I don’t think the iPhone does naive dead reckoning like that, as the sensitivity of the needed inputs is so high for it be reasonable in a consumer handheld. The cost of components would be way to high, then there’s the issues of power consumption, because you would need to poll all your sensors at a very high rate to get enough accurate data to perform naive dead reckoning accurately.

Instead I think the iPhone cheat a bunch, and use a mixture of step counting, magneto and gyro inputs to perform dead reckoning. The iPhone uses periods when you GPS is in use anyway to calibrate your stride length at different speeds, which when mixed with magneto data to estimate a course direction, would allow an iPhone semi-accurately perform dead reckoning, without needing high resolution, wide scale, accelerometer input.

In short, the iPhone cheats. It’s dead reckoning systems take advantage human bio-mechanics to simplify the problem of dead reckoning, at the cost of building a dead reckoning system that only works effectively in a handheld device that semi-permanently lives on your person. A perfectly reasonable assumption for a phone, but a terrible solution for a more generic dead reckoning system.

Any 1000g acceleration is going to be irrelevant to the position of the user of the phone (or they’d be dead!)

If my math is right, assuming an iphone deforms linearly on impact with 1000g deceleration, it would only give way 0,7mm when dropped from 1m height onto concrete. that seems too low a deformation? I would expect it to whobble quite a bit in slow-mo.

> I highly recommend just getting that magnetic ring instead.

But wearing a ring, while convenient and safer, is not biO hAcKiNg. It gives no bragging rights.

Based on a quick search, a small gap (less than 500 μm) between the implant and bone can actually support osseointegration by allowing trabeculae to form. 3D printed titanium has been used a lot for osseointegration.

I tried this once. There was no convincing effect for being able to "detect by feel" AC currents, but you could definitely notice magnetic materials.

Maybe the most fun, though, was just playing around and "picking up" paperclips and tiny screws by flicking my finger at them.

Magnetic force drops off with the 4th power of distance. What is a slight wobble is centimeters away from being a bullet. This is pretty easy to experiment with fridge magnets to build intuition. I would exercise caution if you have embedded magnets though.

"Don't take in ferromagnetic metals that would get pulled by a 3T magnet with 6 inches of separation" is a bit harder to grok for pretty much everyone.

What matters is the field gradient. The field inside the core of the MRI is pretty homogenous, so if the metal is in there when it's switching on/off then it's fine. They will put a precautionary shield over embedded shrapnel, but it's not a big deal- it'll just wiggle a bit, and can cause some image distortion. Heating would be caused by imaging gradients, which are very small precise fields layered over the main field. Imaging gradients can switch very rapidly, which is what could cause heating, but they're so weak (fridge magnet range) that it's a nonissue.

Likewise if you're more than about a meter away, the gradient has also dropped off and forces are much smaller.

Imagine a bubble around the entrance to the core: that's the danger spot where field gradients can be >3 T/m. Fun fact: take B^2/(2*u0) and you get units of Pascals. Magnetic pressure and fluid pressure can be thought about pretty similarly, except that free space is very impermeable and things like iron instead act like "holes" that can create strong flows between areas of different magnetic pressure. You can have a 3 T field separated from you by air, but if you stick a steel rod into it suddenly both ends of the rod will be at 3 T.

3 T works out to about 500 psi of magnetic pressure. If you have a small, 5 mm iron sphere in a 3 T/m gradient, the pressure difference across it is only ~2.5 psi. Size matters a LOT. 2.5 psi may be a slight tug; 50 psi may pull a wire straight through you. And 10 psi may be fine when it's embedded in you already, but given a distance to accelerate it can go pretty fast.

Are you trying to tell me that the anal rail gun story isn’t true? I’m shocked.

Are you real-life Wolverine?

Was the MRI needed to hard reset the NFC chips?

What’s it like having magnet in a fingertip?

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4848040/

> The static magnetic field B_0 of an MRI machine attracts ferromagnetic objects and accelerates them toward the center of the bore of the MRI scanner. Ferromagnetic objects such as coins, hairpins, steel oxygen tanks or scissors can be accelerated or torqued by B_0 [4,10] and become dangerous projectiles [51]. The MRI safety program of the facility needs to warn about the misconception that larger objects will resist attraction to the field and need to emphasize the relationship between object size, material components, and projectile risk. Insufficient MRI safety training of ancillary medical personnel has led to fatal accidents when medical and other equipment was accelerated into the bore of the magnet [50,51].

X-rays would be no problem. Having something that is attracted by a magnet (such as another magnet) may cause an issue.

https://pubmed.ncbi.nlm.nih.gov/38128958/ has more on the "stuff flying into an MRI with concerning force."

Things like a 10p coin imbedding itself 0.5cm into ballistic gel. A spoon going in 3.5 cm is a bit more concerning.

I don’t have an implant, but I have purchased at times copper and metal shirt collar stays.

Once after falling on ice, I had a chest x ray where my shirt was unbuttoned but didn’t need to be removed.

A very anxious tech came to me a few minutes later asking if I had been wounded when I fell, if there was something embedded in me, etc.

Very quickly figured it out when shown the images.

Incredibly comical, and obviously not an issue. (Nor related to implants)

I would love to see how TSA reacts to your scans