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Im having some difficulty with this problem..i know im getting the wrong answer but not sure why.

Question: A uniform plank of length 5.0m and weight 225N rests horizontally on two supports, with 1.1m of the plank hanging over the right support. To what distance x can a person who weighs 450N walk on the overhanging part of the plank before it just begins to tip?

Diagram:

p

-----------------------

i i

i - supports

p - person

----- - plank

the distance between the rightmost i and p is (x)

and the distance between the rightmost i and the edge of the plank is 1.1m

My (wrong) solution:

Sum of the forces in the y direction = 0

So..

0 = N(Normal from first support) + N(normal from second support) - Ww(Weight of the wood) - Wp(Weight of the person)

2N = Ww + Wp

N = (Ww +Wp)/2

Sum of the torques = 0

0 = Wp(x) - Ww(1.4) - N(3.9)

Wp(x) = Ww(1.4) + [(Ww +Wp)/2)](3.9)

subbing in the numerical values...

(450)x = (225)(1.4) + [(225 +450)/2](3.9)

x = 3.625

but this answer is wrong because x has to be less than 1.1 m!!

thanks for your help

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# Homework Help: Torque problems

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