Finally, we have the crane itself. I’ve left out some unimportant forces here – the most critical is the force of the magnet and cart pulling in opposite directions.
The net force of all these bodies is zero. Here’s the important part: zero net force and zero acceleration. The three objects that start with zero velocity will remain at zero. They won’t move. It doesn’t work.
I think part of the reason it’s hard to understand why this doesn’t work is that you can’t directly see the attractive interaction between the magnet and the cart. This makes it look like magic – and magic makes things move.
So here’s a slight variation on the magnet cart, which doesn’t work either: let’s say I fasten the crane to the cart, but I use rubber bands instead of magnets and iron. When stretched, the rubber band also creates an attractive force similar to the interaction between a magnet and iron. However, with a rubber band, you can actually see the interaction. check it out:
Photo: Rhett Allen
It doesn’t move, and I don’t think anyone would expect it to move. But it’s essentially the same thing as a magnet cart.
Wile E. Coyote Understanding Physics
I like to use old comics as an example, in Looney Tunes runner In the series, Wile E. Coyote is always looking for new ways to catch a fast-moving bird — or maybe a goofy rabbit. Usually he does something wrong, but every once in a while he happens to be right. Hmm, kinda right.
In the episode below, he put some bird species with extra iron. After Road Runner eats it, Wile E. attaches a magnet to himself so he can catch the bird.
Of course, this doesn’t work because in the cartoon, the magnet is attracted to the moving train, so Wile E. hits it. And, more realistically, in this case, the magnetic interaction creates an extremely small force, not enough to overcome the frictional force.
However, the main idea is at least based on the real thing. Did you notice that with this Wile E. Coyote method, the magnet and the iron are attached to two different things? This is the important part. Remember that forces come in pairs. The magnet will pull the Road Runner, but the Road Runner will pull the magnet back. Since the power of the magnet is in some parts of Road Runner and not Wile E., which means those forces don’t cancel out, he is able to amplify to his own destruction.
In the end, when things seem too good to be true, they probably are.
