1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Law of conservation of energy

  1. Feb 24, 2014 #1
    1. The problem statement, all variables and given/known data
    Particle goes to free fall from point ##A## to point ##B##. ##\upsilon_{A}=0##. How to define law of conservation of energy. Coordinate system is selected in such way that ##y## coordinate od point ##A## is zero. (See figure energy)


    2. Relevant equations
    For free fall ##\upsilon_{B}=\sqrt{2gy}##.


    3. The attempt at a solution
    My problem is with formulation of law of conservation of energy. Energy of point ##A## is in this case ##0##? Right? Because potential energy is zero (##y## cordinate of point ##A## is zero) and in case of free fall ##\upsilon_{A}=0##.
    And energy of point ##B## is also zero ##-mgy+m\frac{\upsilon^2}{2}=0##.
    Am I right? I'm confused how the energy of the particle could be zero? Can you help me? Give me some other view?
     

    Attached Files:

  2. jcsd
  3. Feb 24, 2014 #2

    DrClaude

    User Avatar

    Staff: Mentor

    What you take to be the zero of energy is arbitrary. At point B, the particle as ##-mgy < 0## potential energy (it has lost some potential energy) and ##mv^2/2 > 0## of kinetic energy (it has gained kinetic energy). (Note: I take it that the y axis is pointing downwards.)

    If you choose point B to be the zero of potential energy, then at point A, you have ##-mgy + 0 > 0## (since ##y < 0##) and at point B ##0 + mv^2/2##, with ##mv^2/2 = -mgy##, which is exactly the same as before.

    In both cases, you see potential energy converted into kinetic energy.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Law of conservation of energy
  1. Conservation Laws (Replies: 2)

  2. Conservation of Energy (Replies: 0)

Loading...