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Help with beginner level physics word problem

  1. Feb 11, 2015 #1
    1.The semester just started and my teacher has assigned us this question: How high of a tower could you build from the people in this class(30 people)? Include diagram. How much force would be pushing down on one of the base person's legs?

    2. Note : Fg = mg

    3. Here is my work so far : https://fbcdn-sphotos-h-a.akamaihd...._=1423753387_e3fafa257d6be865db2e4a070b88de0e

    I created a triangular shape(human pyramid) with 7 people on bottom then 6,5,4,3,2,1,1,1 <- the 1's are on each other shoulders. So far I have calculated the approximate height if the bottom 6 rows are on their hands and knees and if the top 3 are standing up.

    The next part is what I need help with. How would I start calculating the force pushing down? I assumed the 2nd row pushing down on the guy at the base would be 2 halves (weight distributed evenly) and so on. I also assumed the height and weight would decrease as the tower went up. I know the force of gravity is 9.8 but how would I use that and Fg = mg in this situation?

    I hope you can understand this and help me as I am really confused right now D:
     
  2. jcsd
  3. Feb 11, 2015 #2

    SteamKing

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    I don't know where you got the equation Fg = mg.

    The weight of a person is calculated using W = mg, where m is the mass of a person in kg, g is the acceleration due to gravity, 9.81 m/s2, and W is the weight in newtons, the SI unit of force.

    For example, if someone has a mass of 50 kg, his weight is 50 * 9.81 = 490.5 Newtons. That's a force of 245.25 Newtons on each leg.

    To find out the force acting on each row, add up the mass of all the people standing on the rows above.
     
  4. Feb 11, 2015 #3

    haruspex

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    Perhaps by Fg you mean Fg, i.e. the force due to gravity?
    The equal distribution is not realistic. Think of them as facing you. The left hand person (your left hand) in the row of two has someone standing on her left shoulder. To spread that weight evenly between her own two feet would involve leaning out to her right. Standing normally, that person could spread her own weight evenly, but transmit all the weight from above to the left foot only. You could analyse the torques.
    So in the bottom row, there will be more weight in the centre than on the edges.
     
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