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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?

Q7.17     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?

Answers (1)
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The centre of mass of meter stick is at 50 cm.

Let the mass of meter stick be m.

Now according to the situation given in the question, we will use the rotational equilibrium condition at the centre point of the meter stick.

                                           10g(45 - 12)\ -\ mg(50 - 45)\ =\ 0

or                                                               m\ =\ \frac{10\times 33}{5}\ =\ 66\ g

Thus the mass of the meter stick is 66 g. 

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