No idea how to do this... A rod has 3 notches in it as seen below (notches are where the numbers are). The rod is placed upon a fulcrum and your task is to place the 3 masses at the proper positions to achieve static equilibrium. The masses are: 20 grams, 17 grams, and 5 grams.

Question

No idea how to do this... A rod has 3 notches in it as seen below (notches are where the numbers are). The rod is placed upon a fulcrum and your task is to place the 3 masses at the proper positions to achieve static equilibrium. The masses are: 20 grams, 17 grams, and 5 grams. <--4cm--><----7cm----><-----15cm----->

anonymous · Answer

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perl · Answer

add the moments

perl · Answer

at the center of gravity, the moments add up to zero .
A moment is equal to weight*displacement from center of gravity
So we have
 -4*x + -7 * y + 15*z = 0

now x ,y,z are constrained 20,17,5
so plug these in and see if some combination equals to zero

anonymous · Answer

yup, 4cm – 20g
7cm – 5g
15cm – 17g

perl · Answer

wait i made a mistake

perl · Answer

we should have  
-(4+7)*x + -7 * y + 15*z = 0 

-11x -7y + 15z = 0

perl · Answer

ok now I get 
x = 20 
y = 5
z= 17

perl · Answer

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