- #1
nix
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Hi
Im having some difficulty with this problem..i know I am 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
Im having some difficulty with this problem..i know I am 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