UNPh17

School Science Lessons
17. Friction, lubrication, viscosity, Stokes' law
2012-01-27 SP
Please send comments to: J.Elfick@uq.edu.au

Table of contents
17.0.0 Friction
17.0.0 Friction, rolling friction and sliding friction
17.2.0 Reduce friction, lubrication
17.3.0 Viscosity

17.0.0 Friction, rolling friction and sliding friction
17.0.0 Friction, rolling friction and sliding friction
4.180 Friction experiments, UNESCO
17.5.12 Angle of friction with pencil
17.5.8 Angle of repose
17.5.6 Area dependence of friction
17.5.14 Capstans
17.0.1 Compare sliding friction with rolling friction
17.5.10 Cross friction
17.5.1 Direction of friction
23.10.6 Flint and steel
6.13 Forces of friction (Primary)
See pdf: Flip Over Top, "Tippe Top", gyroscopic forces
17.5.2 Friction blocks
17.5.5 Friction blocks down an incline
17.184 Friction blocks on an inclined plane (GIF)
17.185 Friction blocks up an incline, force to pull body up an incline (GIF)
23.10.5 Friction ignition, bow and stick, fire maker, drill and dowel
17.5.9 Friction roller
17.5.16 Gripping rice
17.5.18 Interleaved telephone books, pycrete
17.5.15 Low friction surfaces
17.5.19 Push a wheelbarrow
17.5.17 Raisin cake
17.2.0 Rolling friction
17.5.13 Sliding chain
17.1.0 Sliding friction
17.5.3 Sliding friction machine
17.5.11 Squeaky chalk
17.5.7 Static friction vs sliding friction
17.5.4 Weight dependence of friction

17.2.0 Reduce friction, lubrication
4.9 Heat from rubbing (Primary)
4.183 Mount a box on wheels
4.186 Reduce friction with air stream
4.185 Reduce friction with ball bearings
17.2.1 Reduce friction with ball bearings, marble bearings
17.2.3 Reduce friction with gases and air streams, hovercraft, dry ice
17.2.2 Reduce friction with liquid
17.2.4 Reduce friction with oil

17.3.0 Viscosity
17.3.0 Viscosity, Stokes' law, fluid friction, falling ball in liquid
17.3.1 Air friction
17.4.7 Ball bearing falling in liquid, Stokes' Law
10.6.3 Distil crude oil and collect the fractions, composition of petroleum
17.4.6.1 Electrorheological fluid (ER fluid), cornflour and vegetable oil
3.2.1 Liquids with different viscosity, hydrogen bonding
17.3.02 Newtonian fluids
17.3.03 Non-Newtonian fluids, dilitancy, thixotropy
6.3.3.14 Poise, derived units
17.3.2 Pulling an aluminium plate
17.4.6 Stir-thickening cornflour mixture
17.4.4 Stir-thinning tomato sauce
17.3.3 Syringe water velocity
17.3.6 Terminal velocity coffee filters
17.3.5 Terminal velocity drop balls and specific gravity
17.3.4 Terminal velocity of different diameters
17.4.5 Time to empty the funnel, viscosity
17.3.8 Uniform pressure drop
17.3.13 Viscosity and temperature
17.3.15 Viscosity bubble race
17.3.10 Viscosity disc
17.3.14 Viscous drag
17.3.11 Viscosity in capillary
13.6.0.2 Viscosity, non-Newtonian fluids
17.4.3 Viscosity of engine oil
17.4.2 Viscosity of honey
17.4.1 Viscosity of thick and thin liquids
17.3.9 Viscosity pipe
6.3.3.14 Viscosity, poise (Table of SI derived units)

