Finally, we have the crane itself. I left off some of the unimportant forces here—the most critical are the forces from the magnet and the cart pulling in opposite directions.
All of these objects have a net force of zero. That’s the important part: With a zero net force, there will be a zero acceleration. These three objects that start with a velocity of zero will stay at zero. They won’t move. It won’t work.
I think part of the reason it’s hard to grasp why this won’t work is that you can’t directly see the attractive interaction between the magnet and the cart. That sort of makes it seem like magic—and magic can make things move.
So here is a slight variation of the magnet-cart that also doesn’t work: Suppose I keep the crane attached to the cart, but I replace the magnet and iron with a rubber band. When stretched, a rubber band also produces an attractive force similar to the interaction between the magnet and iron. However, with the rubber band you can actually see the interaction. Check it out:
Photograph: Rhett Allain
It doesn’t move, and I don’t think anyone would expect it to. But it’s essentially the same as the magnet cart.
Wile E. Coyote Understands Physics
I like to use old cartoons as examples, and in the 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 gets things wrong, but every once and a while he just happens to be right. Well, sort of right.
In the episode below, he puts out some bird seed with extra iron. After the Road Runner eats it, Wile E. attaches a magnet to himself so that he can catch that bird.
Of course, it doesn’t work, because in the cartoon the magnet gets attracted to a moving train, and so Wile E. crashes into it. And, more realistically, in this case the magnetic interaction would produce an extremely tiny force that wouldn’t be enough to overcome friction.
However, the main idea is at least based on something real. Did you notice that with this Wile E. Coyote method, the magnet and 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 back on the magnet. Since the force from the magnet is on the Road Runner and not on some part of Wile E., that means those forces don’t cancel and he is able to zoom off to his own destruction.
In the end, when things look too good to be true, they probably are.