NEET Practice Questions (MCQs) with Answers & Solutions

(c) $l=\frac{mgL}{AY}=\frac{1\times 10\times 1.1}{1.1\times {10}^{11}\times {10}^{-6}}m=0.1mm$

(b) To double the length of wire,

Stress = Young's modulus

$\therefore \frac{F}{A}=2\times {10}^{12}\frac{dyne}{c{m}^2}$

If A=$1$ then F=$2 \times {10}^{12}dyne$

(d) When a elastic body is stretched and applied deforming force is removed then the body is expected to return to its original configuration instantaneously. But sometimes some materials take some time to return to its original configuration. This temporary delay in achieving its original configuration is termed as elastic after effect. This elastic after effect is very short for quartz fiber. But elastic after effect is more for glass. 

(c) Breaking Force = µ X Area of cross section of wire ($\mathrm{\pi }$r²)

If radius of wire is double then breaking force will become four times.

(a)

 $$\begin{gathered} Y=3K\left(1-2\sigma \right)\Rightarrow \frac{Y}{3K}=1-2\sigma \\ and Y=2n\left(1+\sigma \right)\Rightarrow \frac{Y}{2n}=1+\sigma \\ On doing \left(1\right)+\left(2\right)\times 2- \\ \frac{Y}{3K}+\frac{Y}{n}=3\Rightarrow \frac{Y\left(n+3K\right)}{3nK}=3\Rightarrow Y=\frac{9nK}{n+3K} \end{gathered}$$

(d) Energy stored per unit volume =$\frac{1}{2}\times Stress\times Strain$

     =$\frac{1}{2}\times Young\text{'}s modulus\times {\left(Strain\right)}^2=\frac{1}{2}\times y\times x^2$

(b) $l=\frac{L^{2}dg}{2Y}=\frac{{\left(8\times {10}^{-2}\right)}^2}{2\times 5\times {10}^8}=9.6\times {10}^{-11}m$

(b) 

$$\begin{gathered} Stress=\frac{force}{Area} \therefore Stress\propto \frac{1}{{\mathrm{\pi r}}^2} \\ \frac{S_B}{S_A}={\left(\frac{r_A}{r_B}\right)}^2={\left(2\right)}^2\Rightarrow S_B=4S_A \end{gathered}$$

(b) Breaking force $\propto$ Area of cross section of wire

i.e. load hold by the wire does not depend upon the length of the wire

(c)

Springs are designed based on the property of elasticity. In the spring the restoring force develops due to deformation. This deformation depends on the elasticity of a material. In the given materials steel is more elastic than copper. So steel is preferred to manufacture a spring.