Physics

71. Which one of the following holds true for a freely falling object ?

Which one of the following holds true for a freely falling object ?

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It moves with a uniform velocity.
It moves with a uniform speed.
It moves with a non-uniform acceleration.
It moves with a uniform acceleration.
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The correct answer is D. A freely falling object near the Earth’s surface, neglecting air resistance, moves with a constant acceleration.
– Free fall is defined as motion under the sole influence of gravity.
– Near the Earth’s surface, the acceleration due to gravity ($\text{g}$) is approximately constant (about 9.8 m/s²).
– This constant acceleration causes the velocity of the object to change uniformly over time.
In reality, air resistance affects falling objects, causing their acceleration to decrease as their speed increases, eventually reaching a terminal velocity where acceleration is zero. However, in standard physics problems where “freely falling” is used, air resistance is usually neglected, assuming movement solely under gravity with uniform acceleration.

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72. If the linear momentum of a moving object changes by two times, then i

If the linear momentum of a moving object changes by two times, then its kinetic energy will change by a factor of

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2
4
6
8
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The correct answer is B, 4.
Linear momentum (p) of an object of mass m and velocity v is given by p = mv.
Kinetic energy (k) of the same object is given by k = ½ mv².
We can express kinetic energy in terms of momentum. From p = mv, we get v = p/m. Substituting this into the kinetic energy equation:
k = ½ m(p/m)² = ½ m(p²/m²) = ½ p²/m.
So, k is proportional to p² (assuming mass m is constant).
If the initial momentum is p₁, the initial kinetic energy is k₁ = ½ p₁²/m.
If the linear momentum changes by two times, it means the new momentum p₂ = 2p₁.
The new kinetic energy k₂ = ½ p₂²/m = ½ (2p₁)²/m = ½ (4p₁²)/m = 4 (½ p₁²/m) = 4k₁.
Thus, the kinetic energy will change by a factor of 4.
This relationship (k ∝ p²) shows that kinetic energy is much more sensitive to changes in momentum than momentum is to changes in kinetic energy (or velocity).

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73. At the time of short circuit, the current in an electric circuit

At the time of short circuit, the current in an electric circuit

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becomes zero
remains same
increases sharply
decreases sharply
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The correct answer is C, increases sharply.
A short circuit occurs when a low-resistance path is created between two points in an electric circuit that are normally at different potentials. This bypasses the intended load (which usually has significant resistance).
According to Ohm’s Law (V = IR), the current (I) in a circuit is directly proportional to the voltage (V) across it and inversely proportional to its resistance (R).
During a short circuit, the resistance (R) in the path becomes very low, ideally approaching zero. If the voltage (V) of the source remains relatively constant, the current (I = V/R) will increase dramatically or sharply.
This sudden surge in current can generate excessive heat, potentially causing damage to the circuit, components, or leading to fires.
Safety devices like fuses or circuit breakers are used in electrical circuits to detect this dangerously high current during a short circuit and quickly interrupt the circuit to prevent damage and hazards.

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74. The magnitude of focal length of a concave lens is 2 m. What is the po

The magnitude of focal length of a concave lens is 2 m. What is the power of the lens?

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+0.5 dioptre
-0.5 dioptre
+2.0 dioptre
-1.0 dioptre
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The correct answer is B, -0.5 dioptre.
The power (P) of a lens is defined as the reciprocal of its focal length (f), when the focal length is measured in meters. The unit of power is the dioptre (D), where 1 D = 1 m⁻¹.
The formula is P = 1/f.
For a concave lens, the focal length is conventionally considered negative.
Given the magnitude of the focal length of the concave lens is 2 m, the focal length f = -2 m.
Power P = 1 / (-2 m) = -0.5 m⁻¹ = -0.5 dioptre (D).
Convex lenses have positive focal lengths and positive power, converging light rays. Concave lenses have negative focal lengths and negative power, diverging light rays. The power of a lens indicates its ability to converge or diverge light; a higher magnitude of power corresponds to a shorter focal length and stronger bending of light.

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75. The electrical device used for converting mechanical energy into elect

The electrical device used for converting mechanical energy into electrical energy is called

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voltmeter
ammeter
motor
generator
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The correct answer is D, generator.
A generator is a device that converts mechanical energy into electrical energy. This conversion is based on the principle of electromagnetic induction, where a relative motion between a conductor and a magnetic field induces an electric current in the conductor. Mechanical work is done to cause this relative motion.
A motor is a device that converts electrical energy into mechanical energy.
A voltmeter is an instrument used to measure electric potential difference (voltage).
An ammeter is an instrument used to measure electric current.
Generators are essential components in power plants, converting mechanical energy from sources like turbines (driven by steam, water, or wind) into electrical energy for distribution.

