UPSC CDS-1

11. The weight of an object is due to

The weight of an object is due to

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the net force acting on it.
the total of all forces acting on it irrespective of their directions.
the force that it exerts on the ground.
its inert property.
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UPSC CDS-1 – 2021
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Weight is defined as the force of gravity acting on an object. In situations where gravity is the only force significantly influencing motion (like free fall), the net force acting on the object is equal to its weight. While weight is a specific type of force (gravitational force) and not the ‘net’ force in all scenarios (e.g., when supported), among the given options, ‘net force acting on it’ is the one most directly related to the force aspect of weight, particularly in simplified contexts where gravity is the primary force considered or its effect on motion is discussed.
Weight (W) is the force exerted by a gravitational field on a mass (m), usually expressed as W = mg, where g is the acceleration due to gravity. This gravitational force contributes to the net force acting on the object. In free fall, the net force acting on the object *is* its weight. The phrasing of option A is imprecise, as weight is a component of the net force in general, but in the context of forces acting on an object, it is the most relevant option compared to others that describe different concepts.
Mass is a measure of the amount of matter and inertia, an intrinsic property of an object. Weight is a force that depends on both the mass of the object and the strength of the gravitational field it is in. The force exerted by the object on a supporting surface (often called apparent weight) is related to, but not always equal to, the true weight. Options B, C, and D are incorrect definitions or concepts related to weight.

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12. The direction of acceleration in uniform circular motion is along the

The direction of acceleration in uniform circular motion is along the

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direction of motion.
tangent to the circle at the point of observation.
direction of velocity.
direction perpendicular to velocity.
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In uniform circular motion, the speed of the object is constant, but its velocity is continuously changing direction as it moves along the circular path. Acceleration is the rate of change of velocity. For an object in uniform circular motion, there is an acceleration directed towards the center of the circle.
The velocity vector in circular motion is always tangent to the circle at the object’s position. The acceleration, known as centripetal acceleration, is always directed along the radius towards the center of the circle. A line tangent to a circle is always perpendicular to the radius at the point of tangency. Therefore, the direction of acceleration (towards the center) is always perpendicular to the direction of velocity (tangent to the circle).
The magnitude of the centripetal acceleration is given by a = v²/r, where v is the constant speed and r is the radius of the circle. Since there is no component of acceleration along the tangent (tangential acceleration), the magnitude of the velocity (speed) remains constant.

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13. How many internal reflections of light take place in the formation of

How many internal reflections of light take place in the formation of primary rainbow ?

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0
1
2
More than 2
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A primary rainbow is formed when sunlight is dispersed by water droplets. The process involves the sunlight entering a raindrop, undergoing refraction at the front surface, reflecting internally off the back surface, and finally refracting again as it exits the raindrop.
The light path for a primary rainbow within a raindrop includes one internal reflection. The sequence is: refraction (entry) → reflection (internal) → refraction (exit). This single internal reflection, combined with refraction and dispersion, separates the light into its constituent colours, forming the rainbow arc.
A secondary rainbow, which is fainter and appears above the primary bow with reversed colours, is formed by sunlight that undergoes two internal reflections within the raindrops. Higher-order rainbows involving three or more internal reflections are theoretically possible but are rarely observed due to significant loss of light intensity.

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14. Calcium oxide reacts vigorously with water to produce slaked lime whos

Calcium oxide reacts vigorously with water to produce slaked lime whose solution is used for white-washing walls. This slaked lime reacts with component (A) in air to form a thin layer of component (B) to give a shiny finish. What are the components (A) and (B) ?

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A – O₂ ; B – CaCO₃
A – CO₂ ; B – Ca(OH)₂
A – O₂ ; B – Ca(OH)₂
A – CO₂ ; B – CaCO₃
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Calcium oxide (quicklime, CaO) reacts with water to form calcium hydroxide (slaked lime, Ca(OH)₂). When slaked lime solution is applied to walls during white-washing, it slowly reacts with carbon dioxide gas present in the air to form a thin, hard layer of calcium carbonate (CaCO₃).
The initial reaction is CaO(s) + H₂O(l) → Ca(OH)₂(aq). This slaked lime solution is used for white-washing. The slaked lime then reacts with atmospheric carbon dioxide: Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l). Component (A) from the air is carbon dioxide (CO₂), and the component (B) formed as a thin layer on the wall is calcium carbonate (CaCO₃). This layer of calcium carbonate gives the walls a smooth, shiny finish after a few days as the reaction completes and the water evaporates.
This process is a form of carbonation. The calcium carbonate layer is chemically similar to limestone and marble, providing a durable and aesthetically pleasing surface. This reaction also helps to ‘set’ the white-wash onto the wall surface.

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15. Which one of the following sodium compounds is used for softening hard

Which one of the following sodium compounds is used for softening hard water ?

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Na₂CO₃
NaHCO₃
NaOH
Na₂SO₄
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Hard water contains dissolved minerals, primarily calcium and magnesium ions. Sodium carbonate (Na₂CO₃), also known as washing soda, is commonly used to soften hard water by removing these ions.
When sodium carbonate is added to hard water, the calcium and magnesium ions react with the carbonate ions to form insoluble calcium carbonate (CaCO₃) and magnesium carbonate (MgCO₃). These precipitates settle down or can be filtered out, effectively reducing the concentration of hardening ions in the water. The overall reaction with calcium ions is: Ca²⁺(aq) + Na₂CO₃(aq) → CaCO₃(s) + 2Na⁺(aq). The resulting sodium ions (Na⁺) do not contribute to water hardness.
Other methods for water softening include using ion-exchange resins, which swap calcium and magnesium ions for sodium ions, or using chelating agents that bind the metal ions. While boiling can remove temporary hardness (caused by bicarbonates), it does not remove permanent hardness (caused by sulfates and chlorides), which sodium carbonate can address.

