I recently caught a question that I realized was far more difficult than expected. Someone asked how hard they would have to throw a Lego brick in order to break a window. The mass of a normal run-of-the-mill 2x4 Lego brick is around 2.3 grams. And looking at wikipedia, they guessed the tensile strength of compressed glass is 50 MPa (which is probably more than normal glass, comparing the value of concrete which was 3 MPa). My instinct was of course that this was sufficient data to calculate the speed necessary with which to hurl the brick at the window in order to break it.(adsbygoogle = window.adsbygoogle || []).push({});

Alas, F=ma, so we've actually got to know the acceleration of the Lego brick as it slows down-- the mass and speed are insufficient data (not even counting the fact that if it hits at some angle other than a perpendicular, it'll throw things all out of whack).

So what else would I need? I imagine there's some fancy calculations I might be able to perform if I knew the elastic constants of the materials involved, plus the thickness of the glass (that at least I could approximate), and possibly constants of elasticity for the specific shape of the Lego brick. But who's got time for that? How would you go about solving this problem in reality? Is there no other solution than by trial? Perhaps I could even get a high-speed camera to determine the approximate acceleration values, or some variety of fancy force meter for the glass, but that still requires some degree of trial (as well as equipment).

Are there any "reasonable" numbers you'd use to approximate? I realized quickly that I had no idea whether the time it took to decelerate would be in hundredths of a second or millionths of a second or somewhere in between-- which is quite a few orders of magnitude in difference!

DaveE

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# Calculating Force

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