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Homework Help: Gravtational Potential Energy

  1. Dec 7, 2009 #1
    1. The problem statement, all variables and given/known data
    A spoon is raised 21.0 cm above a table. If the spoon and it's contents have a mass of30.0g, what is the gravitational potential energy associated with the spoon at that height relative to the surface of the table?

    2. Relevant equations
    I know that the equation for gravitational potential energy is PEg=mgh
    3. The attempt at a solution
    I know that the mass is 30.0g and the height is 21.0 cm. But how would you figure out the free-fall acceleration?
  2. jcsd
  3. Dec 7, 2009 #2
    By knowing that for most bodies in the proximity of the earth, g (the grav constant)=9.8m/s-s. That is the free fall acceleration.
  4. Dec 7, 2009 #3
    would you convert the 30.0g to kilograms? since the answer is suppose to be in joules
  5. Dec 7, 2009 #4
  6. Dec 7, 2009 #5

    For the potential energy answer, yes you must. Inconsistent units of measurements lead to wrong results.

    The importance of specifying units is undercut at times here because of the time involved.
  7. Dec 7, 2009 #6
    so then the equation would look like this?

    .03kg x 9.8 m/s-s x 21.0cm = _________ J
  8. Dec 7, 2009 #7
    Close. The prevailing system uses Kg-meters-seconds. Joules is based on these. Convert cm to m.
  9. Dec 7, 2009 #8
    Ah so .03kg x 9.8 m/s-s x .021m = 6 x 10^3 J
    is that right?
  10. Dec 7, 2009 #9
    pull the handle and see the wheels spin. (my own wish, I assume the right/wrong confirmation is less dramatic).
    Yea you're ok
  11. Dec 7, 2009 #10
    Thankyou so much!!
  12. Dec 7, 2009 #11
    i ment to put 6 x 10^-3 J
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