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# need some help please (bending moments) Watch

1. hi 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.

i have tried working this out but keep getting a really small number, proberly going the wrong way about it but wanted to try myself first. this is what i have done.

first converted weight into newtons 100*9.81=981newtons

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

i need to rearrange this formula -wl^3/3*E*I=0.100

so i ended up with I=(-981*3^3)/(3*12*10^9)/0.100=-7.3575*10^-6

this answer seems to small but when put in this formula the deflection come out right.

-981*3^3/3*(12*10^9*-7.3575*10^-6)=0.100

so the deflection come out correct not sure how to work out the thickness from these any help would be much aprecated.

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