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Re: Friction and area.



Dario Moreno wrote:

Ask a sailor or a cowboy. Both know from experience that if
something-tied-to-a-rope is too heavy (or pushy) to be
held by you, you may turn your end the rope around a tree
or any other pole. You are not supposed to tie a knot on the tree,
just coil the rope around it. If the rope is coiled several turns, then
you need only the little finger to keep the cow.

This is a different situation - in the case of a uniform weight but
different surface areas, the weight is divided up by area. In the case
of tension in a rope around a pole, start by considering the
frictionless case so that the tension is a constant everywhere on the
rope. There is therefore a certain amount of compressive force per unit
length along the rope. The more loops you make the more compressive
force there is. So putting more loops around the tree is analogous to
using a bigger block, not to spreading out the surface area of a given
block.

Somebody else mentioned the large tires on race cars. Isn't this at
least partly to reduce frictional heating and also due to acceleration
considerations (same torque, larger radius => more acceleration)?

/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\

Doug Craigen "Technology with purpose"
http://www.dctech.com