Finally we have the crane itself. I let go of some of the insignificant forces here – the most critical are the forces from the magnet and the carriage pulling in opposite directions.
All these objects have a net force of zero. That’s the important part: with a net force of zero, there will be a zero acceleration. These three objects, starting at a speed of zero, will remain at zero. They will not move. It will not work.
I think part of the reason it’s hard to understand why this does not work is that you can not directly see the attractive interaction between the magnet and the carriage. That kind of makes it seem like magic – and magic can make things move.
So here’s a little variation of the magnet cart, which does not work either: Let’s say I keep the tap attached to the cart, but I replace the magnet and the iron with a rubber band. When stretched, a rubber band also produces an attractive force similar to the interaction between the magnet and the iron. But with the rubber band, you can actually see the interaction. Check it out:
It does not move, and I do not think anyone would expect that. But it’s pretty much the same as the magnetic car.
Wile E. Coyote understands physics
I like to use old cartoons as examples and in Looney Tunes Road Runner series, Wile E. Coyote is always trying to find new ways to catch the very fast-moving bird – or maybe a silly rabbit. Usually he makes mistakes, but once in a while he’s just right. Well, that’s right.
In the section below, he lays out some bird seed with extra iron. After the Road Runner has eaten it, Wile E. puts a magnet on himself so he can catch that bird.
Of course it does not work, because in the cartoon the magnet is attracted to a moving train, and then Wile E. crashes into it. And more realistically, the magnetic interaction in this case would produce an extremely small force that would not be enough to overcome friction.
The main idea, however, is at least based on something real. Have you noticed that with this Wile E. Coyote method, the magnet and the iron are attached to two separate things? That’s the important part. Remember that forces come in pairs. The magnet pulls on the Road Runner, but the Road Runner pulls the magnet back. Since the force from the magnet is on Road Runner and not on part of Wile E., this means that these forces do not cancel and he is able to zoom out to his own destruction.
In the end, when things look too good to be true, they probably are.