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Homework Help: Understanding Work

  1. Dec 4, 2009 #1
    1. The problem statement, all variables and given/known data

    The kinetic energy of a system decreases while its potential energy and thermal energy are unchanged. Does the environment do work on the system, or does the system do work on the environment?

    2. Relevant equations

    W = K1 - K2

    3. The attempt at a solution

    I got the answer to this question wrong, so I'm having trouble understanding exactly what's going on here (as usual the textbook is of no help, and we didn't cover this explicitly in class...).

    Here is what sense I can make in my head, but I just want to make sure I am not making anything up:

    The kinetic energy decreases, so we know the final K (K2) is less than the initial K (K1):
    W = K1 - K2 and K1 > K2 so W > 0
    Since W is positive, the system does work on the environment.

    Is this correct logic? How would you have approached the problem? Thanks!
  2. jcsd
  3. Dec 4, 2009 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    Looks good.

    Another way to think about it: the system lost energy. That energy had to go somewhere, since energy is conserved. The system lost energy by doing work on the environment.
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