1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

B Is it possible to stop a bullet by just using a magnet?

  1. Jul 21, 2017 #1
    Stop it or deflect it? and what are the laws and the effects that I need to know to determine something like this?
  2. jcsd
  3. Jul 21, 2017 #2
    Stopping it would definitely be a technological fete. But, in principle you could do that.
    Although lead is not considered a magnetic material, it can be moved by a magnetic field.

    To give you a sense of what might be required: Set the experiment up in orbit. This eliminates the problem of having to actually hold the bullet up against gravity when the bullet stops. The vacuum will also make it clear that only the magnetic field is responsible for changes in the bullets velocity. Then fire the bullet through a tube that will match the bullet's trajectory. All along the tube would be powerful electromagnets that would be activated as the bullet passed by - to slow the bullet and keep it in from striking the sides of the tube.

    If the bullet has a brass jacket, the effect from the magnets will be better. Here are links to pages with videos showing the effect of magnets on lead and brass.:
    Last edited: Jul 21, 2017
  4. Jul 21, 2017 #3


    Staff: Mentor

    I like @.Scott 's scenario. But in space, there is no minimum relative velocity for a bullet. So it is trivially easy to stop something moving arbitrarily slow. :approve: I could just hold the magnet and let the bullet hit it. Come to think of it, a big heavy magnet can stop a fast bullet if you use the magnet for the target.

    OP, you need more effort to properly phrase your question here on PF.
  5. Jul 21, 2017 #4


    User Avatar
    Science Advisor

    If you fire a metal bullet through a bunch of magnetic coils, eddy current braking should slow down the bullet some, and there will also be a reaction force on the magnets, so they need to be rather robust. But, since bullets are small (which reduces the size of the eddy currents) and very fast, I'm not sure it will be enough.
  6. Jul 21, 2017 #5
    Mythbusters has an episode on that topic, where they try to deflect a bullet with a magnet. The best they manage is by lining up a rail of super strong permanent magnets and then shoot the bullet a millimeter above the rail along it. The deflection they get is still tiny and it grazes the last magnet.
  7. Jul 21, 2017 #6


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    I remember that episode
    If you hadn't mentioned it, I was going to
  8. Jul 22, 2017 #7
    Well, you would have to have an insanely powerful magnet or otherwise increase the duration of the bullet in the magnetic field such that over time the force would eventually overcome the bullets momentum, such a magnet probably does not exist at the moment and since you need a stronger magnet to increase the force field range or in the case of the myth busters more magnets in the path of the bullet I suggest you take this up with science fiction until technology advances a little more.
  9. Jul 22, 2017 #8


    User Avatar
    Science Advisor
    Gold Member

    The deflection was just as likely to have been caused by aerodynamic effects .
  10. Jul 22, 2017 #9
    Thant you guys, I was working on scientific research about it but I need to take it to science fiction till technology advances
  11. Jul 22, 2017 #10
    what about lenz law ?
  12. Jul 22, 2017 #11
    I disagree. If bullets were that susceptible to air fluctuations you couldn't use them for anything.
  13. Jul 22, 2017 #12
    Lenz Law explains why it is possible to stop a non=magnetic, but electrically conductive bullet with a magnetic field.
    As everyone agrees, the issue here is one of technology.
    Also, if you don't put this thing is a zero-G environment, the magnetic field will only be able to stop the bullet's horizontal velocity. Attempting to stop its fall due to gravity would involve large eddy currents - likely strong enough to quickly melt the bullet.
  14. Jul 22, 2017 #13
    In this Mythbuster segment, they rated a James Bond magnetic deflection device as "Busted":

    I couldn't find one involving a rail.
  15. Jul 22, 2017 #14
    Josh you could have the strongest magnet in the universe but magnetic forces do no work thus they cant change an objects kinetic energy,
    Last edited by a moderator: Jul 22, 2017
  16. Jul 22, 2017 #15
    Unfortunately Zachary I strongly disagree as I believe that is not a correct analysis as magnets can do work its just gravity that does no work when an object is at a certain distance to the larger body but since a magnetic field is able to pull a body towards it as work is defined as force * distance in the direction of the force a magnet is by definition is able to do work just check out this site https://van.physics.illinois.edu/qa/listing.php?id=17176 .And on top of that kinetic potential energy is defined as 1/2 mv^2 because this formula includes velocity which is a vector quantity anything that changes the direction of an object and/or the motion therefore changes its kinetic energy. . But my point was that the force needed to stop abullet using a magnet is not achievable right now. :oldbiggrin:
  17. Jul 22, 2017 #16


    User Avatar
    2017 Award

    Staff: Mentor

    Magnets don't do work on electric charges. This thread is not considering electric charges, and to deflect something you don't have to change the speed of the object.

