Chapter 12

At the conclusion of this chapter, the student should be able to:

• 1. Name, define, and use the following terms properly as they apply to linear motion: force, inertia, mass, weight, momentum, and impulse.
• 2. Explain what is meant by the terms magnitude, direction, and point of application of force, and use these terms properly as they apply to internal and external forces.
• 3. Explain the effect of specified changes in the magnitude, direction, and point of application of force on the motion state of a body.
• 4. Define and give examples of the terms linear forces, concurrent forces, and parallel forces.
• 5. Determine the magnitude, direction, and point of application of muscle forces in hypothetical situations in which specific muscles are considered in isolation.
• 6. State Newton’s laws as they apply to linear motion.
• 7. Explain the cause and effect relationship between the forces responsible for linear motion and the objects experiencing the motion.
• 8. Name and define the basic external forces responsible for modifying motion: weight, normal reaction, friction, elasticity, buoyancy, drag, and lift.
• 9. Draw and analyze simple two-dimensional free-body diagrams in which all applicable external forces are properly accounted for.
• 10. Explain the work–energy relationship as it applies to a body experiencing linear motion.
• 11. Define and use properly the terms work, power, kinetic energy, and potential energy.
• 12. Using the concepts that govern linear motion, perform a mechanical analysis of a selected motor skill.

Objects start moving when they are pushed or pulled, that is, when some type of force acts on them. Forces produce motion, stop motion, and prevent motion. They may increase speed, decrease speed, or cause objects to change direction. They may push or pull to cause motion or produce a net effect so that bodies remain stationary. Force is defined as that which pushes or pulls through direct mechanical contact or through the force of gravity to alter the motion of an object. It is the effect that one body has on another. The identification of forces that act to produce motion of the body or of an object is an important element of a kinesiological analysis.

The action of a force may be internal or external. Internal forces are defined as forces exerted by bodies on other bodies within a defined system, whereas external forces are those exerted by bodies within an arbitrarily specified system on bodies outside the specified system. Internal forces cause differences in body shape, and external forces cause displacement of the body. In kinesiology, the system under consideration is the human body. Internal forces are therefore usually classified as muscle forces that act on the various structures of the body, and external forces are those outside the body. The best-known external force is weight, or gravitational force. Wind or water resistance forces, friction, or forces due to other objects acting on the body are also external forces.

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