17.0.0 Friction
Static and sliding friction, F_f = uF_NSee diagram: 17.183: Friction blocks
The direction of friction is such as to oppose motion. When a force is applied to a body and the body is about to move or is moving the friction is called limiting friction. Limiting friction F = µ R, where µ is the coefficient of friction and R is the normal reaction at right angles to the surfaces.
The frictional force opposes the motion of one surface in contact with another. Walking is difficult on a surface covered by ice, so the lack of friction is an inconvenience. Machines are lubricated to reduce friction to allow the machine to do its work.
1. Friction is a force emerging in the relative motion of two objects in contact. When two surfaces arc in contact and one surface moves relative to the other, they produce a frictional force that opposes the motion.
Two types of friction are possible, depending on the nature of the relative displacement of solid objects in contact:
1.1 Sliding friction occurs when an object is sliding over the surface of another object.
1.2 Rolling friction occurs when an object rolls over the surface of another object.
The sliding friction force is directed along the contact surface of objects against the displacement. For the same solid objects, sliding friction is nearly proportional to the force with which one object is pressed against the other, i.e. to the force of pressure of one object on the other. This force is normal to the contact surface between the objects, f = N. The proportionality factor is called the coefficient of sliding friction, µ = f / N.
2. Between solid surfaces, friction acts parallel to the surfaces and in the opposite direction to the motion, or the attempted motion. Sliding friction depends on the materials in contact and their degree of smoothness, is usually less than static friction, is independent of speed of motion and is not dependent on area of contact. Friction is directly proportional to the normal reaction of the surface. For sliding friction, examples of coefficients of friction are as follows: Tyre on dry road = 0.7, Tyre on wet road = 0.5, Steel on steel = 0.6, Wood on wood = 0.3. Newton's second law of motion state F = ma, where F is the resultant accelerating force but the acceleration will be less than expected due to friction. Friction appears not only when one surface slides over another surface but also when you attempt to cause such a slip by applying a force to an object. If the size of the external the force is less than N, no slip occurs between the objects because the force is balanced by static friction that automatically assumes a value equal to the size of the force. When the force attains a value equal to N, the object starts to slide over the surface of the other object, and static friction becomes sliding friction. Static friction may assume values between zero and N.
3. The coefficient of friction depends on the force n and the velocity of slippage. You can assume that the coefficient is constant.
4. When an object moves in a fluid, a force appears that hampers the motion of the object. The fluid particles exert this force on the moving object. The resistance of the fluid medium differs from the friction between two solid surfaces in that no static friction exists because the weakest possible force can displace an object floating in a liquid.
5. The friction acting on an object in a fluid, and the friction between two solid surfaces, is always directed against the motion of the object and depends on velocity.

17.0.1 Compare sliding friction with rolling friction
Wedge a piece of paper in the wheels of a dynamics cart or a roller skate so that it slides instead of rolls when you pull it. Measure the force needed to make it move and keep moving with constant velocity. Remove the paper and measure the force needed to make it roll and keep rolling with constant velocity.

17.1.0 Sliding friction
See diagram 17.183: Friction blocks
Sliding friction between solids, reducing friction with oil, walking, skid, slippery floor, sewing, wear and tear
1. Wrap a house brick in newspaper. Attach the brick to a spring scale with string. Put the brick on a uniform table surface with the largest area down on the table. Practice pulling the spring scale to move the brick with constant velocity while another person reads and records the spring scale measurement. If you pull the house brick horizontally at constant velocity, the applied force equals the frictional force.
2. Friction and different surfaces
Record the frictional force with different kinds of wrapping on identical house bricks, e.g. newspaper, plastic from a plastic bag, sandpaper.
3. Friction and the area of a surface
Turn the wrapped house brick so that a smaller area is down on the table. Adjust the attached string and measure the frictional force.
4. Compare starting motion friction with maintaining motion friction
Measure the frictional force needed to start the brick moving. Compare the force with A2.
5. Friction and speed of motion
Measure the frictional force necessary to move the brick at different constant velocities.
6. Friction and force pressing the surfaces together
Measure sliding friction with stacks of two to five bricks.
7. Attach a string from a heavy box to a spring balance. Move the box across the table by pulling it with the spring balance. Record the force needed to pull the box at constant slow speed. The force of friction opposes the motion. The friction between the bottom of the box and the table is called sliding friction.
8. Put a sheet of glass on the table. Record the force needed to pull the box at a slow constant speed. If the surface of the glass is smoother than the surface of the table, the sliding friction is less.
9. Use chalk or water or oil to lubricate the surface of the table. Record the force needed to pull the box at a slow constant speed.
10. Bind a thick book with string. Place the book on a smooth tabletop, and, on a rough wood, then attach the string to the hook of the spring balance and pull the spring balance to make the book move with uniform velocity. Compare the reading on the spring balance and note when friction is the smallest.

17.2.0 Rolling friction
See diagram 17.261: Pencil rollers
1. Put round pencils under a box to act as rollers. Record the force needed to pull the box at a slow constant speed. This friction is called rolling friction. Remove the pencils. Record the force needed to pull the box at a slow constant speed as it slides along the table. This friction is called sliding friction. Compare the values of rolling friction and sliding friction
2. Place three round pencils under the book near the front of the book. Pull the spring balance to make the book move with uniform velocity. When the book moves off the most back pencil, put it under the book again from the front of the book. Compare the reading on the spring balance with and without pencils under the book.

