MadSci Network: Physics

Re: Why does the bottom sheet of cardboard stick to the top when lifted quickly

Date: Thu Nov 24 08:18:13 2005
Posted By: Calvin Cole, Faculty, Engineering Physics, Northeastern State University
Area of science: Physics
ID: 1132196838.Ph

     The really short answer is, “Most likely the bottom sheet doesn’t get
lifted because it is stuck to the top one.”  Your observation that the
bottom sheet is often lifted along with the top one is of course correct. 
It is an experimentally reproducible fact, the basis of all good science. 
The fun comes when we try to describe, understand or explain the fact.
      Things stick together in the sense that the surface of one object
actually exerts a force one the surface of another to hold them together
for a variety of reasons.  A few common ones are:  They are mechanically
interlocked, think hooks and loops in Velcro or buttons in button holes and
so on.  They “adhere” meaning the molecules in one are attracted to the
differing molecules of the other, water hydrogen bonds to the silicon
dioxide in well cleaned glass, or the sticky stuff on adhesive tape.  One
or both surfaces have a static electric charge or have been magnetized.  
     None of these really apply to your cardboard though so we’re back to
the short answer given above.  What you have discovered is the basis of
operation of suction cups.  If you play with it for a while you should be
able to see that the bottom sheet is being shoved up against the top sheet
by the force of the air beneath it, about 14.7 pounds of force for every
square inch of surface area.  It is only when the air has had time to leak
in between the two pieces and push down on the top of the bottom card as
hard as it has on the bottom of the bottom card all along that the bottom
piece no longer feels a force pushing up on it  and falls away.  Don’t try
to say that last sentence three times fast but read it carefully, sketching
a picture may help.   If you pick the top piece up quickly the experiment
should work much better than if you pick up slowly.  It should also work a
bit better if the bottom piece is resting on a piece of carpet or has
something beneath it to allow an open space for air to occupy before
lifting it.  Here’s one of my favorite variations on this.  Take a nice
slippery new deck of playing cards.  (An old one  is okay but you may get
better results with a new deck.)  Set the deck across a couple of pencils
to support it a short distance above the table top.  Attach a scotch tape
tab to the center of one card for a lifting handle.  If you lay one card on
the table and then pick it up with the card that has the handle attached
you should find it will sometimes “stick” for upwards of two seconds.  If
you place the lifting card on the deck and press then press the deck
together before lifting you will sometimes be able to lift the entire deck
for a fraction of a second! 

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