    A very rough estimate:
    Let's be optimistic and make the bullet itself a strong magnet. 1 cm3, 1 T, 7g. It moves towards a target at 300 m/s, quite slow. Ignore aerodynamics. How can we deflect it? It won't feel a force in a homogeneous field, we need an inhomogeneous field. The best quadrupole magnets in the LHC can provide a peak field gradient of about 200 T/m over a few centimeters. This leads to a force of 160 N. If we can let the bullet travel along 1 meter of these magnets, we get this force for 3.3 milliseconds, for a final vertical velocity of 75 m/s and a deflection of 13 cm.
    That might be enough to make a bullet miss a head, for example. But it means the bullet has to hit the central spot of a meter-long tube with a deviation of not more than 2-3 centimeters. If we know the bullet will hit exactly there, it is much easier to just move the head out of the way.

    If you surround you with a big coil, 10 T/m over a meter is a more realistic (but still optimistic) field gradient, and then you get a deflection of less than a centimeter - towards the coil. On the positive side, this big coil will stop the bullet mechanically, and you don't even have to power it for the mechanical shielding effect...
  18. Jul 23, 2017 #17


    User Avatar
    Science Advisor
    Gold Member

    A projectile skimming very close to a surface certainly can be deflected by aerodynamic effects . Trying to work out which way it will be deflected is the hard part .
  19. Jul 23, 2017 #18

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2017 Award

    This is getting close to "I'll give you this swell magnet if you promise not to shoot me."
  20. Jul 23, 2017 #19
    I always forget that this is applicable to charges. and that magnetic torque on a current carrying loop can do work, now whether this applies to bullets....doesnt seem very likely that one could consider a general bullet to have any amount on non-neglible circulating current...however if one were induced....then bullets would start being made from non conducting materials...and would circumvent any electromagnetic countermeasures
  21. Jul 23, 2017 #20
    also a non-uniform B field will exert a force on any dipole moment , doing non-zero work
  22. Jul 24, 2017 #21
    The figure below shows the force stopping a 9mm bullet as it penetrates ballistic gelatin. The stopping distance is about 30 cm. The peak force is about 10,000 N. Of course, the force is greatest right after the bullet impacts and begins to penetrate. Impact energy is close to 600 J.

    I like to simplify the physics of stopping bullets into work using the work energy theorem. The average force (in N) times the stopping distance (in m) needs to equal the initial energy of the bullet. If the initial energy is 600 J (typical of pistol bullets) and you can exert a force of 10 N, you need to exert the force for a distance of 60 m to stop the bullet. Exerting a magnetic force over 60 m is hard. Exerting a magnetic force of 10 N on a non-magnetic bullet (copper jacketed lead) is hard. Increasing the force to 600 N to reduce the stopping distance to 1 m is hard. Increasing the distance to 600 m to reduce the force to 1 N is hard.
    9mm115DPX Force Dual Scale.png
  23. Jul 24, 2017 #22
    Its ok that's why we are here to learn, right? And that's why science is so interesting because we never stop learning and improving our understanding of it and it continuously grows and surprises us.
  24. Jul 25, 2017 #23
    As Anorlunda stated, you could use a Magnet to stop the bullet, assuming it is strong enough not to break when the bullet hits it, which would be independant of the Bullet's material.

    However, bullets come in a variety of materials, some of which would not be influenced by a magnet, for example a bullet made of plastic or rubber or even copper, so the clear answer to your question is No.

    The only other way I can think of to stop a bullet using a magnet would be to insert the magnet into the barrel of the gun, that would probably stop the bullet.
  25. Jul 25, 2017 #24
    short answer is yes, magnets can stop a bullet. how you do it needs engineering, physics, and a transfer 180 degree vector force equal or greater than the KE of the bullet. with losses, you'll need a ton more input energy to get the net transfer needed.
  26. Jul 25, 2017 #25
    Yes you can deflect a bullet with a magnet but only if you set the magnet in the path of the bullet and let the bullet strike the magnet.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted