# What's wrong with this car model? [duplicate]

Even if every possible attempt to avoid stuff that can be avoided (like it will topple, the connectors will break, it won't overcome the rolling resistance, etc, etc) is made

What if one of the magnet was replaced by iron or something?

• Nothing wrong. It can be only tow with a third magnet – Alchimista Dec 8 '17 at 14:09
• Now, the donkey and carrot setup will work if the donkey is hungry enough. – Lambda Dec 8 '17 at 15:27

The left magnet is pulled or pushed with some force $F$ by the other one, which experience the opposite force $F$. They stay in place because they are held by the struts in the car. So the struts holding the magnet experience the force the magnet experience, and exert an equal counter-force... which means that the magnets will not be accelerating (they are held still). But now the struts will exert forces on the parts of the car holding them, and so on. As you go around the loop this sums to zero: every part is safely held by other parts so that the forces balance. So the net force on the entire car is zero: it will not accelerate.