17.2.1 Reduce friction with ball bearings, marble bearings
See diagram 17.264: Ball-bearing marbles
1. Find two tin cans such as paint cans that have a deep groove around the top. Lay marbles in one groove and invert the other can over the marbles to form a ball bearing. Place a book on top and note how easily the demonstration bearing turns. Oil the marbles and it will turn still more easily.
2. Mount a box on wheels. Record the force needed to pull the box at a slow constant speed. Has the rolling friction decreased? Put ball bearings or marbles under the box. Record the force needed to pull the box at a slow constant speed. Pour oil on the ball bearings or marbles. Record the force needed to pull the box at a slow constant speed. This may be the least force need to pull the box.
3. Use two same round iron boxes. There is a shallow trough near the rim the each cover, for example, iron cans contain biscuit or milk powder. Remove the cover of a box off and place some steel balls or glass marbles children play in the trough on the cover. Pile the other box on the cover with steel balls or glass marbles. This is a ball bearing. Place a heavy book on the box and make them move together. You may feel how easy it is. If you drop a bit of lubrication oil or cooking oil on the balls, they rotate more easily.

17.2.2 Reduce friction with liquid
1. To learn about wet friction, use a large block of glass on a desk. Use small block of glass, for example, the glass on a small picture frame. Place the small block of glass under the large one. Push the small glass with your fingers to make it move under the large one. Experience the magnitude of the force you exert. Wet the contact surface between two blocks of glass. Make sure the surface completely wet. Then push the small glass again and experience the magnitude of the force you exert.
2. Repeat the experiment but using oil instead of water. Which is easier? If wipe oil on your hand then push the small glass. Note what you feel. Compare the experiences.

17.2.3 Reduce friction with gases and air streams, hovercraft, dry ice
See pdf: Build a hovercraft
To learn the fundamentals of a gas supported boat, hovercraft, use a block of sponge and put it on a large table. Use a small wooden spool and cut it into two halves. Cut off a round card of diameter 10 cm. Dig a hole at the centre of the card. The diameter of the hole is not more than the inner diameter of the spool. Paste a half spool to the card hole to hole. Do not leave any crack at the connecting. If it does not, fill it with paste. Use a short tube just to fit inserting into the spool. Insert one end of the tube into the spool pasted on the card but the tube does not spread out of the card. Cover a balloon on the other end of the tube. Using rubberized fabric to twine the connecting tight to air proof. Blow balloon, from the hole on the card, through the short tube. When the balloon is bouffant, quickly cover the hole on the card then place the whole set on the sponge. As soon as leave your hand off the set, gently spring the card out. It will pass by the tabletop nearly without friction. Use a light, thin, plastic box with a cover. Drill 0.1 cm holes in the bottom of the box. Put some dry ice into the box and cover it well. The dry ice vaporizes at normal temperature then spreads out of the holes on the bottom of the box so that the box floats on the gas. Gently spring the box. Observe the box's moving on a large table. If without fit plastic box, you may make a box with cardboard instead. Both the above experiments show the fundamental of gas supported boat. Now a kind of gas supported boat, both used on land and water, has been applied. Its speed may reach 10 km per hour.

17.2.4 Reduce friction with oil
Lay two panes of glass side by side and place a few drops of oil on one. Feel the difference when you rub a finger back and forth on the unoiled pane and on the oiled pane. Repeat the experiment by substituting other liquids for oil.

