Agricultural Science 1B Principles & Processes in Agriculture Mike Wheatland (m.wheatland@physics.usyd.edu.au)
Outline - Lectures weeks 9-12 Chapter 6: Balance in nature - description of energy balance in the atmosphere Chapter 7: Properties of water - surface tension, pressure, specific heat capacity Chapter 8: Materials: Elasticity & Viscosity - stress and strain, rheology Chapter 9: Farm machinery: Friction & Lubrication - friction Chapter 10: Farm machinery: Stability - Newton s laws, torque Chapter 11: Farm machinery: Vibrations - oscillations, resonance
Chapter9:9:Farm Materials: Elasticity & Chapter machinery: Viscosity Friction & Lubrication
Friction Sliding a heavy object is harder than rolling it along - a frictional force opposes the motion - the force is larger in the sliding case Sliding friction depends on the surfaces involved - e.g. it is small in skiing, skating
Roles of friction A nuisance in many contexts - energy is lost overcoming friction in engines - friction wears engine parts out - lubrication reduces friction in this context In other contexts it is useful/essential - walking, rolling depend on traction (friction) - braking depends on friction - a knot holding depends on friction
Empirical laws of sliding friction Formulated by Leonardo da Vinci using apparatus shown - the weight is increased until the block just begins to slip - this weight equals the maximum frictional force F max block pulley weight
Trying blocks with different areas leads to the law: 1. F max is independent of the area of contact Trying heavier blocks leads to the law: 2. F max is proportional to the weight of the block - the weight W of the block is balanced by an upwards force N due to the table (the normal reaction force) so W = N and hence: 2. F max = µ s N, where µ s is the coefficient of friction
Direction of friction
Static versus kinetic friction If you apply a force F to a block on a table and increase the force with time - the block is stationary until F = F max = µ s N, when it starts to slide - the frictional force then decreases: a smaller force than F max is needed to sustain motion - the force is still proportional to N In the static situation the frictional force is f s µ s N and in the moving (kinetic) case f k = µ k N - µ s and µ k are coefficients of static and kinetic friction - µ k µ s
From University Physics, Young & Freedman
Coefficients of friction From University Physics, Young & Freedman
Stopping distance for a vehicle Assuming deceleration is -F max /m d = u2 2µg for an initial speed u, with g = 9.8 m/s For non-skid braking µ = µ s - if the car skids, µ = µ k and the stopping distance is longer by about 20% For u = 60 km/h 17 m/s without skidding: d 14 m - skidding: d 18 m
http://www.nrma.com.au/pub/nrma/motor/car-research/stop_distance.shtml
Measuring friction on an incline Increase θ until tractor just slips down incline tan" = µ s - so for µ s 1, θ 45 degrees
Origin of friction On microscopic scale, even smooth surfaces are rough - for some surfaces, polishing increases friction - friction must originate in the real area in contact Real area is not related to apparent area (Law 1) Real area is small and increases with load - hence friction is proportional to weight or N (Law 2) Strong adhesion (cold welding) occurs at contact points F max is force required to just shear the points of contact
The frictional laws are phenomenological laws: they are not fundamental - more complicated behaviour may be seen, e.g. µ may depend on N due to surface layers - materials may deform elastically (as well as plastically at points of contact) When objects slide on one another, kinetic energy is converted into heat - ancient method of starting a fire - very high temperatures may be produced locally and for short times
Disc brakes Brake pads press on a rotor: friction halts motion Rotor is cast iron or ceramic; pads are a material designed to provide friction and withstand heat Disc brakes replaced drum brakes: pads pressed on inner surface of a drum - drum expanded due to heat: `brake fade
Lubrication Friction may be reduced by introducing a material, e.g. oil between two surfaces: lubrication Hydrodynamic lubrication: surfaces completely separated by a layer of fluid (oil or grease) - µ 10-3 may be achieved; depends on viscosity - examples: journal bearing, ice skates Journal bearing bearing shaft
Boundary lubrication: a slow journal comes into contact with the bearing - lubricant in localised patched - friction depends on chemical properties and not viscosity
Summary Phenomenological laws of sliding friction Static versus kinetic friction Origin of sliding friction Disc brakes Lubrication