Stopping the Unstoppable: When an unstoppable force meets a dashpot snubber

The snubber only has resistance in one direction. There's a check valve that lets air flow easily in the other direction. An ordinary dashpot without a check valve provides the same damping in both directions.

Dashpots and snubbers are not used much as emergency devices. They are for repetitive use. When you desperately need to dissipate a lot of energy, but collisions are rare, expendable devices are usually used. The article shows a pile of sand. In the early days of BART, the control system was so bad that large piles of sand were put at the end of each line for years. Truck runaway areas on mountain grades are piles of sand. The barrels seen at the gore of freeway exits are filled with sand.

Another trick is to force a collision deform metal. When US cars had a 5 MPH bumper standard, the energy absorbing device was often a metal pin forced through a too small hole. The front ends of cars are designed to crush without damaging the passenger compartment. That's why modern car accidents usually show a crushed front end, yet even the windshield is intact.

The buffer is the final stage of an emergency stop. Something else has to handle the slowdown phase. One of the more impressive solutions is how high speed trains stop. They have a set of permanent magnets above the rails, held clear of the rails in normal operation. In an emergency, they drop to just above the rails, where they act as eddy current brakes. The energy of the moving train goes into heating the rails, which can take a lot of heating, rather than brake shoes, which can burn up. Braking force increases with speed. No wear. Unaffected by rain, snow, leaves, or loss of power. Roller coasters also use eddy current brakes.

But because eddy current braking force increases with speed, it's nearly zero at the end. You need something else to get to a complete stop.