Resistance in Mechanical Systems

1
Resistance in Mechanical Systems

• 4.1

2
Newtons 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)
• SINewtonN (1 kgm/s2)
• EnglishPoundlb (1slugft/s2)

3
Calculating Weight and Mass

• Fggravitational force or weight
• When gravity is the only force acting on an
  object, it accelerates in the direction of
  gravitygravitational acceleration (g). On
  earths surface, the direction of g is the same
  as Fg9.80 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 Frictionthe 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 togethernormal 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
  systemsbrakes, friction clutch, conveyer belt
  systems