Moving metal object in a magnetic field

[alikim]

Let's say I have a bullet flying by a magnet, the magnet will attract the bullet and change its trajectory so it will turn slightly. Does that trajectory change take energy? If it does, where does it come from? The kinetic energy of the bullet?

 

[scottdave]

It's just like gravity, the attractive force acts on both objects. So if the magnet attracts the bullet with a force, the magnetic force is also pulling on the magnet. If the magnet is bolted to the earth, then the Earth's motion may change slightly. But since the Earth is much more massive than the bullet, the change will be barely noticed.

I don't know if you are assuming that the bullet is made of a magnetic material. It could be something like copper, but you can still have it affected by the magnet, due to the conductor's motion in a magnetic field. See Lenz's Law.

 

[alikim]

I understand that there are forces at play there. I'm asking about energy, see my OP.

 

[berkeman]

If the magnet is bolted to the earth, then the Earth's motion may change slightly. But since the Earth is much more massive than the bullet, the change will be barely noticed.

I understand that there are forces at play there. I'm asking about energy, see my OP.

He did answer your question. In the first case the energy comes from the Earth. There is a force applied through a small distance (the change in the Earth's position and/or rotation), which is equivalent to the work done to change the trajectory of the bullet.

It's the same as when you bolt a magnet to a table and place a ferrous ball nearby. The ball is pulled to the magnet because the Earth is exerting a force on the magnet which changes the Earth's motion imperceptibly.