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