Static Friction, Kinetic Friction, Wheel, Axle, Roads, Steel Wheels, Steel Track, Coefficient of Friction and Intermodal Transportation

Posted by PITHOCRATES - November 28th, 2012

Technology 101

Friction Pushes Back against us when we try to Push Something

Have you ever done any landscaping?  Buy some decorative rocks to cover the ground around your flowers and shrubs?  If you go to a home improvement store with a garden center you probably bought your decorative rocks by the bag.  And those bags are pretty heavy.  Say you have a pickup truck.  And the good people at the garden center bring out a pallet of stone bags on a pallet jack.  Placing it down next to your truck.  Before loading it in your tuck do this experiment.

Don’t really do this.  Just imagine if you did.  Squat down behind the pallet.  Place your hands on the pallet.  And push with all of your might.  What do you think would happen?  Would you send that pallet sliding across the pavement?  Or would you fall on your face as your feet slipped out from underneath you?  You’d be kissing the pavement.  And possibly giving yourself a good hernia.  Now if they had put that pallet of stone into your pickup truck and you put the truck into neutral and tried pushing that what do you think would happen?  You may still get a hernia but that truck would probably move.

A pallet of stone may be too heavy to push.  But a pickup truck with a pallet of stone in it may not be too heavy to push.  How can that be?  In a word, friction.  It’s that thing that pushes back when we try to push something.  The heavier something is and the more surface area in contact with the ground the more friction there is.  Which is why that pallet is hard to push.  The force of friction is so great that we can’t overcome it.  But something that can be almost 10 times heavier sitting on 4 rubber tires bolted onto a greased axle?  That’s a different story.

The Two Basic Types of Friction are Static Friction and Kinetic Friction

There are two basic types of friction at play here.  Static friction.  Which prevents us from pushing that pallet of stone.  And kinetic friction.  Which we would have experienced with that pallet of stones if we were able to overcome the static friction.  Kinetic friction is what we encounter when sliding something across the ground.  Static friction is greater than kinetic friction.  As it takes more effort to get something moving than keeping something moving.

Now here’s why we are able to push a pickup truck easier than a pallet of stones.  With a pallet there is 48″X40″ of surface area in contact with the ground producing a large amount of static friction to overcome.  Whereas on the pickup truck the only thing that slides are the axles in highly greased bearings.  Which offer very little static friction.  The rubber tires offer some static friction due to the immense weight of the truck pushing down on them, flattening the bottom of the tires somewhat.  Once the resistance of the flattened tires is overcome the rubber tires offer kinetic friction in the direction of travel.  While offering static resistance perpendicular to the direction of travel.  Keeping the truck from sliding away from the direction of travel.  Which works most times on dry and wet pavement.  But not so good on snow and ice.  As snow and ice offer little friction.

The wheel and axle changed the world.  Allowing people to move greater loads.  People could grow wheat and other food crops in distant areas and load them onto carts to transport them to cities.  Which is what the Romans did.  Using their roads for their wheeled transportation.  Which increased the speed and ease they could pull these large loads.  Sections of Roman roads have survived to this day.  And in them you can see centuries old wheel ruts worn into them.

Intermodal Transportation combines the Low Cost of Rail and the Convenience of Trucking

The basic wooden-spoke wheel remained in use for centuries.  From Roman times and earlier.  To 19th century America.  While we were still using the wooden-spoke wheel we began using something else that offered even less friction.  Iron wheels on iron rails.  Allowing great loads to be transported over great distances. The friction of an iron wheel on an iron track was so low that the drive wheels would slip when starting to pull a heavy load.  Or going up any significant grade.  To prevent this slip trains carried sand and deposited it on the track in front of the drive wheels.  To increase the friction of the drive wheels for starting and travelling on inclined grades.   Iron wheels and iron track gave way to steel wheels and steel track.  Allowing trains to pull even greater loads.

There is no more cost-efficient way to move heavy freight over land than by train.  Thanks to exceptionally low coefficients of friction.  And the less friction there is the less fuel they need to pull those heavy loads.  Which is the reason why so many of our roads are pocked with potholes.  Roads are only so strong.  They can only carry so much weight before they break apart.  Which is why the heavier load a truck carries the more axles they must distribute that weight over.  Putting more tires on the pavement.  Increasing the friction to overcome.  Requiring greater fuel consumption.  Which is why a lot of truckers cheat.  And try to get by on fewer axles.  Increasing the weight per axle.  Which hammers potholes into the pavement.

The reason why we use trucks to transport so much freight is that there aren’t railroad tracks everywhere.  But we can still make use of the railroad tracks that are near our shipping points.  By combining rail and truck transportation.  We call it intermodal.  Using more than one means of conveyance.  Putting freight into containers.  Then putting the containers onto truck trailers.  Then driving them to an intermodal yard.  Where they take the containers from the truck trailers and put them onto rail cars.  Where they will travel great distances at low friction.  And low costs.  Then at another intermodal yard they’ll transfer the containers back to truck trailers for a short ride to their final destination.  Getting the best of both worlds.  The low-cost of rail transport thanks to the low friction of steel wheels on steel rail.  And the convenience of truck transportation that can go where the rails don’t.

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