1)

Two metal wires P and Q  of same length and material are stretched  by same load. Their masses are in the ratio  m1 : m2 . The ratio of elongations of wire  P to that of Q is 


A) $m_1^2:m_2^2$

B) $m_2^2:m_1^2$

C) $m_2:m_1$

D) $m_1:m_2$

Answer:

Option C

Explanation:

We know that

 young's modulus (Y)$=\frac{Fl}{A\triangle l}\Rightarrow\triangle l=\frac{Fl}{AY}$

 where, A= area of cross section of wire 

     $\triangle$ l= change in the length of the wire,

     l= length of the wire

 and F= applied force

According to the question, F, l and Y are same for both wires, i.e,

 $\triangle l \propto \frac{1}{A}$    ......(i)

But m= $\rho$ V

 where, $\rho$ = density, V= volume

 m= $\rho$ Al        [$\because$   V= Al]

  $m \propto A$    .......(ii)

 From Eqs. (i) and (ii), we get

$\frac{\triangle l_{1}}{\triangle l_{2}}=\frac{A_{2}}{A_{1}}=\frac{m_{2}}{m_{1}}$