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Homework Help: Few questions before exam

  1. May 2, 2004 #1
    I am looking over my review and a few questions I am lost on:

    1. The disk shaped head of a pin is 1.0mm in diameter. What is the best estimate for the number of atoms on the top layer of the pinhead.
    -I have no idea where to begin with this... any ideas?

    2.How does density fit in when the absolute temperature of a sample of monatomic ideal gas is doubled at constant volume?
    - pV=nRT , Where does the density fit in?

    3. WHat is the KE of a rock just before it hits the ground?

    - I used the conservation of energy and didn't come up with the right answer... the correct answer is KEi + mgh... I don't know how to derive this though?

    Thank ya.
  2. jcsd
  3. May 19, 2004 #2
    About the KE of the rock..remember that any loss in potential enengy is a gain in kinetic energy. At its peak just as it's getting ready to drop, all of its energy is potential. Think of it like this - potential energy is highest when the rock is where it doesn't want to be. So halfway down, it's energy is half potential and half kinetic. And just before it hits the ground, it's energy is all kinetic. Maybe you can use KE= .5m(v-squared). You might use the rock's weight to calculate its mass. You also might have to use your free-fall equations to find its final velocity.
  4. May 19, 2004 #3
    For #1, maybe use 10^(-10) m as an estimate of an atom's diameter. I'm not sure why, but that figure just comes to mind.

    For #2, I'm not sure what you mean by "where does the density fit in". The density just varies with the inverse of the volume; if the volume doubles and the number of molecules is constant, the density is halved. Is that what you had in mind?