • Explain why the horizontal and vertical components of projectile motion are analyzed separately.

  • Distinguish between average and instantaneous quantities and identify the circumstances under which each is a quantity of interest.

  • Select and use appropriate equations to solve problems related to linear kinematics.

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Why is a sprinter's acceleration close to zero in the middle of a race? How does the size of a dancer's foot affect the performance time that a choreographer must allocate for jumps? At what angle should a discus or a javelin be thrown to achieve maximum distance? Why does a ball thrown horizontally hit the ground at the same time as a ball dropped from the same height? These questions all relate to the kinematic characteristics of a pure form of movement: linear motion. This chapter introduces the study of human movement mechanics with a discussion of linear kinematic quantities and projectile motion.

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LINEAR KINEMATIC QUANTITIES

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Kinematics is the geometry, pattern, or form of motion with respect to time. Kinematics, which describes the appearance of motion, is distinguished from kinetics, the forces associated with motion. Linear kinematics involves the shape, form, pattern, and sequencing of linear movement through time, without particular reference to the forces that cause or result from the motion.

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kinematics

the form, pattern, or sequencing of movement with respect to time

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Careful kinematic analyses of performance are invaluable for clinicians, physical activity teachers, and coaches. When people learn a new motor skill, a progressive modification of movement kinematics reflects the learning process. This is particularly true for young children, whose movement kinematics changes with the normal changes in anthropometry and neuromuscular coordination that accompany growth. Likewise, when a patient rehabilitates an injured joint, the therapist or clinician looks for the gradual return of normal joint kinematics.

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Kinematics spans both qualitative and quantitative forms of analysis. For example, qualitatively describing the kinematics of a soccer kick entails identifying the major joint actions, including hip flexion, knee extension, and possibly plantar flexion at the ankle. A more detailed qualitative kinematic analysis might also describe the precise sequencing and timing of body segment movements, which translates to the degree of skill evident on the part of the kicker. Although most assessments of human movement are carried out qualitatively through visual observation, quantitative analysis is also sometimes appropriate. Physical therapists, for example, often measure the range of motion of an injured joint to help determine the extent to which range of motion exercises may be needed. When a coach measures an athlete's performance in the shot put or long jump, this too is a quantitative assessment.

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Movement kinematics is also referred to as...

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