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Q1. The density of a substance is defined as
Density is defined as the mass of a substance per unit volume. The formula for density is ρ = m/V, where ρ is density, m is mass, and V is volume. Weight per unit volume is specific weight, volume per unit mass is specific volume, and force per unit area is pressure. So density is correctly defined as mass per unit volume.
Q2. Objects float in a liquid when their density is
Objects float in a liquid when their density is less than the density of the liquid. This means the object’s weight is less than the weight of the liquid it displaces, so the buoyant force is greater than the weight, causing the object to rise and float. If density is equal, the object remains suspended; if greater, it sinks.
Q3. Density comparison with water gives
Relative density (also called specific gravity) is the ratio of the density of a substance to the density of water. It tells us how many times a substance is denser than water. Relative density has no unit because it is a ratio of two similar quantities.
Q4. Density is considered a characteristic property because it
Density is considered a characteristic property because different substances have different densities under fixed conditions. For example, iron has a different density than wood, and water has a different density than oil. This makes density useful for identifying substances.
Q5. The buoyant force increases when
The buoyant force depends on the density of the fluid and the volume of fluid displaced. According to Archimedes’ principle, buoyant force equals the weight of the displaced fluid. If the fluid density increases, the weight of the displaced fluid increases, so the buoyant force also increases.
Q6. The SI unit of density is
The SI unit of density is kilograms per cubic metre (kg/m³ or kg m⁻³). This comes from the definition of density as mass (kg) divided by volume (m³). g/cm³ is also a common unit but is not the SI unit. N/m² is the unit of pressure, and kg/m² is not a standard unit.
Q7. Objects sink in a liquid when their density is
Objects sink in a liquid when their density is greater than the liquid’s density. This means the weight of the object is greater than the weight of the liquid it displaces, so the buoyant force is insufficient to support the object, and it sinks. If density is less, it floats; if equal, it remains suspended.
Q8. The net force on the string decreases because
When an object is lowered into water, the net force on the string (tension) decreases because the water exerts an upward buoyant force on the object. This upward force partially supports the weight, reducing the tension. The mass, density, and gravity do not change.
Q9. Thin straps of school bags hurt more because
Thin straps hurt more because they have a smaller area of contact with the shoulder. Pressure is defined as force per unit area (P = F/A). For the same force (weight of the bag), a smaller area results in higher pressure. This higher pressure causes discomfort and pain.
Q10. The density of gold is approximately
The density of gold is approximately 19,300 kg/m³ (or 19.3 g/cm³). This high density makes gold one of the densest common metals. 1000 kg/m³ is the density of water, 2700 kg/m³ is the density of aluminium, and 7800 kg/m³ is the density of iron.
Q11. As the stone is lowered into water, the elongation of the string
As the stone is lowered into water, the elongation of the string decreases. This is because the buoyant force from the water acts upward on the stone, reducing the tension in the string. The stone feels lighter in water, so the string stretches less.
Q12. Density of a given substance under fixed conditions
The density of a given substance under fixed conditions (temperature and pressure) remains constant. It is an intrinsic property of the material and does not depend on the amount of the substance or its shape. Density changes only when temperature or pressure changes.
Q13. A body experiences buoyant force when it is
A body experiences buoyant force when it is fully or partially immersed in a fluid (liquid or gas). The buoyant force is due to the pressure difference between the top and bottom of the immersed object. In vacuum, there is no fluid to exert the force; on the ground or in air, the immersion is not complete.
Q14. Buoyant force acts in which direction?
Buoyant force always acts in the upward direction. It is the force exerted by a fluid on an object immersed in it, and it opposes the downward weight of the object. This upward force is what makes objects feel lighter and allows floating.
Q15. The elongation in the string is initially due to
The elongation in the string is initially due to the weight of the stone when it is suspended in air. The stone’s weight pulls the string downward, stretching it. Buoyant force acts only when the stone is immersed in a fluid. Air resistance is negligible, and water pressure is not acting in air.
Q16. Density of silver is calculated by
Relative density (specific gravity) is the ratio of the density of a substance to the density of water. To find the actual density, we multiply the relative density by the density of water (1000 kg/m³). For silver, relative density is 10.8, so density = 10.8 × 1000 = 10,800 kg/m³.