17.3.0 Viscosity, Stokes' law, fluid friction, falling ball in liquid
See 3.12.0: SI Derived units, Viscosity | See 13.6.0.3: Shear-thinning, stir-thinning, thixotropy
Fluid friction, ball bearing falling in a liquid
Viscosity is the property of a fluid that resists the force tending to cause the fluid to flow. Viscosity is the property of a fluid that tends to prevent the movement of one portion of the fluid relative to an adjacent portion, or prevent the motion of any body through the liquid. Viscosity is the internal friction of a fluid, produced by the movement of its molecules against each other. Viscosity causes the fluid to resist flowing. Absolute viscosity, η is a measure of this resistance, equal to the tangential stress on a liquid undergoing streamline flow divided by its velocity gradient. It is measured in newton seconds per metre².
Dynamic viscosity, the coefficient of velocity of a fluid at a given temperature, has symbol η (eta) and µ (mu). Dynamic viscosity measures the resistance to flow of a liquid under shear stress in pascal seconds..
The SI unit is the pascal.second, Pa.s, in newton.second / metre². 1 Pa.s = 1.00 g.cm^-1.s^-1. Dynamic viscosity of water at 20^oC = 0.001001 Pa.s.
The cgs unit of dynamic viscosity is the poise, P (for some technologies centipoise, cP). Dynamic viscosity of water at 20^oC = 1.002.0 cP.
Viscosity: Pa s: Pascal second, N s m^-2, a measure of viscosity replacing the c.g.s. unit, poise = 0.1 Pa s.
Kinetic viscosity, (kinematic viscosity), ν (nu) is the ratio of the viscosity of a liquid to its density ρ (rho).
The SI unit, ν = µ / ρ, where µ is measured in m² / s, and ρ is measured in kg / m³.
The cgs unit is stokes, St, or centistokes, cSt. Kinetic viscosity of water = 1 cSt.
At 20^oC the viscosity of water is 1.002 mPa·s and its kinematic viscosity (ratio of viscosity to density) is 1.0038 mm² / s.
Lubricating oil may be classified using a Viscosity index (VI) for change of viscosity with temperature.
When a liquid flows slowly through a pipe, the rate of flow depends on the viscosity of the liquid. However, when velocity exceeds a critical velocity, ν, the flow becomes turbulent and the rate of flow depends on the density of the liquid and not the viscosity. Critical velocity, ν = kη / ρr, where k = constant (Let water = 1000), η (eta) = coefficient of velocity of a fluid at a given temperature, ρ (rho) = density of liquid, r = radius

17.3.01 Stokes' law
Stokes' law (George Stokes 1851) describes the frictional force on a spherical ball falling through liquid, force = 6 × π × radius × velocity × viscosity of liquid. A falling ball accelerates until it reaches a constant terminal speed, settling velocity. F = gravitational force on the spherical ball less the upthrust. Stokes' law can be applied to liquid and gas mediums. The falling sphere viscometer uses Stokes' law to calculate viscosity of a fluid from the size and density of a sphere falling in it, the density of the liquid and the terminal velocity of the sphere. Drop ball bearings or a sphere with known density and weight into treacle or glycerine to determine their viscosity.

17.3.02 Newtonian fluids
Deformation occurs when a force is applied to a volume of material. If two plates, area A, are separated by a fluid distance apart, separation height H, are moved relative to each other at velocity V, by a force F, the shear stress, force divided by area parallel to the force, F / A, is proportional to the shear strain rate, V / H. An example of shear strain rate is the rate of stirring. The proportionality of shear stress to shear strain rate follows Newton's laws so you can refer to Newtonian fluids which follow this law of proportionality. In a Newtonian fluid the velocity gradient is directly proportional to the shear stress, the analogue for gooey materials of Hooke's Law for springs.

17.3.03 Non-Newtonian fluids
1. For dilatant (shear thickening) fluids, the faster the liquid moves, the more viscous it becomes, shear viscosity increases with applied shear stress, the viscosity increases with the rate of shear strain, e.g. mixture of cornstarch and water (oobleck). When walking on wet sand the sand immediately below the foot is dry due to rearrangement of the grains from a close packed to a less close packed structure.
2. For thixotropic fluids, the faster the liquid moves, the less viscous it becomes, the material becomes more fluid with increasing time of applied force, e.g. quicksand (so do not struggle to get out of quicksand!) The proportionality constant is called the dynamic viscosity η (eta). Viscosity is the tendency of a fluid to resist flow. Viscosity increases, "becomes thicker", with increase in concentration or increase in molecular weight of a solute.

17.3.1 Air friction
Drop crumpled and flat sheets of paper.

17.3.2 Pulling an aluminium plate
Use a string and pulley to a mass to pull an aluminium plate out of a viscous fluid, e.g. silicone fluid.

17.3.3 Syringe water velocity
Squirt water out of a syringe. The water moves faster through the constriction.

17.3.4 Terminal velocity of different diameters
Three steel balls of different diameters are sealed in a 10 cm diameter.

17.3.5 Terminal velocity drop balls and specific gravity
Drop steel, glass and lead balls of different diameters in a tall cylinder filled with glycerine. Use a precision ball in a precision tube. Measure the terminal velocity in water and glycerine. Drop steel balls in large 1 metre test-tubes or graduated cylinders filled with water and filled with glycerine. Drop four balls of the same diameter with different specific gravity into glycerine. Drop Styrofoam balls in a 5 m tall glycerine cylinder. Use Dylite beads reach to attain terminal velocity quickly in water and when expanded by heating in boiling water. Also, use Dylite bead to show terminal velocity in air.

