[Physics] Doesn’t magnetism violate Newton’s First Law

electromagnetismnewtonian-mechanics

If you place two magnetic objects close to each other, they will attract or repel depending on the position. Yet no energy is added to this system, so how is it possible?
Another example is a levitating magnet in a box. It fights against the force of gravity without any force put into the system itself.

Please explain

Best Answer

These cases have nothing to do with Newton's first law. Newton's first law is:

The first law states that if the net force (the vector sum of all forces acting on an object) is zero, then the velocity of the object is constant.

(It's stated in many ways. That's the phrasing used on Wikipedia)

Note that the first law says absolutely nothing about energy or what forces are allowed to be on the system. All it states is that if the sum of the forces on an object is 0, its velocity doesn't change.

Ignoring Newton's first law, here's the things you were asking about in your examples:

  • When you place two magnets close together and they attract/repel, the energy comes from the potential energy in the magnetic field. You put that energy into the system as part of the process of placing those magnets into place.
  • Opposing a force can be done without expending energy. You will learn that the work done on an object is equal to the force applied to it times the distance it moves. In the case of a levitating magnet, it's not moving at all, so there's no work being done, meaning no energy is needed.
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