# Stopping a bullet with a field

1. ### Bacat

151
So, I was thinking a while ago about force fields. Don't stop reading though, hear me out.

It occurred to me that you could change the course of a bullet in midair if you had a sufficiently powerful electric or magnetic field. The problem, of course, is that it is probably not economically feasible to build such a shield. My question to the physics community is, for fun, to calculate how much energy would be needed to create a field that can do the following:

1a) Deflect a speeding bullet away from centerline of a human being's chest (assume this requires a deflection of 25 cm). This deflection must happen over no more than 1 m of bullet trajectory.

OR

1b) Slow down a speeding bullet so that if it strikes the human being, he does not suffer permanent injury (use your best judgment)

Assume that the bullet has a kinetic energy of 500 Joules and has a velocity of 350 m/s (see http://blog.dotphys.net/2008/10/bullets-have-a-lot-of-kinetic-energy-apparently). Assume the bullet is made of a material that is convenient for interaction with the field, but limited to metals commonly used in bullet manufacturing (such as brass, lead, copper, and steel alloys). You are welcome to ignore air resistance and turbulence effects to simplify the problem.

Once we get some estimates for the amount of energy required to generate such a field, I'd like to consider how this field could be generated from a backpack.

Alternatively, we could come up with some good reasons why this is infeasible using the principles of physics.

NOTE: I don't expect this thought experiment to result in a viable technology. Kevlar makes a lot more sense (and is cheaper) than whatever this field result will do. I'm just curious to consider the physics. If you disagree with doing a thought experiment for fun, don't bother posting.

835
3. ### Bob S

I think that a very large (extremely large) alternating transverse magnetic field will (nearly) stop it. Because the bullet is metallic, eddy currents will be generated in it when it enters a transverse magnetic field because of dB/dt = dB/dx * dx/dt = v dB/dx.

If the bullet were copper, 500 joules in a 8.16 gram bullet would heat it up about 159 degrees C (the heat capacity is 0.385 joules per gram deg. C) before it stopped.

Bob S

### Staff: Mentor

Mythbusters checked the idea.

5. ### Bacat

151
Bob, that sounds like what they use as breaks on levitating trains. I hadn't thought about an alternating field.

I like it!

7. ### Bob S

I was thinking about rapidly alternating transverse stationary magnetic fields from permanent magnets, such as those used in undulators used in synchrotron light sources:
http://www-xfel.spring8.or.jp/cband/e/Undulator.htm
This one has a maximum vertical field of 0.59 Tesla, and a period of 0.032 meters. So
By=0.59 sin(kx) where k = 2 pi/0.032 =196 radians per meter. So then (see my previous post)
dBy/dx = 0.59 k cos(kx)
The induced eddy currents (using Faraday Law induction of current loops in the metal) are given by the induced voltage squared, so the energy loss by the bullet is (see my prevoious post) proportional to vx [dBy/dx]2, where vx is the bullet velocity.
Bob S
 See Smythe "Static and Dynamic Electricity" 3rd edition Chapter X for a thorough discussion of eddy currents, and section 10.14 for calculating eddy currents in a rotating conducting disk in a stationary magnetic gap.

Last edited: Oct 23, 2009