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# Bridge held up by the cars that cross it

Use the energy of the vehicles that cross the bridge to counteract the force of gravity
 (+1, -1) [vote for, against]

Think about it: a bridge that collects energy from cars to hold itself up. Basically the bridge is a giant treadmill, so that the cars that move across it generate energy. When you go onto the bridge, there's a bunch of little rollers linked to generators built into the road. Driving across them would "steal" some of your speed and pipes it into turbines on the bottom of the bridge that push it up. Energy could also be stored for later, and might even be supplemented by solar panels.

The resistance of the rollers can also be adjusted individually, so speed limits can be strictly enforced. Going to fast? Nope! You're just donating more energy to the bridge.

Once electric cars take over, and this idea becomes the norm (as it inevitably will), power can be pulled directly from the batteries of the cars.

If you think about it, this isn't that much different than normal bridges. Typically, when you pay to cross a bridge, you're really paying for the materials whose energy keeps the bridge standing. In this way, the money you spend on gas that goes to powering the bridge is paying for the energy needed to keep it standing.

 — mace, Oct 24 2017

Why bother with the bridge with that much energy? [RayfordSteele, Oct 24 2017]

 We suggest that you study the aerodynamics and energetics of helicopters, and then redraft the idea.

 A typical car has nowhere near the power-to-weight ratio to lift its own mass.

If it could, then it wouldn't need a bridge; it could fly across.
 — 8th of 7, Oct 24 2017

So, who gets to be the lucky car to cross it first?
 — RayfordSteele, Oct 24 2017

[8th of 7] huh. That's a great way of thinking about it. I knew my idea seemed wrong, but couldn't figure out how. Thank you for proving this to be truly half-baked
 — mace, Oct 24 2017

Also, why design a bridge that needs constant power to hold itself up, rather than a simple bridge that just sits there?
 — MaxwellBuchanan, Oct 24 2017

Next you'll be telling me that the sea is held down by the boats that cross it.
 — Wrongfellow, Oct 24 2017

The approaching car spins its wheels on the approach-ramp rollers until it has gifted enough energy to pump out the submerged pontoons. It then has enough time to cross the water before the precision-engineered leaks re-sink them.
 — pertinax, Oct 25 2017

//A typical car has nowhere near the power-to-weight ratio to lift its own mass// - this sounds right, but intuitively, I would think that a car has sufficient power to, for example, power an air pump that could keep a bridge constructed with 'bouncy castle' technology inflated to a high enough pressure to support its weight.
 — hippo, Oct 25 2017

You could use the treadmill method for powering a transporter bridge.
 — Gordon Comstock, Oct 25 2017

Pontoons that a car can be driven onto and is then propelled by power from rollers under the rear wheels have been built, but are not WKTE.
 — 8th of 7, Oct 25 2017

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