Q17. The decrease in elongation indicates the presence of
A decrease in elongation of the string when a stone is dipped in water indicates the presence of an upward force by water (buoyant force). This upward force reduces the effective weight of the stone, causing the string to stretch less.
Q18. Lactometers are used to determine
Lactometers are instruments used to determine the purity of milk by measuring its density. They work on the principle of buoyancy. Pure milk has a higher density than diluted milk because the addition of water reduces the density. The lactometer floats higher in pure milk.
Q19. When the stone is fully immersed, buoyant force depends on
When the stone is fully immersed, the buoyant force depends on the volume of fluid displaced (which is equal to the volume of the stone) and the density of the fluid. Colour and shape do not affect the buoyant force. Depth does not affect it once fully immersed.
Q20. Archimedes discovered his principle by observing
Archimedes discovered his principle when he noticed water overflowing as he got into a bathtub. He realized that the volume of water displaced was equal to his body volume. This led to the principle that the buoyant force on an object equals the weight of the fluid displaced.
Q21. Hydrometers are used to measure
Hydrometers are instruments used to measure the density (or relative density) of liquids. They work on the principle of buoyancy. A hydrometer floats in a liquid, and the depth to which it sinks indicates the density of the liquid. They are commonly used in batteries, wine making, and industrial applications.
Q22. The density of water is
The density of water is 1000 kg/m³ (or 1 g/cm³) at 4°C. This is the standard value used as a reference for relative density. 100 kg/m³ is for cork, 19,300 kg/m³ is for gold, and 10,000 kg/m³ is not a standard density.
Q23. The buoyant force on a given body depends on
The buoyant force on a given body depends on the density of the fluid and the volume of fluid displaced. Shape and colour do not affect the buoyant force, and the temperature of the body is not directly relevant (though it can affect fluid density).
Q24. Archimedes’ principle explains the
Archimedes’ principle explains the magnitude of the buoyant force: it states that the buoyant force on an object is equal to the weight of the fluid displaced by the object. This principle does not explain shape, colour, or falling speed.
Q25. Archimedes’ principle is used in designing
Archimedes’ principle is used in designing ships, submarines, and other floating vessels. It helps engineers determine how much water a ship must displace to support its weight. Submarines use the principle to control buoyancy by filling or emptying ballast tanks.
Q26. Relative density has
Relative density (specific gravity) has no unit because it is the ratio of the density of a substance to the density of water. Since both quantities have the same units, they cancel out, leaving a dimensionless number. For example, the relative density of silver is 10.8, with no units.
Q27. Buoyant force exists in
Buoyant force exists in all fluids—both liquids and gases. Any fluid exerts an upward buoyant force on objects immersed in it. In liquids, it is very noticeable; in gases like air, it is present but weaker (e.g., helium balloons rise due to buoyancy in air).
Q28. Archimedes’ principle applies to
Archimedes’ principle applies to all fluids—both liquids and gases. It states that any object immersed in a fluid experiences an upward buoyant force equal to the weight of the fluid displaced. This is true for water, oil, air, and any other fluid.
Q29. An iron nail sinks in water because
An iron nail sinks because the density of iron is greater than water. This means the weight of the nail is greater than the weight of the water it displaces. The buoyant force is insufficient to support the nail, causing it to sink. Gravity is still acting, and the buoyant force is actually less, not greater.
Q30. Archimedes was a
Archimedes was a Greek scientist who lived in Syracuse (in present-day Sicily, Italy) from about 287 to 212 BC. He is considered one of the greatest mathematicians and physicists of antiquity, known for his work on buoyancy, levers, and mechanics.
Q31. Archimedes used his knowledge to determine
Archimedes used his knowledge of buoyancy to determine the purity of a gold crown. The king suspected that the goldsmith had mixed silver into the crown. Archimedes compared the crown’s density with pure gold using the principle of displacement, confirming the fraud.
Q32. According to Archimedes’ principle, the buoyant force is equal to
According to Archimedes’ principle, the buoyant force acting on an object is equal to the weight of the fluid displaced by the object. This is the key statement of the principle. The density of the fluid alone is not the buoyant force; it is the weight (mass × gravity) of the displaced fluid.
Q33. The word “Eureka” means
The word “Eureka” is Greek for “I have found it” or “I have got it.” According to legend, Archimedes shouted this when he discovered the principle of buoyancy in his bathtub. This exclamation has become famous for moments of sudden discovery.
