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Homework Help: Finding Energy Given Mass and Height

  1. Oct 15, 2012 #1
    1. It is claimed that large trees can evaporate as much as 918 kg of water per day. Evaporation takes place from the leaves. To get there, the water must be raised from the roots of the tree. Assuming that the water must rise an average of 13.8 m from the ground, how much energy must be supplied per day to raise the water?

    Any help would be much appreciated
  2. jcsd
  3. Oct 15, 2012 #2


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    Welcome to PF!

    Hi ChEgirl22! Welcome to PF! wink:

    show us what you've tried, and where you're stuck, and then we'll know how to help! :smile:
  4. Oct 15, 2012 #3
    I've tried to set U=mgh and used 9.81 m/s^2 for g and that didn't work. Then I assumed that because the water is being pulled at a constant acceleration the force would be equal to the mass of the water; W=mh. That also did not work. Finally, I set mgh=1/2mv2 and solved for velocity to be 16.45 m/s and then plugged that into 2w=v^2 and that value did not work as well.
  5. Oct 15, 2012 #4


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    it should work :confused:

    show us what you did :smile:
  6. Oct 15, 2012 #5
    I did U=(918kg)(9.81m/s^2)(13.8m)
    U= 124277 J
    This was not an accepted answer. I also converted kg to g and it was also incorrect.
  7. Oct 15, 2012 #6

    I like Serena

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    Hi ChEgirl22!

    Your answer is correct, so your fight is with the system that is supposed to accept the answers.

    In particular you should round your answer to 3 significant digits, since all the numbers have 3 significant digits.
    Perhaps you should try 124 kJ?
  8. Oct 15, 2012 #7


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    I get the same answer (don't convert kg to g -- stay in the mks system of units).

    Maybe there is some problem with listing too many significant figures in the answer...? Not sure what is wrong. Maybe check with your teacher tomorrow?

    EDIT -- Dangit! Beaten to the punch by ILS again! :smile:
    Last edited: Oct 15, 2012
  9. Oct 15, 2012 #8
    It finally accepted 124.3E3 J Thank you everyone for your help!!
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