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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 slidi
by 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.