What physics concepts support the observation of the book and paper falling together?

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If you place the book on the desk and a piece of paper on it, if there is no wind turbulence it will stay there until the cleaning lady comes.

Why? because the gravitational force keeps it there, and in addition frictional forces, i.e. electromagnetic interactions between the paper molecules and book molecules . This would be true in vacuum too. Atmospheric pressure on top of the paper and "vacuum", i.e. displacement of air between the book and paper contact surface adds to the stability.

When both fall, unless a turbulence lifts a side of the paper and air enters underneath, they will fall together. If falling from a great enough height, turbulence will separate them inevitably. Just needs some air to intervene between the two surfaces.

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Updated on February 09, 2020

Comments

  • user73782
    user73782 over 3 years

    you place a sheet of paper on a book, it falls with it rather than being separated & slowed by air resistance. What physics concepts support the observation of the book and paper falling together?

    • ACuriousMind
      ACuriousMind over 8 years
    • DanielSank
      DanielSank over 8 years
      Newton's law: $F=ma$. Force of gravity: $F = m g$ where $m$ is the object's weight and $g$ is a constant which depends on the size of the thing generating the gravity (i.e. the Earth). Therefore $a = F/m = g$, so the acceleration of the falling object does not depend on it's mass. The reason a book normally falls faster than paper is that there is another force which we ignored here, air resistance.
    • N. Virgo
      N. Virgo over 8 years
      I think the comments and close votes are misunderstanding the question. I interpret it as saying that if you place a sheet of paper on a book, it falls with it rather than being separated and slowed by air resistance.
    • docscience
      docscience over 8 years
      Good catch Nathaniel - yes it is a different question.
    • user73782
      user73782 over 8 years
      yes Nathaniel. That is what I tried to ask. When a sheet of paper is placed on a book, it falls with it...
    • ACuriousMind
      ACuriousMind over 8 years
      Ah, I see. I have retracted my close vote.
    • Sofia
      Sofia over 8 years
      @user73782 they fall together because there is no air between them. In order to be acted upon by the buoyancy force the paper needs some air beneath itself, at the atmospheric pressure. But, what is beneath the paper is the book.
    • dmckee --- ex-moderator kitten
      dmckee --- ex-moderator kitten about 8 years
      This is an important demonstration. I show it even to the conceptual physics students. Book and paper (I actually use a basket-style coffee filter with a couple of paperclips in it) fall differently when alone. Then you allege that it is atmospheric drag prevent the paper from falling briskly. Then show that this is the case by having the book push the air aside as they fall one atop the other.