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Physics MCQs for NEET — Practice Questions with Answers

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If x longitudinal strain is produced in a wire of Young's modulus y, then energy stored in the material of the wire per unit volume is-

$y x^{2}$

$2 y x^{2}$

$\frac{1}{2} y^{2} x$

$\frac{1}{2} y x^{2}$

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Explanation

(d) Energy stored per unit volume = $\frac{1}{2} \times S t r e s s \times S t r a i n$

= $\frac{1}{2} \times Y o u n g' s m o d u l u s \times {(S t r a i n)}^{2} = \frac{1}{2} \times y \times x^{2}$

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The Young's modulus of a rubber string 8 cm long and density $1.5 k g / m^{3}$ is $5 \times 10^{8} N / m^{2}$ , is suspended on the ceiling in a room. The increase in length due to its own weight will be

$9.6 \times 10^{- 5} m$

$9.6 \times 10^{- 11} m$

$9.6 \times 10^{- 3} m$

9.6 m

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Explanation

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

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A and B are two wires of same material. The radius of A is twice that of B. They are stretched by the same load. Then the stress on B is

Equal to that on A

Four times that on A

Two times that on A

Half that on A

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Explanation

(b)

$S t r e s s = \frac{f o r c e}{A r e a} ∴ S t r e s s \propto \frac{1}{{πr}^{2}} \frac{S_{B}}{S_{A}} = {(\frac{r_{A}}{r_{B}})}^{2} = {(2)}^{2} \Rightarrow S_{B} = 4 S_{A}$

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If the length of a wire is reduced to half, then it can hold the ......... load

Half

Same

Double

One fourth

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Explanation

(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

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Why the spring is made up of steel in comparison of copper

Copper is more costly than steel

Copper is more elastic than steel

Steel is more elastic than copper

None of the above

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Explanation

(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.

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Steel and copper wires of same length are stretched by the same weight one after the other. Young's modulus of steel and copper are $2 \times 10^{11} N / m^{2} a n d 1.2 \times 10^{11} N / m^{2} .$ The ratio of increase in length

$\frac{2}{5}$

$\frac{3}{5}$

$\frac{5}{4}$

$\frac{5}{2}$

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Explanation

(b)

$l = \frac{F L}{A Y} \Rightarrow \frac{l_{s}}{l_{e u}} = \frac{Y_{e u}}{Y_{s}} (F, L a n d Y a r e c o n s \tan t) ∴ \frac{l_{s}}{l_{e u}} = \frac{1.2 \times 10^{11}}{2 \times 10^{11}} = \frac{3}{5}$

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The temperature of a wire of length 1 metre and area of cross-section $1 c m^{2}$ is increased from 0°C to 100°C. If the rod is not allowed to increase in length, the force required will be $(α = 10^{- 5} / ° C a n d Y = 10^{11} N / m^{2})$

$10^{3} N$

$10^{4} N$

$10^{5} N$

$10^{9} N$

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Explanation

(b) F = force developed= YA $α (∆ θ)$

$= 10^{11} \times 10^{- 4} \times 10^{- 5} \times 100 = 10^{4} N$

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The force constant of a wire does not depend on

Nature of the material

Radius of the wire

Length of the wire

None of the above

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Explanation

(d) $K = \frac{Y A}{L} = \frac{Y \times {πr}^{2}}{L} \Rightarrow K \propto \frac{Y r^{2}}{L}$

i.e. force constant of a wire depends on young's modules (nature of the material), radius of the wire and length of the wire.

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The length of a wire is 1.0 m and the area of cross-section is $1.0 \times 10^{- 2} c m^{2}$ . If the work done for increase in length by 0.2 cm is 0.4 joule, then Young's modulus of the material of the wire is

$2.0 \times 10^{10} N / m^{2}$

$4 \times 10^{10} N / m^{2}$

$2.0 \times 10^{11} N / m^{2}$

$2 \times 10^{10} N / m^{2}$

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Explanation

(c) $W = \frac{1}{2} \frac{Y A δ^{2}}{L} \Rightarrow 0.4 = \frac{1}{2} \times \frac{Y \times 1^{- 6} \times {(0.2 \times 10^{- 2})}^{2}}{1} ∴ Y = 2 \times 10^{11} N / m^{2}$

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The quality of the material which opposes the change in shape, volume or length is called

Intermolecular repulsion

Intermolecular behaviour

Viscosity

Elasticity

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Explanation

(d)

Elasticity of a material resists change in shape, size, and volume of material.

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