|MadSci Network: Physics|
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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