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Old Jul 21st 2017, 06:06 AM   #1
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Post Is it possible to stop a bullet by just using a magnet?

Stop it or deflect it? and what are the laws and the effects that I need to know to determine something like this?
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Old Jul 21st 2017, 06:43 AM   #2
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A fundamental problem is that most bullets are lead and lead is not affected by magnets!

Assuming you have a ferrous bullet then the laws you need to use are "F= ma" and "F= K/r^2", where K is the strength of the magnet. Together those give ma= K/r^2 or a= K/mr^2. The deceleration of the bullet is directly proportional to the strength of the magnet and inversely proportional to the mass of the bullet and the square of the distance from the bullet to the magnet.

Writing v as the speed of the bullet at each moment, we have a= dv/dt= (dv/dr)(dr/dt)= v(dv/dr) so we can write the equation as v(dv/dr)= K/mr^2 and the v dv= (K/m)r^-2 dr. Integrating both sides (1/2)v^2= C- (K/m)r^-1. C is the "constant of integration" that depends upon the initial speed of the bullet. So v^2= 2C- (2L/m)r^-1 and v= dr/dt= sqrt(2C- (2L/m)r^-1), r= D+ integral of (2C- (2L/m)r^-1). D is another "constant of integration" that depends upon the initial distance of the bullet from the magnet. That last integral is an "elliptic integral" that cannot be done in terms of elementary functions.

The magnet will stop the (ferrous) bullet if v becomes 0 before r does.
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Old Jul 21st 2017, 07:38 AM   #3
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Originally Posted by OmarIbrahim View Post
Stop it or deflect it? and what are the laws and the effects that I need to know to determine something like this?
Maybe. It depends on what the bullet is made of and how powerful the magnetic field used to stop it is. It'd have to be a mighty powerful field. One so powerful that its never be created on Earth as of yet.

Magneto stops a bullet X-men: First Class. Could that be where your question come from? Myth busters addresses this question here: https://prezi.com/lcctn8mblfaf/mythb...eeding-bullet/

The answer will depend on whether the bullet is made of a magnetic material, which most are. T

Magnetizable material - Typically bullets (the projectile) are made of lead which is magnetizable. All magnetizable materials are effected by magnets and magnetic fields. A magnetic field can be used to create a magnetic force on a body which is composed of magnetizable material.

Conductive material - A conductive material can have eddy currents set up inside them. If a bullet is made of such a material then eddy current braking may be used to stop them.

There are two effects which can be used to either stop or deflect a bullet:

1) Magnetic Force: This may be how Magneto stops a bullet X-men: First Class. If so then by imposing a magnetic force on it by using a magnetic field on a bullet made of magnetizable a bullet can be either stopped or deflected.

For details please see: https://prezi.com/lcctn8mblfaf/mythb...eeding-bullet/

2) Eddy current brake - A device which can be used on non-magnetic material which is a conductor. Such a device can slow or stop a moving object by dissipating its kinetic energy as heat. For details please see: https://en.wikipedia.org/wiki/Eddy_current_brake

If the bullet is made of lead, which is a diamagnetic material, then it may be deflected or even stopped. The magnetic field required to do so would have to be huge in order to stop it in a short distance such as 1 meter. But lead is only weekly affected by a magnetic field.

The laws are called Maxwell's equations aka the laws of electrodynamics (ED). In ED there are two kinds of material: ferromagnetic, paramagnetic, and paramagnetic. Ferromagnetic materials are strongly attracted to magnets, paramagnetic materials are weekly attracted to magnets and diamagnetic materials are repelled by them.

For details see:
https://en.wikipedia.org/wiki/Ferromagnetism
https://en.wikipedia.org/wiki/Paramagnetism
https://en.wikipedia.org/wiki/Diamagnetism
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Old Jul 22nd 2017, 07:10 AM   #4
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Originally Posted by HallsofIvy View Post
A fundamental problem is that most bullets are lead and lead is not affected by magnets!

Assuming you have a ferrous bullet then the laws you need to use are "F= ma" and "F= K/r^2", where K is the strength of the magnet. Together those give ma= K/r^2 or a= K/mr^2. The deceleration of the bullet is directly proportional to the strength of the magnet and inversely proportional to the mass of the bullet and the square of the distance from the bullet to the magnet.

Writing v as the speed of the bullet at each moment, we have a= dv/dt= (dv/dr)(dr/dt)= v(dv/dr) so we can write the equation as v(dv/dr)= K/mr^2 and the v dv= (K/m)r^-2 dr. Integrating both sides (1/2)v^2= C- (K/m)r^-1. C is the "constant of integration" that depends upon the initial speed of the bullet. So v^2= 2C- (2L/m)r^-1 and v= dr/dt= sqrt(2C- (2L/m)r^-1), r= D+ integral of (2C- (2L/m)r^-1). D is another "constant of integration" that depends upon the initial distance of the bullet from the magnet. That last integral is an "elliptic integral" that cannot be done in terms of elementary functions.

The magnet will stop the (ferrous) bullet if v becomes 0 before r does.
Thanks for doing the hard part and explaining to me I will do it and try
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