17.3.6 Terminal velocity coffee filters
Drop a coffee filter and it falls with low terminal velocity. Crumple a coffee filter, drop it and it falls with a greater terminal velocity

17.3.8 Uniform pressure drop
Make water flows in a horizontal glass tube with three pressure-indicating stand pipes fitted with wooden floats.

17.3.9 Viscosity pipe
Make a series of small holes in a long water pipe or gas pipe to show pressure drop due to friction. Run a water pipe around the laboratory with pressure gauges at the top and bottom of each side and examine the difference between static pressure and kinetic pressure.

17.3.10 Viscosity disc
Hang a horizontal disc on a single thread and spin a second disc below it to cause deflection. Spin a disc between two parallel plates of a platform balance and note the deflection. A horizontal disc is hung on a single thread and a second is spun below it causing deflection. Mount a metal sheet and a disc parallel in a container of fluid, rotate the disc and observe the displacement of the sheet.

17.3.11 Viscosity in capillary
A Mariotte flask with a capillary out on the bottom permits varying the pressure.

17.3.13 Viscosity and temperature
Invert tubes filled with motor oil and silicone oil at room temperature and after cooling. Rotate a cylinder of castor oil in a water bath on a turntable at different temperatures.

17.3.14 Viscous drag
Mount 2 coaxial cylinders are separated by a fluid. As you rotate the outer cylinder, you can observe the drag induced motion of the inner cylinder.

17.3.15 Viscosity bubble race
Use a bubble race to compare viscosity in different tubes of different liquids when inverted. Almost fill identical test-tubes, or plastic tubes with stoppers, with the same volume liquids having different viscosity, e.g. cooking oil, lubricating oil, dishwashing detergent, glycerol, water, methylated spirit. The height of liquids in the test-tubes must be identical. Put corks in the test-tubes. Put the test-tubes in a test-tube stand and attach them to it so that when the test-tube stand is inverted the test-tubes will not fall down. Quickly invert the test-tube stand and note the speed of the bubbles passing up through the liquids. Note the sequence of the bubbles hitting the bottom of the inverted test-tubes. The liquid that wins the bubble race is the least viscous.

17.4.1 Viscosity, thick and thin liquids
(After Donna Bennett, The Queensland Science Teacher, Volume 24, No. 2)
Heating a liquid makes it thinner, less viscous, because the spaces between the molecules get bigger and the molecules can slide over one another more easily.

17.4.2 Viscosity of honey
1. Dip a knife into a jar of honey at room temperature. Lift the knife out and keep lifting to make a stream of honey. Observe the stream of honey as the knife gets higher and higher.
2. Raise the temperature of the honey by standing it in a bowl of warm water for 15 minutes. Dip a knife into a jar of honey at room temperature. Lift the knife out and keep lifting to make a stream of honey. Observe the stream of honey as the knife gets higher and higher. Observe how high can you raise the knife before the stream of honey becomes drips? Note the direction the stream of honey coils.

