How does a bridge over which a train travels move?

Answer

Dear Willy,

it can all be explained with forces and reaction forces. If you look at the train and the bridge separately and consider the forces acting on both, your questions are immediately answered:

* if the train continues to drive over the bridge at the same speed, there will be almost no forces in the direction of travel. Of course the bridge must bear the weight of the train, but you will be able to speak much less about “push” or “attract”.

* if the train brakes on the bridge, friction will cause the bridge to be “pulled” in the direction in which the train is traveling. This is because the braking force experienced by the train is actually provided by the bridge. The bridge therefore experiences an opposite force that is equal to that experienced by the train. Those forces are quite large (just imagine how much force is needed to slow down such a colossus), but are spread over a long (braking) distance.

* if the train accelerates on the bridge, the friction will push the bridge backwards, with the same force as the train is pushed forward.

These forces can all be calculated theoretically. Fortunately, the bridges are built in such a way that they can handle all these forces without any problems (a safety factor is then taken into account). By choosing the construction of the bridge correctly, most of the forces will be transferred to the bottom and the flanks between which the bridge stands, without you seeing anything move or being able to measure (with a lath and the like). It’s only when you make the bridge just strong enough that you’ll be able to see such things. Of course, with a lot of (sophisticated) measuring equipment it is possible to measure which forces the construction undergoes.

It is therefore both demonstrable (by making theoretical calculations) and measurable (by special equipment).