Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Center of mass of NH3 molecule, not quite there

  1. Oct 17, 2011 #1
    1. The problem statement, all variables and given/known data

    An amonia NH3 molecule in the figure has three hydrogen atoms, which form a equilateral triangle as the base of a pyramid. At the top of the pyramid is the nitrogen atom.

    The length between any two hydrogen atoms is 9.4x10^-11, the length from nitrogen to any of the hydrogen atoms is 10.14x10^-11.

    The mass ratio of nitrogen to hydrogen is 13.9.

    The xaxis runs through the bottom of the pyramid, one of the points lies on it. The y axis runs through the center of the pyramid vertically.

    2. Relevant equations

    3. The attempt at a solution

    I am getting a close answer, but off enough to think I'm doing something wrong.

    x center of mass is 0, by symmetry/intuition.

    For y, what i did was try to find how high nitrogen is with the pythagorean theorem. I got that nitrogen should be at a height of 3.8x10^-11 along y axis.

    I multiplied this number by 13.9, then divided by 16.9.

    My result is 3.125x10^-11 for the ycom, but the correct answer is 3.08x10^-11. Why am I off by that amount?
  2. jcsd
  3. Oct 17, 2011 #2

    Doc Al

    User Avatar

    Staff: Mentor

    What triangle did you use?
  4. Oct 17, 2011 #3
    The reason for the error is that I took their word for it and just used 1 and 13.9, while they used the actual periodic table masses of N and H. 13.9 is pretty darn close to the actual ratio of mass between the two, but not close enough.
  5. Oct 18, 2011 #4

    Doc Al

    User Avatar

    Staff: Mentor

    Good. Glad you figured it out.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook