Physics Questions & Answers - Free Online Practice

86.

A simple pendulum, has a period of 5.77 seconds. When the pendulum is shortened by 3 m, the period is 4.60 seconds. Calculate the new length of the pendulum

5.23 m

6.42 m

4.87 m

7.26 m

View answer & explanation

Correct answer is A

Let the original length L=xm

;New length =( x -3 ) m

$T_1$ = 5.77s; $T_2$ = 4.60s,

$T^2$ α L

⇒ $T_2) = kL where K is constant ⇒K = \(\frac{T^2_1}{L_1}$ = $\frac{T^2_2}{L_2}$

⇒ $\frac{5.77^2}{x}$ = $\frac{4.60^2}{x-3}$

⇒ $\frac{33.29}{x}$ = $\frac{4.60^2}{x-3}$

⇒ 33.29(x-3) = 21.16x

⇒ 33.29x - 99.87 =21.16x

⇒12.13x = 99.87

;x = $\frac{99.87}{12.13}$ = 8.23m

New length of the pendulum

=x-3 = 8.23-3

=5.23m

87.

Light of wavelength 589 nm in vacuum passes through a piece of fused quartz of index of refraction n = 1.458. What is the frequency of the light in fused quartz?

[Speed of light c = $3.00×10^8 ms^{-1}$ ]

$5.09×10^{15}$ Hz

$5.09×10^{14}$ Hz

$1.77×10^{15}$ Hz

$1.77×10^{14}$ Hz

View answer & explanation

Correct answer is B

n=1.458, c= $3.00 ×10^8 ms^-1$ ,λo = 589nm; f=?

Speed of light in a medium (v)= $\frac{c}{n}$ where n is the refractive index of the medium

⇒λn= $\frac{589}{1.458}$ = 404nm

v=fλ

⇒f= $\frac{v}{λ}$

= $\frac{2.06×10^8}{404×10^-9}$ $1nano=10^{-9}$

∴f= $5.09×10^{14}$ Hz

88.

A 400 N box is being pushed across a level floor at a constant speed by a force P of 100 N at an angle of 30.0° to the horizontal, as shown in the the diagram below. What is the coefficient of kinetic friction between the box and the floor?

0.19

0.24

0.40

0.22

View answer & explanation

Correct answer is A

W = 400 N; P = 100 N; θ = 30o; μ = ?

Frictional force (Fr) = μR (where R is the normal reaction)

The forces acting along the horizontal direction are Fr and Px

∴ Pcos 30° - Fr = ma (Pcos 30° is acting in the +ve x-axis while Fr in the -ve x-axis)

⇒ 100cos 30° - μR = ma

Since the box is moving at constant speed, its acceleration is zero

⇒ 100cos 30° - μR = 0

⇒ 100cos 30o = μR ----- (i)

The forces acting in the vertical direction are W, Py and R

∴ R - Psin 30° - W = 0 (R is acting upward (+ve) while Py and W are acting downward (-ve) and they are at equilibrium)

⇒ R - 100sin 30° - 400 = 0

⇒ R = 100sin 30° + 400

⇒ R = 50 + 400 = 450 N

From equation (i)

⇒ 100cos 30° = 450μ

⇒μ=100cos30°

N = $\frac{100cos30°}{450}$

= μ = 0.19

89.

Which of the following thermometers measures temperature from the thermal radiation emitted by objects?

Pyrometer thermometer

Platinum resistance thermometer

Thermocouple thermometer

Constant pressure gas thermometer

View answer & explanation

Correct answer is A

A pyrometer is a type of remote-sensing thermometer used to measure the temperature of a surface. It does this by measuring the thermal radiation or infrared energy being emitted from the object. Therefore, a pyrometer thermometer measures temperature from the thermal radiation emitted by objects.

90.

What is the amount of heat required to raise the temperature of a 0.02 kg of ice cube from $-10^oC$ to $10^oC$ ?

[specific latent heat of fusion of ice = 3.34 x $10^5$ $Jkg^-1$ , Specific heat capacity of water = 4200 $Jkg^-1$ $k^-1$

Specific heat capacity of ice = 2100 $Jkg^-1$ $k^-1$

6680 J

1680 J

7520 J

7940 J

View answer & explanation

Correct answer is D

Quantity of heat required to raise the temperature of the ice cube from $-10^oC$ to $0^oC$
⇒H=mc►ø = 0.02 × 2100 × 0 -(-10)

⇒ H = 0.02 × 2100 × (0 + 10) = 0.02 × 2100 × 10

⇒ H = 420 J

quantity of heat required to melt ice at $0^oC$

⇒ H = mL = 0.02 × 3.34 × $10^5$

⇒H=6680 j