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A metre stick is balanced on a knife edg...

A metre stick is balanced on a knife edge at its centre. When two coins, each of mass 5 g are put one on top of the other at the 12.0 cm mark, the stick is found to be balanced at 45.0 cm. What is the mass of the metre stick?

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Suppose a mass of metre stick is M and its centre at E.
If a metre stick is balanced on a knife edge from distance 45 cm, then new gravitational centre is D.
`therefore` Distance `DE=OE-OD=50-45`
= 5 cm
When two coins are put one on the top of the other at the 12.0 mark, then distance between C and D is = `45-12`
= 33 cm
From rotational equilibrium
`MxxDE=(2xx5)xxCD`
where mass of each coin is 5g
`therefore Mxx5=10xx33`
`therefore M=66g`
`therefore` mass of metre stick is 66.0 g
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