Machinery's Handbook, 31st Edition
156
Mechanics and Strength of Materials MECHANICS
Throughout this section of the Handbook, both English and metric SI data and for mulas are given to cover the requirements of working in either system of measure ment. Except for the passage entitled Use of Metric SI System in Mechanics Calcula- tions , formulas and text relating exclusively to SI are given in bold face type. Terms and Definitions Definitions.— The science of mechanics is concerned with the effects of forces in causing or preventing motion. Statics is the branch of mechanics that deals with bodies in equilib- rium, i.e., the forces acting on bodies cause them to remain at rest or to move with uniform velocity. Dynamics deals with bodies not in equilibrium, i.e., the forces acting on bodies cause them to move with non-uniform velocity. Kinetics is the branch of dynamics that concerns both the forces acting on bodies and the motions that they cause. Kinematics is the branch of dynamics that deals only with the motions of bodies without reference to the forces that cause them. Definitions of certain terms and quantities as used in mechanics follow: Force may be defined simply as a push or a pull, which may result from the force of con- tact between bodies or from a force, such as magnetism or gravitation, in which no direct contact takes place. Matter is any substance that occupies space; gases, liquids, solids, electrons, atoms, molecules, etc., are all classified as matter. Inertia is that property by which matter tends to resist any change in its motion or state of rest. Mass is a measure of the inertia of a body. Work, in mechanics, is the product of force and distance and is expressed by a combi nation of units of force and distance, expressed as foot-pound, inch-pound, meter-kilo- gram, etc. The metric SI unit of work is the joule (J), which is the work done when the point of application of a force of one newton is displaced through a distance of one meter in the direction of the force. Power, in mechanics, is the rate of change of work per unit time; it is the product of force and distance divided by time; it measures the amount of work performed in a given time period. It is the rate of doing work and is expressed in foot-pound per minute, foot-pound per second, kilogram-meter per second, etc. The metric SI unit is the watt (W), which is equivalent to one joule per second. Horsepower is the unit of power that has been adopted for engineering work. One horse power (hp) is equal to 33,000 foot-pounds per minute or 550 foot-pounds per second. The kilowatt (kW), used in electrical work, equals 1.34 horsepower; or 1 horsepower equals 0.746 kilowatt. However, in the metric SI, the term “horsepower” is not used, and the basic unit of power is the watt. This unit, and the derived units milliwatt and kilo watt, for example, are the same as those used in electrical work. Torque or moment of a force is a measure of the tendency of a force to rotate a body upon which it acts about an axis. The magnitude of the moment due to a force acting in a plane perpendicular to some axis is obtained by multiplying the force by the perpendicular distance from the axis to the line of action of the force. (If the axis of rotation is not perpen dicular to the plane of the force, then the components of the force in a plane perpendicular to the axis of rotation are used to find the resultant moment of the force by finding the moment of each component and adding the component moments algebraically.) Moment or torque is commonly expressed in pound-feet, pound-inches, kilogram-meters, etc. The metric SI unit is the newton-meter (N·m). Velocity is the rate of change of distance; it is expressed as distance divided by time, that is, feet per second, miles per hour, centimeters per second, meters per second, etc.
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