Car mounts are a great point, thanks! I hope those magnets are strong though.
How fast you ask?
Why itβs an inverse square law! Like gravity. 1/r^2.
At the risk of being pedantic (or misremembering college classes), there's more to it than that. First, at the relevant distance scales, the magnets aren't point particles, they have an extent and very close to that surface it should be 1/r. Secondly, magnets are dipoles and at greater distances the two poles cancel out to first order and you have a 1/r^4.
Additionally, it looks like the goal is to let your phone be held vertically, while attached only to the magnet. This makes sense for car mounts, where you may want to mount your GPS via magnet and charge it simultaneously.