3 ms·

I don't quite see what's amiss. > Static and kinetic friction is only a resistive force to a train if there is slippage between the wheels and the track. No,

by mankyd 12y ago

I don't quite see what's amiss. > Static and kinetic friction is only a resistive force to a train if there is slippage between the wheels and the track. No, static friction affects the train _until_ it starts moving. If there is any slippage or movement or anything, you've moved into the realm of kinetic friction, whether it be between the wheels and the track, or the axle and its bearings. > Axle bearings have pretty low friction ... if they didn't, they would get hot, the grease would melt Axle bearings have low _kinetic_ friction. They can, in theory, have a much higher static friction, and this is what needs to be overcome to get the train moving.

msandford 12y ago
It doesn't make any sense to talk about static vs kinetic friction for the cars because they have roller bearings. Static friction is when things are not sliding and kinetic friction is when they are sliding. Since the cars have roller bearings (not bushings) the wheel doesn't slide relative to the track and the axle doesn't slide relative to the axle truck. There might be slightly more start-up torque to get a bearing moving but that's more as a result of the static friction of the seals or the roller cage or the grease. The reason that bearings work so well is that they DON'T slide but rather roll. They're an ingenious hack around the idea that you have to pay the price somewhere for friction. Before they existed everyone bitched about friction but "hey what can you do?!" Afterwards it seemed obvious but the world spent many thousands of years with wheels and no bearings.
- tanzam75 12y ago
  > It doesn't make any sense to talk about static vs kinetic friction for the cars because they have roller bearings. Well, modern trains don't have cabooses either. So the scenario wouldn't even come up. But suppose that this is a train from 1940, with a caboose, with journal bearings instead of roller bearings, and with a steam engine. Can it start? I wonder if maybe this was a legitimate problem from the steam era, which led to the procedure to compress the train and the start it one car at a time. It's conceivable that we'd keep passing along the received wisdom, even after roller bearings and electric motors changed the picture.

by mankyd 12y ago

msandford 12y ago

It doesn't make any sense to talk about static vs kinetic friction for the cars because they have roller bearings. Static friction is when things are not sliding and kinetic friction is when they are sliding. Since the cars have roller bearings (not bushings) the wheel doesn't slide relative to the track and the axle doesn't slide relative to the axle truck. There might be slightly more start-up torque to get a bearing moving but that's more as a result of the static friction of the seals or the roller cage or the grease. The reason that bearings work so well is that they DON'T slide but rather roll. They're an ingenious hack around the idea that you have to pay the price somewhere for friction. Before they existed everyone bitched about friction but "hey what can you do?!" Afterwards it seemed obvious but the world spent many thousands of years with wheels and no bearings.

tanzam75 12y ago
> It doesn't make any sense to talk about static vs kinetic friction for the cars because they have roller bearings. Well, modern trains don't have cabooses either. So the scenario wouldn't even come up. But suppose that this is a train from 1940, with a caboose, with journal bearings instead of roller bearings, and with a steam engine. Can it start? I wonder if maybe this was a legitimate problem from the steam era, which led to the procedure to compress the train and the start it one car at a time. It's conceivable that we'd keep passing along the received wisdom, even after roller bearings and electric motors changed the picture.