How are trains able to start with so much cargo?

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Just passed by a train with a hundred or so cargo crates. how do they start? I understand how they continue moving, but how do they overcome the inertia of starting it? It has to be like thousands of tons, and I can’t imagine a bunch of coal being able to start moving that. unless of course it can, in which ELI5

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Anonymous 0 Comments

1. Slowly
2. Using a lot of power.
3. On an extremely low friction surface thanks to steel-to-steel contact between wheels and rails (and good bearings).

1+3 is also why the “strongman pulling a train wagon” works.

Inertia is actually much less of a problem than friction in many cases. Speed is acceleration times time, and acceleration is force divided by mass.

If you pretend that there is no friction at all, even the tiniest force will very slowly accelerate the train. If you apply one kg-force (about the force that you can *comfortably* apply with your little finger) to a metric ton (1000 kg), it will accelerate by 0.01 m/s every second. Apply that force for a minute, and your metric ton will be moving at 0.6 m/s – about 2 km/h, or about half of a slow walking speed.

In reality there is “stiction” (a certain amount of friction force that needs to be overcome, otherwise a non-moving object doesn’t start to move), which is why you can’t *actually* get a train moving just by pushing it with your finger for a few minutes, and *some* friction (that you need to overcome constantly or the train will slow down again), but the steel-wheel-on-rails setup makes this very small compared to most other things you’re used to pushing or pulling.

If you ever pull something that’s on either ice, hovering on an air pillow, or in water, you can experience this yourself. For example, a moderate amount of force applied to a line (rope) going to a multi-ton boat over seconds to tens of seconds will make it move, and often it’s much more practical to push/pull twice as long rather than twice as hard.

Locomotives are also quite powerful – thousands of horsepowers, but unlike your car where you usually don’t use those horsepowers except for brief periods of acceleration, the locomotives do use them for much longer periods of time as they very slowly accelerate the train. They’re also optimized for applying a lot of force, and power is force *times distance* – so if the train isn’t moving much, a small amount of *power* is a lot of *force*. (This is what makes levers and gearboxes work.)

So imagine your car, times ten, in a gear much, *much* lower than first gear, absolutely flooring it, not just for a few seconds but for minutes.

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