17.4.3 Viscosity of engine oil
See 10.6.3: Distil crude oil and collect the fractions
1. Viscosity index
The viscosity change is called the viscosity index. The higher the viscosity index the less change in viscosity as the temperature rises. The SAE (Society of Automotive Engineers) classification is an unbiased way of classifying the viscosity of "light", "medium" and "heavy" engine oils. For example SAE 10 W is "thin" oil that you use to start engines in cold weather and SAE 30 is "thick" oil that you use to start worn engines in hot weather.
2. Centistokes
The common denominator for Kinematic Viscosity measurement is "Centistokes", measured at 40^oC, and 100^oC, and at sub zero temperature, e.g. SAE 15W40 engine oil measures 13 to 15 Centistokes at 100^oC, but above 16.3 Centistokes you rate the oil as SAE 50 Grade.
3. Engine oil additives
Additives to engine oil include the following: oxidation and / or corrosion inhibitors to retard engine deposits, detergent disperses to keep dirt oil soluble, rust preventives, anti-wear agents to retard wear of moving parts, foam inhibitors to reduce froth, pour point depressants to allow oil to flow at low temperatures, and viscosity index improvers to reduce difference of viscosity of hot and cold oil.
4. Friction and wear
Engines rely on moving parts to function. The surfaces that move over each other wear with use. Friction is the resulting force encountered at the common boundary between two bodies when, under the action of an external force, one body moves, or tends to move, relative to the surface of the other. Wear is progressive damage, involving material loss, which occurs to the surface of a component because of relative motion at the surface of that component. Wear is a gradual process so the associated costs and inconvenience are not as apparent as failure due to brittle fracture or fatigue. Lubrication is the application of lubricants to reduce friction and wear. Boundary lubrication mode is reached every time machinery comes to a standstill. Most of the wear takes place at start-up. Hydrodynamic lubrication is reached within moments of the machinery starting up. The oil pump begins to push the oil in circulation. The oil pressure lifts each movable component into operational relationship with the mating component. Low viscosity oils, light oils, flow freely and high viscosity oils, heavy oils, flow slowly.
5. Viscosity index
Viscosity changes with temperature, the higher the temperature the lower the viscosity. The Viscosity Index is an empirical numbering system that describes the rate of change of viscosity of an oil within a given temperature range. A low viscosity Index signifies a relatively large change while a high Viscosity Index shows a relatively small change, i.e. the higher the Viscosity Index the less oil thins out with increasing temperature and vice versa. Fluid lubricants include specially formulated Engine Oils, Transmission Oils (Manual and Automatic), Heavy Duty Gear Oils suitable for differentials, Hydraulic Fluids, Compressor Oils.
6. Engine greases
Greases are semi-solid lubricants. A variety of viscosity ranges are produced. The load bearing capacity is linked to both the viscosity (or thickness) and adherence or retention characteristics. The speed of rotation (RPM) of the journal in a bearing will dictate the viscosity needs of the grease. The same factors apply in gearboxes or transmission. The rule is as follows: The faster the movement of the components, the lighter the viscosity of the gear / transmission oil, and the slower the movement of the components, the heavier the lubricant needs to be. If you use a heavy viscosity oil, e.g. SAE 25W60, in winter condition, the engine may not start without being preheated. Engines usually need much lighter viscosity oil, e.g. SAE 5W30, to enable easy cold starting. Similarly, if you use SAE 5W30 oil in tropical conditions where pavement temperatures may be above 55^oC, premature wear may occur because it is too thin or "light" for such high temperature conditions.
7. "Speciality oils"
The OEMs (Original Equipment Manufacturers) are asked to produce engines with finer tolerances because the lighter viscosity engine oils will result in less internal fluid friction and diminish the amount of fuel required. A survey of over 100 major transport companies in USA established that 80% of power is used initially in overcoming friction. They can fiction minimize friction by increasing the film strength of the lubricants. An interstate heavy transport operator who has used ordinary commercial brand lubricants until recently decided to try a "speciality" brand gear oil in the differential of his prime mover to reduce the operating temperature. The result was lower operating temperature because of the higher load carrying capability of the gear oil by comparison with the oil he had previously used. In another example, a used car given an oil change oil at 10 000 km was given a "speciality" brand of engine of the same viscosity (SAE 15W40). After the oil change it was travelling 120 km further on every tankful of fuel so saving over 15% of the fuel costs. In both of these examples less fuel is being burnt, so less harmful exhaust gas emissions pollute the atmosphere.
1. Compare the viscosity of two engine oils by shaking two identical bottles at the same temperature by the same amount. Invert the bottles and compare the speed bubbles rise. The higher the speed the lower the viscosity.

17.4.4 Stir-thinning tomato sauce
Stir-thinning liquids that get thinner, decrease in viscosity, when you add force to them by stirring or shaking.
1. Pick up a bottle of tomato sauce and without shaking it. Try to pour it out. Note what happens.
2. Shake the bottle of tomato sauce ten times. Try to pour it out. Note what happens.
3. Leave the tomato sauce bottle to stand still for 20 minutes. Try to pour it out. Note what happens.
4. Shake the bottle of tomato sauce ten times. Try to pour it out. Note what happens. After tomato sauce has fallen on food and stopped moving it becomes thick again
5. Repeat the experiment using hair gel.

