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Physics MCQs for NEET — Practice Questions with Answers

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A wire of length L and radius r is rigidly fixed at one end. On stretching the other end of the wire with a force F, the increase in its length is l. If another wire of same material but of length 2L and radius 2r is stretched with a force of 2F, the increase in its length will be

$\frac{l}{2}$

$\frac{l}{4}$

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Explanation

(a)

$l = \frac{F L}{A Y} = \frac{F L}{{πr}^{2} Y} ∴ l \propto \frac{F L}{r^{2}} (Y = c o n s \tan t) ∴ \frac{l_{2}}{l_{1}} = \frac{F_{2}}{F_{1}} \times \frac{L_{2}}{L_{1}} {(\frac{r_{1}}{r_{2}})}^{2} = 2 \times 2 \times {(\frac{1}{2})}^{2} = 1$

$∴ l_{2} = l_{1}$ i.e. increment in its length will be l.

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In steel, the Young's modulus and the strain at the breaking point are $2 \times 10^{11} N m^{- 2}$ and 0.15 respectively. The stress at the breaking point for steel is therefore -

$1.33 \times 10^{11} N m^{- 2}$

$1.33 \times 10^{12} N m^{- 2}$

$7.5 \times 10^{- 13} N m^{- 2}$

$3 \times 10^{10} N m^{- 2}$

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Explanation

(d) Breaking stress = strain $\times$ Young's modulus

$= 0.15 \times 2 \times 10^{11} = 3 \times 10^{10} N m^{- 2}$

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Which one of the following substances possesses the highest elasticity

Rubber

Glass

Steel

Copper

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Explanation

(c)

Modulus of elasticity gives the idea of elasticity, and steel has highest modulus of elasticity.

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Which one of the following quantities does not have the unit of force per unit area

Stress

Strain

Young's modulus of elasticity

Pressure

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Explanation

(b) Because strain is a dimensionless and unitless quantity.

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A copper wire and a steel wire of the same diameter and length are connected end to end and a force is applied, which stretches their combined length by 1 cm. The two wires will have

Different stresses and strains

The same stress and strain

The same strain but different stresses

The same stress but different strains

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Explanation

(d) $S t r e s s = \frac{F o r c e}{a r e a}$

In the present case, force applied and area of cross-section of wires are same, therefore stress has to be the same.

$S t r a i n = \frac{S t r e s s}{Y}$

Since the Young’s modulus of steel wire is greater than the copper wire, therefore, strain in case of steel wire is less than that in case of copper wire.

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The possible value of Poisson's ratio is

0.9

0.8

0.4

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Explanation

(d) Poisson’s ratio varies between – 1 and 0.5.

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Which of the following affects the elasticity of a substance

Hammering and annealing

Change in temperature

Impurity in substance

All of these

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Explanation

(d) The elasticity of substance is affected by impurity of substance hammering and anneasing change in the temperature.

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A wire of diameter 1mm breaks under a tension of 1000 N. Another wire, of same material as that of the first one, but of diameter 2 mm breaks under a tension of

500 N

1000 N

10000 N

4000 N

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Explanation

(d) Breaking force $\propto$ r²

If diameter becomes double then breaking force will become four times i.e. 1000 × 4 = 4000 N

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The force required to stretch a steel wire of cross-section $1 {cm}^{2}$ to 1.1 times its length would be $(Y = 2 \times 10^{11} N m^{- 2})$

$2 \times 10^{6} N$

$2 \times 10^{3} N$

$2 \times 10^{- 6} N$

$2 \times 10^{- 7} N$

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Explanation

(a) $F = A \times Y \times s t r a i n = 1 \times 10^{- 4} \times 2 \times 10^{11} \times 0.1 = 2 \times 10^{6} N$

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When a weight of 10 kg is suspended from a copper wire of length 3 metres and diameter 0.4 mm, its length increases by 2.4 cm. If the diameter of the wire is doubled, then the extension in its length will be

9.6 cm

4.8 cm

1.2 cm

0.6 cm

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Explanation

(d)

$l \propto \frac{1}{r^{2}} (F, L a n d Y a r e c o n s \tan t) \frac{l_{2}}{l_{1}} = {(\frac{r_{1}}{r_{2}})}^{2} = {(\frac{1}{2})}^{2} \Rightarrow l_{2} = \frac{l_{1}}{4} = \frac{2.4}{4} \Rightarrow l_{2} = 0.6 c m$

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