Cable Car Energy Conservation


I have a school project in which I have to build a "Cable Car" to transport people cheaply across a river. I was thinking of having something bent in the shape of a flat spiral, which, when the one car comes down tightens. Then when one needs to go back up again one needs only release the brake and Bon Voyage. I wonder if you could tell me in your opinion if this would work and if you could maybe give me some kind of idea of what something to use for the spiral etc.

Also if you don't think it would work could you possibly give me an idea on how to refine it.


You are thinking the right way. It is a good plan to have something store the energy created when your vehicle moves from a higher to lower elevation, so that it may be applied to the return trip. That is why real cable cars always come in pairs, one moving up while the other moves down. That way the prime mover only needs to supply the energy lost to friction and that arising from differences in weight of the loaded cars.

The idea of using a spring which a descending vehicle winds up has some problems if applied on a full scale, people carrying vehicle. The spring would have to be huge to store sufficient energy. Also springs are not totally free of internal friction so that considerable energy would be lost each cycle, heating the spring and causing problems. If we look at all the possible energy storing schemes: springs, flywheels, air pressure, electric storage batteries and so on and so forth, there is only one in which energy is not lost in the storage mechanism itself and that is gravity. There is essentially zero loss of energy stored in an elevated weight. Neglecting air resistance which is small for slow moving vehicles, all the energy that went into elevating the weight is available. If we are going to lift a weight we may as well make it useful by making the weight a second car.