17.4.5 Time to empty the funnel, viscosity
1. Collect liquids from the home, e.g. oil, honey, sauce, water, milk.
2. Put your finger over the bottom end opening of a small kitchen funnel and fill the funnel with one of the liquids.
3. Hold the funnel over a larger container, remove your finger and observe the time taken for the funnel to empty. Note which liquid takes the longest to leave the funnel. You can measure viscosity by how easily things move through a liquid or how easily the liquid flows. In the more viscous liquids the molecules are sticking together more making it harder for them to pass quickly through a narrow space

17.4.6 Stir-thickening cornflour mixture
1. Put cornflour into a plastic container. Slowly add water to make a thick sludge. If you add too much water it will be too thin. Add some food colouring, e.g. cochineal.
2. Let the cornflour mixture run through your fingers and observe how fast it runs.
3. Slap the mixture with your fist. Let the cornflour mixture run through your fingers and observe how fast it runs. Note whether it is still "runny"
4. Take a handful and add a force to it so that it forms into a ball. Stop the force and note that the mixture will snap like a solid. The cornflour mixture is a stir-thickening substance. The viscosity increases when a force of stirring or thumping is applied.
5. Repeat the experiment with cream.

17.4.6.1 Electrorheological fluid (ER fluid), cornflour and vegetable oil
See 31.1.02: Electrostatic series, triboelectric series, ranking of insulators
Mix 3 parts of vegetable oil with 1 part of cornflour (maize flour, cornstarch) to produce a colloidal mixture. Leave the mixture to chill in a refrigerator then remove the mixture to warm until it starts to flow. Rub wool or hair on a block of polystyrene or PVC or a blown-up rubber balloon to give the object a negative charge. Place the charged object near the flowing mixture. The mixture stops flowing and pieces of mixture may break off. The mixture is an electrorheological fluid, (ER fluid), i.e. a mixture of small non-conducting particles in an electrically insulating fluid. Under the electric field, electrorheological fluids form fibrous structures which are parallel to the applied field so the fluid increases in viscosity. The dielectric cornflour particles separated by the oil form positive to negative chains aligned with the applied electric field. The ER fluid gets thicker because the charged dielectric cornflour particles in the chains attract due to positive negative attraction. Similarly, melted chocolate may stop flowing in an electric field. ER fluids are used in brakes and shock absorbers.

17.4.7 Ball bearing falling in liquid, Stokes' Law
1. Use household liquids with different viscosity, e.g. honey, petroleum oil, vinegar, water, olive oil, fruit juice cordial concentrate. Drop a large ball bearing into the liquids and note the time to fall a certain distance. The relative times is one method of calculating viscosity based on Stokes' Law.

17.5.1 Direction of friction
Friction always opposes the motion. This statement is often heard or read and, indeed, friction does oppose the motion in very many situations. Now consider the following situation, with a block sitting on top of another block and the system moving to the right under the action of an applied force. If you think about the forces on the system, you indeed find that friction that friction between the lower block and the table is opposing the motion. What about the upper block? What is causing it to move? In this case, the upper block is moving because of friction between it and the lower block. Friction is in the same direction as the motion! In fact, you often meet this question. The simple act of walking is a demonstration of friction acting in the direction of motion. Push back on the earth and the friction force between your feet and the earth moves you forward.

17.5.2 Friction blocks
See diagram 17.183: Friction block
Pull a block along different surfaces with a spring scale, e.g. Teflon, wood sandpaper, and rubber.

17.5.3 Sliding friction machine
A spring scale is attached to an object on a rotating table.

17.5.4 Weight dependence of friction
Pull a friction block with a spring scale add a second equal block to the first and repeat.

17.5.5 Friction blocks down an incline
A loaded cart rolls down an incline and hits a barrier The load continues sliding on a second incline until it stops The mass on the slider is varied to show stopping distance is independent of mass.

17.5.6 Area dependence of friction
A friction block has a rectangular shape with one side twice as big as the other. The friction block is pulled along the bench top while resting on either the narrow or wide face.

17.5.7 Static friction vs sliding friction
Kinetic friction is the friction experience by two surfaces sliding against each other with a known relative speed. Static friction has values from zero to a maximum value. Friction coefficients are dimensionless.
See diagram 17.183: Friction block
1. Measure static friction by noting the reading on the scale of a spring balance just before the block starts to slide over a rough surface.
If reading on spring scale = F, and force of static friction = f_s, then F = f_s, until block starts to move and f_s has maximum value. Once the block starts moving the friction is kinetic friction, f_k, i.e. sliding friction.
2. Measure sliding friction by continuing to pull the block at constant speed.
3. Change the rough surface and note the different values for static friction and sliding friction.

