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The article has the physics completely wrong. Static friction is what opposes movement between two surfaces that are static next to each other and kinetic fric

by hvm 12y ago

The article has the physics completely wrong. Static friction is what opposes movement between two surfaces that are static next to each other and kinetic friction is what slows down movement between two surfaces. Static friction is always >= kinetic friction. This is explained very well in here (the top picture says it all): http://hyperphysics.phy-astr.gsu.edu/hbase/frict2.html http://hyperphysics.phy-astr.gsu.edu/hbase/frict2.html Think about how when you start your car from a standstill you need to be careful about your clutch release but when the car moves even the tiniest bit you can even start in the second gear if you want. So, the engine car's wheels get kinetic friction (because they're rolling) and the other cars' wheels (which are stopped) are blocked by static friction. When you have slack and you move one car at a time you only need to beat the static friction of one car at a time. Even if the difference is 10%, if you have 10 cars and it starts to add up. I'm not saying slack is required for starting the train, just that there is an increased difficulty in doing so without slack.

hvm 12y ago
Also, this is why doing a wheelspin is never a good idea when you have trouble starting your car/train. The traction when the wheels are blocked but the engine tries to move them is always a lot better than when they're spinning in place.
- Gracana 12y ago
  To be super-pedantic (because the topic is interesting), a little spin is good for an automobile. Rubber friction is a combination of adhesion, mechanical deformation, and wear.[1] In order to maximize traction with a tire you will have some optimal slip, increasing the "wear" component beyond what is seen in "normal" driving conditions. Of course, too much sliding/wheelspin and you reduce the other components. It's a balance that depends on rubber compounds, tire construction, and road conditions. [1] http://insideracingtechnology.com/tirebkexerpt1.htm http://insideracingtechnology.com/tirebkexerpt1.htm

by hvm 12y ago

hvm 12y ago

Also, this is why doing a wheelspin is never a good idea when you have trouble starting your car/train. The traction when the wheels are blocked but the engine tries to move them is always a lot better than when they're spinning in place.

Gracana 12y ago
To be super-pedantic (because the topic is interesting), a little spin is good for an automobile. Rubber friction is a combination of adhesion, mechanical deformation, and wear.[1] In order to maximize traction with a tire you will have some optimal slip, increasing the "wear" component beyond what is seen in "normal" driving conditions. Of course, too much sliding/wheelspin and you reduce the other components. It's a balance that depends on rubber compounds, tire construction, and road conditions. [1] http://insideracingtechnology.com/tirebkexerpt1.htm http://insideracingtechnology.com/tirebkexerpt1.htm