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# need some help please (bending moments) Watch

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1. hi there i have been given this problem

A diving board 3 m long is made of glass fibre and is required to support the load of a person weighing 100 kg without deflecting more than 10 cm. The glass fibre used has a modulus of elasticity of 12 GPa if the board is to be 50 cm wide how thick should it be
Assume that the board is uniform thickness throughout and is erected horizontal before any load is applied. The concentrated load of the board can also be ignored as it is negligible.

first thing i did was convert the kg to newtons

100*9.81=981newtons

then used this equation deflection=(-wl^3)/(3*E*I)

but i dont know (I) second moment of area.

so i rearranged the formula I=(981*3^3)/(3*12*10^9)/0.100

but the answer i get is 7.3575*10^-6 this is to small but if put in this equation the max deflection is correct. (-981*3^3)/(3*(12*10^9*7.3575*10^-6))=0.100

how would i work out thickness of board im lost i know what i have done is not right, thought i would give it a go, any help would be much apprecated.

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Updated: December 13, 2010
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