Friction coefficients	Kinetic friction, µ_k	Static friction, µ_s
Steel on steel	0.57	0.74
Glass on steel	0.40	0.94
Steel on ice	0.06	0.10

17.5.8 Angle of repose
Lift one end of an inclined plane until a block begins to slide.
1. Put the following different objects at one end of a smooth tray
1.1 Cubes of different substances
1.2 Knife fork and spoon
1.3 Identical packets of salt with,
1.3.1 smallest area down,
1.3.2 medium-sized area down,
1.3.3 largest area down.
Slowly lift the tray by raising the end where the objects are placed. Note the angle or height of that end of the tray when the objects start to slide. Note the sequence of objects starting to slide.
2. Repeat the experiment with another tray with a different surface or attach a new surface to the first tray, e.g. a sheet of sand paper or emery paper. The objects will start to slide at a greater height than before but note whether the sequence of objects starting to slide is the same.

17.5.9 Friction roller
A cylinder in a yoke can be rolled or locked and slid as it is pulled by a spring scale. A cylinder is pulled along and perpendicular to its axis by a yoke with a spring scale. Front and rear brakes A toy car rolls down an incline with either front or rear wheels locked

17.5.10 Cross friction
Push a block across the slope of an incline and the block will move with a straight line trajectory Knock a coin across and it will move in a curved path.

17.5.11 Squeaky chalk
You don't have to break chalk to eliminate squeaking because if you use friction you can hold the chalk at an angle that eliminates it.

17.5.12 Angle of friction with pencil
Tilt a pencil until it slides along the table.

17.5.13 Sliding chain
Hang a chain over the edge of the table until the weight of the chain makes it slide. This simulates the action of a siphon.

17.5.14 Capstans
Examine the frictional force vs the number of turns around a rod. Falling flask capstan: Attach a 4 litre round bottom flask at the other end of a ball on a string and drape the flask around a horizontal rod 1 m high. Falling keys capstan: Hang a set of keys from a string draped around a pencil and when the string is released the keys don't hit the floor. Friction experiments with the cord wrapped around a cylinder

17.5.15 Low friction surfaces
Teflon surface: Cut up a Teflon coated cookie sheet for an inexpensive Teflon surface. Teflon pulley: Teflon sheet bent around corner replaces a pulley. Dylite beads on a rimmed glass surface window pane provide a low friction surface.

17.5.16 Gripping rice
Almost fill a narrow neck plastic jar with rice. Note the level of the rice in the jar. Stab down into the rice with a sharpened pencil. Keep stabbing down, sometimes deeply to the bottom of the plastic jar and sometimes shallowly. When the rice starts to grip the rice, slowly with great force push the pencil down to touch the bottom of the plastic jar. The conical end of the pencil pushes rice sideways when the pencil is pushed into the jar. When the pencil is raised and removed from the jar trice falls into the space left by the pencil so the level of rice in the jar decreases slightly. The action of the repeated stabbing with the pencil is to pack the rice more closely. Eventually the rice is so closely packed to increase friction between the rice grains that you cannot raise the pencil without the jar of rice also being raised.

17.5.17 Raisin cake
Coat raisins with flour before adding them to a batter to make a raisin cake. The flour increases the surface friction of the raisins so that not all of them will fall down to finish at the bottom of the cake but be more equally distributed throughout the cake.

17.5.18 Interleaved telephone books
1. Put two identical telephone books on the table side by side, push them to touch then interleave the pages, i.e. any page of one book has a page of the other book above it and below it. After interleaving, push the books together to maximize contact between the pages. Place a heavy weight on the interleaved telephone books then later remove the weight and try to pull the books apart. A considerable force is needed to pull the books apart and even heavy machinery may be needed.
2. The considerable friction between the pages of the telephone book is augmented by "Chinese finger lock" mechanisms used by police to restrain suspects before "patting down" to detect weapons or other objects. Finger lock techniques are also used in ju-jitsu (jiu-jitsu) Japanese unarmed combat contests.
3. A mixture of 14% wood pulp and water can be frozen to produce "pycrete" that is much stronger than the same volume and shape of a piece of ice. The pycrete should contain no air bubbles. The friction between the particles of wood pulp causes the extra strength of pycrete.
4. If the edge of a piece of paper cuts your finger the cut is very painful because of the multitude of jagged edges from the wood fibres in the paper.

17.5.19 Push a wheelbarrow
Raise the handles of a wheelbarrow full of sand and push it towards a small hillock. It may be difficult to push the wheelbarrow over the hillock unless you lower the the handles until they are almost parallel to the ground. By lowering the handles you are applying a force in the direction of motion, i.e. tangential to the slope. If you push the wheelbarrow with handles raised, a component of the force you apply is into the hillock which also increases frictional loss, with only the component of force parallel to the slope being utilized to move the wheelbarrow over the hillock.