I have used a couple of online calculators to estimate the attractive force between two cylindrical bar magnets. Using d=1"; t=1"; x=0.1"; Neo35 (Br=12,300 Gauss):

source 1 gives 63.5 lbf
source 2 gives 29.6 lbf

source 2 explains data is based on measured data, so this accounts for the difference from source 1. But in general, I think this is the right ballpark number.

I then tried to reproduce these numbers myself, using a formula I found on Wikipedia. I could not get close to the right answer - my answer is about 4000 lbf. Could someone please explain what I did wrong in my calculation?


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EDIT Tough crowd. Let me try and be more specific.

The problem itself is pretty straightforward. I did not really mean to imply I need someone to check the numbers. I mean more on if I am applying the right approach to the problem:

  1. Is the formula above valid when trying to replicate results in the calculators I linked? (i.e. "Calculate the Pull and Repulsion Forces Between Two Disc Magnets")

  2. Perhaps "real life" is significantly different than theory for this application? In which case, the engineering calculators would apply a de-rating factor that is not represented in the Wikipedia formula. I did find any references to this, but perhaps someone with more knowledge could advise?

  3. Assuming the formula is valid here, I think Magnetic Flux Density (B_0) is the most likely source of my error. It is my understanding that this is a physical property of the material. Is that correct? The value I found for Neo35 is Br = 12,300 Gauss (CGS units). I converted this to SI 1.23 Tesla using a 10^-4 conversion factor. Is this the correct interpretation of B_0?


closed as off-topic by Kyle Oman, ACuriousMind, Kyle Kanos, Ali, user10851 Jul 29 '14 at 0:18

This question appears to be off-topic. The users who voted to close gave this specific reason:

  • "Homework-like questions should ask about a specific physics concept and show some effort to work through the problem. We want our questions to be useful to the broader community, and to future users. See our meta site for more guidance on how to edit your question to make it better" – Kyle Oman, ACuriousMind, Kyle Kanos, Ali, Community
If this question can be reworded to fit the rules in the help center, please edit the question.

  • 1
    $\begingroup$ Why the downvote? Also, this is not homework (I rolled back the edit.) $\endgroup$ – Roberto Jul 28 '14 at 23:36
  • $\begingroup$ Hi Roberto. Welcome to Phys.SE. If you haven't already done so, please take a minute to read the definition of when to use the homework tag, and the Phys.SE policy for homework-like problems. $\endgroup$ – Qmechanic Jul 28 '14 at 23:39
  • $\begingroup$ @Qmechanic Thanks. In that case I now understand "homework" in this context. I still don't get the downvotes though; I clearly did some background research and attempted an answer. Am I missing something here? $\endgroup$ – Roberto Jul 29 '14 at 0:20
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    $\begingroup$ In particular, the policy states, "It's not enough to just show your work and ask where you went wrong. If you just need someone to check your work, you can always seek out a friend, classmate, or teacher." Even if you aren't in school and thus can't easily confer with someone in person, the fact remains we aren't a check-my-work site. As for downvotes, each user has their reasons, but for what it's worth this has far fewer downvotes than a typical "here's my homework, do it for me" question. $\endgroup$ – user10851 Jul 29 '14 at 0:21
  • $\begingroup$ @ChrisWhite - OK. Thanks for the clarification. To make it clear I am not looking for a "do it for me" response, I edited the post with specific questions. $\endgroup$ – Roberto Jul 29 '14 at 1:33