MadSci Network: Physics

Re: Why my soles are having holes?

Date: Mon Jan 1 08:56:11 2001
Posted By: Arnold Anderson, Staff, Tribology/Friction systems, retired (Ford Scientific Laboratory)
Area of science: Physics
ID: 977890489.Ph

You were correct that the wear of your shoes resulted from dragging, but 
it appears that your brother missed something in your explanation.  We are 
able to walk and run only because of the friction between the soles of our 
shoes and the ground.  This friction is needed to start, stop, turn, and 
maintain our balance.  

Our heels wear most from walking traction.  The heels contact the ground 
first, and their friction is used to move us forward.  As the leg moves, 
our weight is transferred to the ball of the foot.  We continue to drive 
forward with friction from this region of our soles.  Wear of the sole is 
concentrated where the load is greatest—usually at the ball of the foot.  

A substantial part of the wear from the sole comes from the braking action 
when walking down hill. Under these conditions, the sole contacts first. I 
suspect there are many hills in your area of Greece.  Walking or running 
down hills requires the sole to work like a brake lining.  The soles slide 
a small amount on each step.  Sharp stone surfaces cut into the soles, 
making the contact surface rough.  Eventually, small pieces are pulled out 
from this roughened sole.  

Stick a piece of adhesive tape to the sole of your shoe, and then pull it 
off.  Look closely at the tape.  You will see small wear particles of 
sole, and some of the sharp stones that cut them.  If there is a part of 
the sole that has not worn (perhaps near the heel), try this again.  Much 
less will stick to the tape where there has not been the cutting and 
roughing. You might also leave a piece of tape on your sole, and watch how 
it wears as you walk on level ground, up hills and down hills.

If you do these simple experiments with your brother, you both may learn 
more about wear.

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