Cookie Lab
Adam Seering

  In this lab, we determined the amount of force necesary to separate the two halves of a cream-filled cookie.

  The idea behind this lab was to better understand the concept of force of separation, by trying to pull two halves of a cookie apart.  It was a fairly straightforward, and simple, lab.  The main difficulty was in designing the mechanism to hold the cookie in place while it was being pulled; most of the effort that we put into this lab went into this, as all of the other steps were quite simple and easy.  Their simplicity does not reduce their benefit, however; this lab effectively and tastily demonstrated the effects of a steadily increasing force on a layered object; namely, a cookie.

  The first step of the lab was to build a harness in which to hold the cookie.  We had to build this harness so that one side of the cookie could be attached to the force probe, and we could hold on to and pull on the other side.  Also, it had to pull the cookie cleanly apart, instead of just breaking off parts around the edge, and it could not damage the cookie filling, because doing so would make the filling weaker and invalidate the force measurement.  We tested several designs, and used the most effective of these for the test.

  Here is a picture of one side of our harness:

  The other side was the same; it was just difficult to get it on for the picture, without attaching it to something to keep it under tension.

  As you can see, the harness works by having the straight ends of partially unbent paper clips wrap around the edge of the cookie; a piece of string connects the opposite end of each paperclip together so that they can be pulled simultaneously and evenly.  The unwrapped ends of the paperclips touch the cream filling, but they don't push into it, so they don't affect the integrity of the connection.

  We then set up the force-measurement system.  We attached the cookie to a force probe, and set up a computer to graph force as measured by the probe vs. time.  We then pulled on the side of the cookie not connected to the probe until the cookie separated.

  Here is the resulting graph from that activity:

  As this graph shows, we exerted a steadily increasing force for the first second; at exactly one second after graphing began, we were exerting 5N of force on the cookie.  At this point, the cookie cleanly separated in half; we had the probe set up to measure only horizontal force, so when the half connected to the force probe fell vertically downward, the probe registered a net horizontal force of zero.

  This means that Oreo cookies can withstand an outward force of 5 newtons before they separate.  This, of course, assumes that the cookie is not being twisted; if it is, you have a whole different situation.  But with a straight, simple linear force, one can pull apart a cookie with 5N of force.

  That is, of course, assuming that the cookie doesn't desintegrate first.  The major difficulty with this lab was keeping the cookie intact and uneaten until it was time to test it, and even then, to only have the cream break, as opposed to part of one of the two crunchy halves.  We went through a number of cookies trying to get one to work right.  But other than this, there were really no significant sources of error in this lab.  Short of the cookie breaking improperly (or having an unusually weak or artificially weakened cream filling), there really was no major source of error in this lab.  If many of the cookie measurements are different, I would account for that by simply stating that all cookies are different; two cookies are rarely exactly identical, so they would probably tend to have at least slightly differing properties.