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76. The optical phenomenon responsible for the blue colour of sky is

The optical phenomenon responsible for the blue colour of sky is

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dispersion
reflection
refraction
scattering
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The correct answer is D, scattering.
The blue colour of the sky is primarily caused by the scattering of sunlight by molecules and tiny particles in the Earth’s atmosphere.
This phenomenon is explained by Rayleigh scattering, which states that the amount of scattering is inversely proportional to the fourth power of the wavelength. Shorter wavelengths (like blue and violet) are scattered much more effectively than longer wavelengths (like red and orange).
When sunlight enters the atmosphere, the blue light is scattered in all directions by the atmospheric particles, making the sky appear blue. Our eyes are more sensitive to blue light than violet, contributing to the perception of a blue sky.
Dispersion is the splitting of white light into its constituent colours due to different wavelengths having different refractive indices in a medium (like in a prism or rainbow). Reflection is the bouncing of light off a surface. Refraction is the bending of light as it passes from one medium to another. While refraction plays a role in atmospheric phenomena like mirages, it’s scattering that causes the blue sky.

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77. Which one of the following materials *cannot* be used to make a convex

Which one of the following materials *cannot* be used to make a convex lens?

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Aluminium
Glass
Sapphire
Water
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The correct answer is A, Aluminium.
A lens works by refracting (bending) light as it passes through the material. For a material to be used to make a lens, it must be transparent or at least translucent to the type of light intended to be used (e.g., visible light for optical lenses).
Glass, Sapphire, and Water are all transparent or translucent materials that can refract light, making them suitable for manufacturing lenses.
Aluminium is a metal that is opaque to visible light. Light does not pass through it but is reflected or absorbed. Therefore, Aluminium cannot be used to make a convex lens for visible light.
Lenses are typically made from materials like glass, plastic, quartz, sapphire, fluorite, or even liquids like water. The choice of material depends on the desired optical properties (like refractive index, dispersion), cost, durability, and the specific wavelength range of light for which the lens is intended.

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78. Which one of the following is *not* the unit of pressure?

Which one of the following is *not* the unit of pressure?

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pascal (Pa)
N/m²
J/m²
bar
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The correct answer is C, J/m².
Pressure is defined as force per unit area (Pressure = Force/Area).
The SI unit of force is Newton (N) and the SI unit of area is square meter (m²). Therefore, N/m² is a unit of pressure.
Pascal (Pa) is the SI unit of pressure, defined as 1 Pa = 1 N/m².
Bar is a non-SI unit of pressure, widely used (1 bar = 10⁵ Pa).
Joule (J) is the unit of energy or work. J/m² represents energy per unit area. While pressure has the same dimensions as energy density (J/m³), J/m² represents pressure multiplied by distance (since J = N.m, J/m² = (N.m)/m² = N/m * m = Pressure * Distance). Therefore, J/m² is not a unit of pressure.
J/m³ (Joule per cubic meter) represents energy density and has the same units as pressure (N/m² or Pa). This is because Energy = Force × Distance, so J = N.m. Thus, J/m³ = N.m/m³ = N/m², which are units of pressure. However, J/m² is not dimensionally equivalent to pressure.

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79. Two objects, x and y, have equal mass and are moving with speeds u and

Two objects, x and y, have equal mass and are moving with speeds u and 3u respectively. Their kinetic energies k_x and k_y are related as

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k_x = k_y
2k_x = k_y
9k_x = k_y
3k_x = k_y
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The correct answer is C, 9k_x = k_y.
The kinetic energy (k) of an object with mass (m) and speed (v) is given by the formula k = ½ mv².
For object x, mass is m and speed is u, so k_x = ½ mu².
For object y, mass is m and speed is 3u, so k_y = ½ m(3u)² = ½ m(9u²) = 9 (½ mu²).
Comparing k_y with k_x, we find k_y = 9k_x, or 9k_x = k_y.
Kinetic energy is directly proportional to the mass of the object and the square of its speed. Doubling the speed of an object quadruples its kinetic energy (if mass remains constant). Tripling the speed increases the kinetic energy by a factor of nine.

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80. Which one of the following is a semi- conductor device that is used to

Which one of the following is a semi- conductor device that is used to increase the power of the incoming signals by preserving the shape of the original signal?

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Transistor
Flip-flop
Diode
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The correct answer is B) Transistor.
A transistor is a semiconductor device commonly used as an electronic switch or amplifier. When used as an amplifier, it takes a small input signal and produces a larger output signal, effectively increasing the power (voltage, current, or both) while aiming to preserve the original signal shape.
A register is a small, fast storage location in a CPU. A flip-flop is a basic memory element that can store a single bit of binary data. A diode is a semiconductor device that primarily allows current to flow in one direction. While semiconductors are involved in all these devices, the function described (increasing signal power while preserving shape) is characteristic of a transistor used as an amplifier.

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