Q34. When the stone is fully immersed, further decrease in elongation is not observed because
When the stone is fully immersed, the buoyant force becomes constant because the volume of fluid displaced no longer changes. Further depth does not change the displaced volume, so the buoyant force remains the same. As a result, the elongation of the string also remains constant.
Q35. When buoyant force equals weight, the object
When the buoyant force equals the weight of the object, the object floats. The net force on the object is zero, so it remains at rest at the surface. If the buoyant force were greater, the object would rise; if it were less, the object would sink.
Q36. For the same body, buoyant force in different fluids is
For the same body, the buoyant force is different in different fluids because it depends on the density of the fluid. In a denser fluid, the buoyant force is greater. For example, the same object experiences a greater buoyant force in salt water than in fresh water.
Q37. Buoyancy acts opposite to
Buoyancy acts opposite to gravity. While gravity pulls objects downward, the buoyant force pushes them upward. This opposition is what makes objects feel lighter in fluids and allows floating. Buoyancy does not directly oppose pressure, tension, or friction.
Q38. For a floating object, buoyant force is
For a floating object, the buoyant force is equal to its weight. The two forces balance each other, so the net force is zero. This is why a floating object remains at rest on the surface of the fluid. If the buoyant force were greater, the object would rise; if less, it would sink.
Q39. When an object sinks, its weight is
When an object sinks, its weight is greater than the buoyant force. This means the downward force is stronger than the upward force, resulting in a net downward force that pulls the object to the bottom. If the buoyant force were greater, the object would rise.
Q40. If relative density of silver is 10.8, it means
Relative density of silver is 10.8, which means silver is 10.8 times denser than water. Since relative density = density of substance/density of water, a relative density greater than 1 means the substance is denser than water. This is why silver sinks in water.
Q41. The purity of substances can be determined using
The purity of substances can be determined using density because density is a characteristic property. For example, the purity of milk is checked using a lactometer, which measures density. Impure or diluted substances will have different densities compared to pure substances.
Q42. Archimedes’ principle helps in understanding
Archimedes’ principle helps in understanding floating and sinking by explaining the magnitude of the buoyant force. It shows why some objects float and others sink in fluids. Heat transfer, sound, and light are not related to Archimedes’ principle.
Q43. The upward force exerted by water on an immersed body is called
The upward force exerted by water (or any fluid) on an immersed body is called the buoyant force (or upthrust). Pressure is force per unit area, weight is the downward force due to gravity, and thrust is the perpendicular force. So buoyant force is the correct term.
Q44. The weighing machine shows apparent mass because
A weighing machine shows the apparent mass rather than the true mass because the buoyant force of air acts on the object. This upward force reduces the effective weight, making the reading slightly less than the true mass. For large and less dense objects, the effect is more significant.
Q45. Cork floats on water because
Cork floats because its density is less than water. This means the buoyant force is greater than the weight, causing it to rise. The mass is not zero, volume is not the reason, and shape is not the main factor.
Q46. Cotton bag and iron bar show same mass on weighing machine because
Cotton and iron of the same mass show the same reading on a weighing machine because the weighing machine measures mass by comparing the effective weights. The buoyant force of air acts differently on them due to their different volumes, but the scale is calibrated to give the mass. The buoyant force differs, but the reading is corrected.
Q47. Buoyant force is a type of
Buoyant force is a contact force because it acts only when the object is in contact with the fluid. The fluid molecules exert pressure on the surface of the object, creating the force. It is not a non-contact force like gravity, nor is it nuclear or magnetic.
Q48. Relative density is defined as the ratio of
Relative density is the ratio of the density of a substance to the density of water. It tells how many times the substance is denser than water. It is a dimensionless quantity (no unit). Mass to volume is the definition of density itself.
Q49. The heavier of cotton bag and iron bar in reality is
If a cotton bag and an iron bar have the same mass on a weighing machine, their actual masses are equal. The buoyant force of air acts differently—more on the cotton bag (larger volume)—but the machine reading corrects for this. In reality, the masses are equal, but the cotton bag is heavier in air due to the buoyant force correction.
Q50. A fully immersed object displaces
A fully immersed object displaces a volume of fluid equal to its own volume. This is because the object occupies space in the fluid, pushing the fluid out of the way. The mass of the displaced fluid depends on the fluid density and the volume displaced. This is the key principle behind Archimedes’ principle.