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Resistance in Mechanical Systems

This article discusses Newton's second law of motion, calculating weight and mass, friction forces, a linear model for friction forces, lubricants, and rolling friction in mechanical systems.

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Resistance in Mechanical Systems

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  1. Resistance in Mechanical Systems 4.1

  2. Newton’s Second Law of Motion • The acceleration of an object is directly proportional to the net force acting on the object and inversely proportional to the mass of the object. • F = ma (Force = mass X acceleration) • SI—Newton—N (1 kg∙m/s2) • English—Pound—lb (1slug∙ft/s2)

  3. Calculating Weight and Mass • Fg—gravitational force or weight • When gravity is the only force acting on an object, it accelerates in the direction of gravity—gravitational acceleration (g). On earth’s surface, the direction of g is the same as Fg—9.80 m/s2 or 32.2 ft/s2 toward the center of the earth. • to calculate weight: F = maFg = mg

  4. Friction Forces • Friction is a result of irregularities in the surfaces of objects. • To move an object, the force you apply must be greater than the opposing force of friction. • Static Friction—the force required to overcome the initial electrical force of attraction between atoms of the two surfaces • Kinetic Friction —force to overcome this attraction between the moving surfaces to maintain constant speed. • Static friction is usually greater than kinetic friction.

  5. A Linear Model for Friction Forces • Friction opposes motion (opposite forces). • Friction force depends on whether it is static or kinetic. • Friction force depends on the surface material. • Friction force depends on how hard the surfaces are pressed together—normal force. • The magnitude of the friction force is proportional to the normal force (N). The constant of proportionality is called the coefficient of friction (μ). • Fstatic ≤ μsN • Fkinetic ≤ μkN • (Maximum friction force = coefficient of friction X normal force)

  6. Lubricants • Reduce friction by keeping the two sliding surfaces apart with a thin layer of fluid • The internal friction in the fluid is called viscosity.

  7. Rolling Friction • Rolling, rather than sliding reduces friction because there is less movement between surfaces. • Bearings are mechanical devices used to reduce friction in rotating systems. Lubricating them reduces friction even more. • Friction is needed in some mechanical systems—brakes, friction clutch, conveyer belt systems

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