Resistance in Mechanical Systems

4.1

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)

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

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.

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)

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.

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