Moving metal object in a magnetic field

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Discussion Overview

The discussion revolves around the interaction between a moving metal object, specifically a bullet, and a magnet, focusing on the energy implications of this interaction. Participants explore concepts related to magnetic forces, energy transfer, and the effects of mass on motion within a magnetic field.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions whether the change in trajectory of the bullet due to the magnet's attraction requires energy and, if so, where that energy originates.
  • Another participant draws an analogy to gravity, suggesting that the magnetic force acts on both the bullet and the magnet, implying that the Earth's motion may change slightly as a result.
  • A participant highlights the possibility of the bullet being made of non-magnetic material, such as copper, and references Lenz's Law to explain how it could still be affected by the magnet due to its motion in the magnetic field.
  • There is a reiteration of the energy question, emphasizing the need to focus on energy rather than just forces at play.
  • One participant asserts that if the magnet is fixed to the Earth, the energy required to change the bullet's trajectory comes from the Earth, as the Earth's position or rotation changes imperceptibly due to the force exerted by the bullet's interaction with the magnet.
  • Another participant compares the scenario to a ferrous ball being attracted to a magnet bolted to a table, suggesting that the energy transfer is similar in nature.

Areas of Agreement / Disagreement

Participants express differing views on the energy dynamics involved in the interaction between the bullet and the magnet. There is no consensus on the specifics of energy transfer or the implications of mass on the system's motion.

Contextual Notes

Participants have not fully resolved the assumptions regarding the material properties of the bullet or the extent of energy transfer involved in the interaction.

alikim
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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?
 
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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.
 
I understand that there are forces at play there. I'm asking about energy, see my OP.
 
scottdave said:
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.
alikim said:
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.
 

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