User Name Remember Me? Password

 Electricity and Magnetism Electricity and Magnetism Physics Help Forum Jul 21st 2017, 05:06 AM #1 Junior Member   Join Date: Jul 2017 Posts: 2 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?   Jul 21st 2017, 05:43 AM #2 Senior Member   Join Date: Aug 2010 Posts: 434 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. OmarIbrahim likes this.   Jul 21st 2017, 06:38 AM   #3
Physics Team

Join Date: Apr 2009
Location: Boston's North Shore
Posts: 1,576
 Originally Posted by OmarIbrahim 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   Jul 22nd 2017, 06:10 AM   #4
Junior Member

Join Date: Jul 2017
Posts: 2
 Originally Posted by HallsofIvy 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    Tags bullet, deflecting, electromagnet, magnet, stop Thread Tools Show Printable Version Email this Page Display Modes Linear Mode Switch to Hybrid Mode Switch to Threaded Mode Similar Physics Forum Discussions Thread Thread Starter Forum Replies Last Post fbaker Kinematics and Dynamics 1 Aug 17th 2010 10:34 PM kubombelar Kinematics and Dynamics 1 Nov 10th 2009 01:05 AM rjsci13 Kinematics and Dynamics 2 Sep 28th 2009 07:52 PM lindsayboyle Kinematics and Dynamics 1 Apr 16th 2009 10:07 AM bfritz88 Kinematics and Dynamics 0 Dec 5th 2008 02:10 PM