Why can’t a magnet-driven truck really work?

Finally, we have the faucet itself. I’ve omitted some of the unimportant forces here – the most critical being the forces of the magnet and the cart pulling in opposite directions.

All these objects have a net force of zero. That’s the main thing: with a net force of zero, there will be zero acceleration. These three objects that start at zero speed remain at zero. They won’t move. It will not 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. In that way it looks like magic – and magic can move things.

So here’s a little variation on the magnet cart that doesn’t work either: Let’s say I’m holding the faucet to the cart, but I’m replacing 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 the iron. However, with the rubber band you can see the interaction. See:

Photo: Rhett Allain

It doesn’t move, and I don’t think anyone would expect it to. But it is essentially the same as the magnetic 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 huh gets things wrongBut sometimes he’s just right. Well, somewhat right.

In the episode below, he puts out some birdseed with extra iron. After the Road Runner eats it, Wile E. attaches a magnet to himself so he can catch that bird.

Of course that doesn’t work, because in the cartoon the magnet is 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 small force that would not 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 is the most important. Remember that forces come in pairs. The magnet pulls on the Road Runner, but the Road Runner pulls back on the magnet. Since the magnet’s power is on the Road Runner and not on any part of Wile E., that means those powers don’t cancel out and he’s able to zoom out to his own destruction.

Ultimately, if things seem too good to be true, they probably are.

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