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Homework Help: I am in 8th grade and I am stuck on a simple physics question

  1. Jan 1, 2005 #1
    Can anyone help me:

    Lifting a sail up a mast. Sail is 825 kg, the distance is 5 meters. What's the force required (in newtons)?

    Thanks to anyone helping - L.

    I calculated this a 8085 newtons, using gravity at 9.8 m/sec2. I need help understanding the relationship between acceleration and height (pulling the mast up)
    Last edited: Jan 1, 2005
  2. jcsd
  3. Jan 1, 2005 #2


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    Please,give the original text of the problem.The way u formulated it,the force required is simply 'mg' (the weight).Now if it's only that,why on earth,moon and sun do they give u a distance???

  4. Jan 1, 2005 #3

    Andrew Mason

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    Are you lifting the whole sail up the mast or are you raising the sail along the mast (which I expect is the case)? In doing the latter, the force will vary, so it is not a very good question. The force at any given time depends of the shape of the sail, which is not given.

    If they really want you to raise the whole sail up 5 metres (which is odd), the minimum force would be constant. In that case, the minimum force required is the force of gravity, mg (assuming no friction). But it does not depend on the distance the sail is raised so the 5 metres is a useless piece of information. I think you understand this and find it confusing. It is. So, again, it not a very good question.

    The only way this question makes any sense is if they are really asking you for the average force required to raise the centre of mass of the sail up the mast by 5 metres. In that case: Work done = Average Force x h = change in potential energy of sail = mgh, so Average Force = mg. In the end, this does not depend on the height either, so again, it is a poor question. You seem to understand the physics part quite well. Get someone to give you better questions.

  5. Jan 1, 2005 #4
    Thanks - here is the original question

    Vizzini was hired to start a war between Florin and Guilder, which is a prestigious line of work
    with a long and glorious tradition. Vizzini's plan is to abduct Princess Buttercup, Prince
    Humperdinck's fiance, and kill her on the Guilder frontier, thus implying that Guilder had made the
    first aggressive move. This being the case Florin would have no choice but to attack Guilder.
    Everyday Princess Buttercup would ride her horse completely alone. One day Vizzini and his
    hired thugs, Fezzik, the giant, and Inigo Montoya, one of the worlds greatest swordsmen,
    approached the Princess on her solo ride through the forest.
    Vizzini stopped the princess and asked, "We are but poor, lost circus performers. Is there a
    village nearby?"
    "There is nothing nearby; not for miles, " was the princess's response.
    "Then there will be no one to hear you scream" said Vizzini with an evil grin. He nods to
    Fezzik, who merely reaches over, touches a nerve on Buttercup's neck. She attempts to scream, but
    falls unconsciousness.
    The three malcontents then place a Guilder crest on her horse and load her up onto an small
    awaiting ship and make way for the Guilder frontier.
    In order to make way the small sail needs to be hoisted. This very heavy sail has a mass of
    825 kg. It needs to be lifted 5.00 meters to the top of the mast.
    1. What is force required to lift the mast?
    2. How much work is done to lift the mast?
  6. Jan 1, 2005 #5
    Siginficant digits???

    Folks - I have been told by my teacher 8085 newtons is wrong becuase of significant digits - I understand that gravity is a constant - but why is 8085 N the wrong answer - please help, my teacher is not cooperative.
  7. Jan 1, 2005 #6


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    Blah,blah,blah...You were right about the force.Only gravity needs to be "beaten"and so the force is equal in modulus with the gravity force and opposite sense.
    To compute the work,use the formula which gives u the work done by a force "F" to move an object over a distance "d".


    PS.U should be getting round about 40000J.
  8. Jan 1, 2005 #7


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    He's right,partially.Take g=10 ms^{-2}.U'll get the gravity force:
    [tex] F=mg=8250 N [/tex]

  9. Jan 1, 2005 #8
    The distance given is to help you calculate the work done which is basically Force(Newtons) multiplied by distance(meters).

    As for significant digits, the way I learned is in the problem identify the number with the least number of digits, in this problem both numbers given have 3 digits, so therefore there are 3 significant digits so you ned to round your answer to 8090 N
  10. Jan 1, 2005 #9
    Use scientific notation, which still requires you to round, but leaving that zero there, it's still significant, because it's not behind a decimal. The correct answer is 8.09 x 10 to the third.
  11. Jan 1, 2005 #10
    My physics book has us do it either way with scientific notation or using 0's, the zeros immediatly to the left fo the decimal are not considered significant digits but if there after the decimal they are.
  12. Jan 1, 2005 #11

    Andrew Mason

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    There is no way that anyone is going to lift this sail without rigging. The rigging, consisting of ropes and pulleys, will allow the hoisting of the sail with much less force. So tell your teacher that you can't tell how much force is needed to lift the sail without knowing what the mechanical advantage of the rigging is.

    You could also ask why anyone should answer this question correctly anyway because it would make one an accomplice to the murder of Princess Buttercup. Scientists have to think about the moral consequences of their work.

    Tell your teacher to write ethical questions.:wink:

  13. Jan 2, 2005 #12
    wow...ur teacher must have a lot of time on your hands to create an entire story line with speech and descriptions. Hope she doesn't make you copy the question out at the top of your work.

    Your in grade 8? wow, impressive, i'm in gr 11 IB (international baccalauriate) higher lvl physics and thats the stuff we did at the beginning of this year.
  14. Jan 2, 2005 #13
    :tongue2: That sounds suspiciously like the story in the beggining of "The Princess Bride"
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