For a skateboard, friction works between the wheels of the board and the axis, as well as between the wheels of the board and the ground when the board is in motion. Since friction works in a direction opposing motion, it is only in effect when the when the board or the wheels of the board are in motion.
when it is worn down to the point where the metal grinds the road
You really never wear in your skateboard but you get used to it. Your bearings will start spinning faster when broken in, Your trucks won't turn by themselves when worn in, and your shoes will feel comfortable and unstiff
well-worn, about $2 lightly-worn, about $10 uncirculated, about $75
Numismedia lists the following approximate retail values as of 05/2009: Very worn condition - 8¢ Moderately worn - 32¢ Slightly worn - 65¢ Almost no wear - $1.00 Uncirculated - $3.25+
well-worn = $1 lightly worn = $5 uncirculated = $20
Un-circulated probably quite a bit. In circulation and worn with age it is worth a penny.
A circulated penny may appear worn or dull due to being handled frequently. It may have scratches, discoloration, or slight fading of the design. The edges of the penny might also be slightly rounded or dented.
A 1883 Indian Head penny would be worth $3 if worn, $16 if none circulated.
penny hardaway and Steve francis
Look for any obvious signs of damage such as bulges or cracks. Next test the tread using a penny. Place a penny head down in the tread. If you can still see part of Lincoln'shead, the tire is worn.
Shock absorbers are worn, wheels/tires need balancing.