This is a common problem given in first year physics majors. One of the interesting things is creating an repulsor field that would keep pretty much any matter outside of it in this way. There was a working (albeit accidental) example of one at a 3M factory for making tape [1]<p>[1] <a href="http:&#x2F;&#x2F;amasci.com&#x2F;weird&#x2F;unusual&#x2F;e-wall.html" rel="nofollow">http:&#x2F;&#x2F;amasci.com&#x2F;weird&#x2F;unusual&#x2F;e-wall.html</a>