# COMMUNITY : FORUMS : TECH

## Magnetic spec

I was wondering how big a cylinderical grade N42 NIB magnet i would need to pull a copper weight of 20 grams from 3inches? I dont know how to calculate so... If you could, that'd be great

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Javin0078 years ago
I don't know if this will be any help, but when they say "56 lbs force" they mean it. I bought two of the 56 lb 1"x1"x3" and received them today. After finally prying them apart, they slammed back together crushing my ring finger. The nail's already solid black. On impact they threw sparks and shattered. (I'm out \$30 pretty quickly.) I've now decided that magnets are scary, and I'm going to go back to playing with a glue gun and balsa wood.
westfw8 years ago
Late breaking news:
KJ Magnetics has been putting together graphs of "pull force vs distance" for the magnets they sell... I won't be terribly useful; they measure force in pounds, so a 1/2*1 inch magnet goes from 16 lbs at 0 to "essentially 0" at 1.2 inches, even though we know from experience that small forces extend well beyond that distance.
Now all you have to do is figure out what "pull a weight" means: just overcome sliding/rolling friction, lift it straight up (20g of force), something else?
PKM8 years ago
I don't know how to calculate
Neither do I, and I have an A-level and a bit of a degree in Physics. My suggestion is buy one of those big tubs of magnets from ThinkGeek and experiment.

"Static" magnetic is way harder to do simple calculations like this with than electricity.
bumpus8 years ago
Copper isn't magnetic..