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16. Which one of the following is used in soda-acid fire extinguishers ?

Which one of the following is used in soda-acid fire extinguishers ?

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Sodium chloride
Sodium hydrogen carbonate
Calcium hydroxide
Acetic acid
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Soda-acid fire extinguishers utilize a chemical reaction to produce carbon dioxide gas, which is used to extinguish fires. The primary components involved in this reaction are an acid (commonly sulfuric acid) and sodium hydrogen carbonate (NaHCO₃), also known as sodium bicarbonate or baking soda.
When the fire extinguisher is activated, the acid mixes with the sodium hydrogen carbonate solution. The reaction is: 2NaHCO₃(aq) + H₂SO₄(aq) → Na₂SO₄(aq) + 2H₂O(l) + 2CO₂(g). The rapid production of carbon dioxide gas builds up pressure, expelling the mixture out of the nozzle. The CO₂ gas also helps smother the fire by displacing oxygen, and the water cools the burning material.
Soda-acid fire extinguishers are effective against Class A fires (ordinary combustibles like wood, paper, and cloth) but are not suitable for Class B (flammable liquids), Class C (electrical equipment), or Class D (combustible metals) fires. Using them on electrical fires is particularly dangerous due to the presence of water.

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17. Which one of the following naturally occurring acids is found in abund

Which one of the following naturally occurring acids is found in abundance in tomato ?

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Acetic acid
Citric acid
Oxalic acid
Tartaric acid
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Tomatoes contain several organic acids, including citric acid, malic acid, and oxalic acid. While citric acid is often the most abundant acid overall in tomatoes and contributes significantly to their flavour, oxalic acid is also notably present in tomatoes, distinguishing them from many other fruits in terms of acid composition that might be highlighted in specific contexts (e.g., dietary oxalate intake). Among the options provided, Oxalic acid is a naturally occurring acid found in abundance (meaning significant quantity, though perhaps not always the single most abundant) in tomatoes, along with other acids. Citric acid is also present, and often in higher concentration, but oxalic acid is a recognized constituent in tomatoes, particularly in discussions of dietary oxalates. Considering the potential ambiguity and common association, Oxalic acid is a plausible answer if the question intends to highlight a specific, notable acid present in significant amounts.
Tomatoes contain several organic acids. Oxalic acid is a naturally occurring acid found in tomatoes, along with citric and malic acids.
The relative abundance of acids in tomatoes can vary depending on the variety, ripeness, and growing conditions. While citric acid is often considered the principal acid by weight, oxalic acid is also a significant component. Other acids like malic acid are also present. The presence of oxalic acid in tomatoes is relevant for individuals concerned about oxalate intake and kidney stones.

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18. Which one of the following is the structure of a cardiac muscle cell ?

Which one of the following is the structure of a cardiac muscle cell ?

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Cylindrical, Unbranched and Multinucleate
Spinal shaped, Uninucleate
Spinal shaped, Branched and Uninucleate
Cylindrical, Branched and Uninucleate
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Cardiac muscle tissue forms the walls of the heart. Its cells, called cardiomyocytes, are cylindrical and branched. They are typically uninucleate (containing one nucleus), although some may have two nuclei. They are also striated due to the arrangement of contractile proteins. Skeletal muscle cells are cylindrical, unbranched, and multinucleate. Smooth muscle cells are spindle-shaped and uninucleate.
Cardiac muscle cells are characterized by being cylindrical, branched, and predominantly uninucleate.
Intercalated discs, which are specialized junctions between cardiac muscle cells, are another characteristic feature of cardiac muscle tissue.

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19. Which one among the following is a free living animal ?

Which one among the following is a free living animal ?

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Liver fluke
Wuchereria
Plasmodium
Planaria
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A free-living animal is an organism that is not parasitic or sessile (attached to a substrate), but lives independently. Liver fluke (Fasciola hepatica), Wuchereria (Wuchereria bancrofti), and Plasmodium are all internal parasites that require a host organism to survive and reproduce. Planaria, a type of flatworm (Platyhelminthes), is commonly found in freshwater environments and is a free-living predator/scavenger.
Free-living organisms are independent and not parasitic or sessile. Planaria is a well-known example of a free-living flatworm.
Parasites live on or inside a host organism and benefit at the host’s expense. Sessile organisms are fixed in one place.

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20. ‘Sleeping sickness’ is caused by

‘Sleeping sickness’ is caused by

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Trypanosoma
Leishmania
Plasmodium
Paramecium
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Sleeping sickness, also known as Human African Trypanosomiasis (HAT), is a parasitic disease caused by protozoa of the genus Trypanosoma. Specifically, it is caused by Trypanosoma brucei, transmitted through the bite of infected tsetse flies.
Sleeping sickness is a parasitic disease caused by Trypanosoma brucei.
Leishmania causes leishmaniasis, Plasmodium causes malaria, and Paramecium is a harmless freshwater protozoan.

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