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So I saw this picture on my google+ feed and I immediately know why it wont work. But I'm having trouble explaining to myself and others exactly why. Without using anything overly complex, can anyone explain the flaw.
Magnet pulling truck
P.S. Please don't say it won't work because of the law of conservation of energy. I know it can't work, I want to know why this specific scenario won't work.
Thanks for the help.

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Newton's third law.

For the force the magnet exerts on the metal, there must be an equal an opposite force on the magnet exerted by the metal. Since both form one system (metal + truck + magnet), the net force on the system is zero, and it won't move.

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