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Q1. 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 while stepping into a bathtub and noticing the water overflow. It has since become a famous expression for moments of sudden discovery.
Q2. Density of water is
The density of water is 1000 kg/m³ (or 1 g/cm³) at 4°C. This is the standard reference value used for relative density calculations. 10.8 kg/m³ is not correct, 500 kg/m³ is too low, and 19,300 kg/m³ is the density of gold.
Q3. The buoyant force depends on
Buoyant force depends on the density of the fluid and the volume of fluid displaced. According to Archimedes’ principle, the buoyant force equals the weight of the displaced fluid, which depends on the fluid’s density. Temperature affects density, but directly it is the density that matters.
Q4. The apparent loss of weight is due to
The apparent loss of weight when an object is immersed in a fluid is due to the buoyant force. The buoyant force acts upward, partially supporting the object, so the effective weight (apparent weight) is less than the actual weight. Pressure causes the force, but the loss is due to buoyancy.
Q5. All objects experience buoyant force when
All objects experience buoyant force when they are immersed in a fluid (liquid or gas). In air, buoyancy is present but weak. In a vacuum, there is no fluid, so there is no buoyant force. Being at rest does not create buoyant force; immersion does.
Q6. Density of a substance depends on
Density depends on the nature of the substance—its material composition. Under fixed conditions of temperature and pressure, density is a characteristic property of a substance. It does not depend on size, shape, or weight alone. Different substances have different densities.
Q7. Which instrument is based on Archimedes’ principle?
A lactometer is based on Archimedes’ principle. It measures the density of milk to determine its purity. The lactometer floats in milk, and the depth to which it sinks indicates the density. Hydrometers, used for measuring liquid density, are also based on this principle.
Q8. An object sinks in water because
An object sinks when its weight is greater than the upthrust (buoyant force). The low density of water is related, but the direct reason is that the downward force (weight) exceeds the upward force (buoyancy). Gravity is present, and the buoyant force is not greater.
Q9. Archimedes’ principle states that buoyant force equals
Archimedes’ principle states that the buoyant force acting on an object is equal to the weight of the fluid displaced by the object. This is the core statement of the principle. It does not equal the mass or weight of the body, nor just the volume.
Q10. Archimedes used his principle to determine
Archimedes used his principle to determine the purity of a gold crown. The king suspected the goldsmith had mixed silver into the crown. By comparing the density of the crown with pure gold using water displacement, Archimedes confirmed the fraud.
Q11. No further decrease in elongation is seen after full immersion because
After full immersion, the buoyant force becomes constant because the volume of fluid displaced is fixed (equal to the object’s volume). Further depth does not change the displaced volume, so the buoyant force remains constant. Therefore, the elongation of the string also does not change.
Q12. A ship made of iron floats because
An iron ship floats because its average density (including the air inside) is less than water. Although iron is dense, the ship is hollow, so the overall density is reduced. This allows it to displace enough water to create a buoyant force greater than its weight.
Q13. A hydrometer is used to measure
A hydrometer is used to measure the density (or relative density) of liquids. It works on the principle of buoyancy. The hydrometer floats in the liquid, and the level at which it floats indicates the liquid’s density.
Q14. Density of silver is calculated by
To find the density of silver, we multiply its relative density (10.8) by the density of water (1000 kg/m³). So density = 10.8 × 1000 = 10,800 kg/m³. This is because relative density is the ratio of the substance’s density to water’s density.
Q15. A body displaces more fluid when
A body displaces more fluid when more of it is immersed. The volume of fluid displaced equals the volume of the immersed part of the object. As the object is pushed deeper, more volume is submerged, displacing more fluid. This increases the buoyant force.
Q16. Archimedes discovered his principle after observing
Archimedes discovered his principle when he observed water overflowing from a bathtub as he stepped in. He realized that the volume of water displaced was equal to his body volume. This led to the principle that the buoyant force equals the weight of the displaced fluid.
Q17. The upward force exerted by a fluid on an immersed body is called
The upward force exerted by a fluid on an immersed body is called the buoyant force (or upthrust). Tension is the force in a string, pressure is force per unit area, and thrust is the perpendicular force. Buoyant force is the correct term for the upward force in a fluid.
Q18. Density helps in determining
Density helps in determining the purity of a substance because pure substances have characteristic densities. For example, the purity of milk is checked using a lactometer, which measures density. Impurities or dilution change the density.
Q19. Relative density of silver is given as
The relative density of silver is 10.8. This means silver is 10.8 times denser than water. 7.8 is the relative density of iron, 19.3 is for gold, and 8.9 is for copper.
Q20. Objects of density less than a liquid will
Objects with density less than a liquid will float. The weight of the object is less than the weight of the liquid it displaces, so the buoyant force is greater, causing the object to rise and float. Dissolving, breaking, and sinking are not related to this condition.
Q21. Pressure in a fluid is transmitted
According to Pascal’s law, pressure applied to a confined fluid is transmitted undiminished in all directions. This principle is used in hydraulic systems, such as car brakes and hydraulic lifts. The pressure acts equally in all directions within the fluid.
Q22. The SI unit of density is
The SI unit of density is kilograms per cubic metre (kg/m³ or kg m⁻³). g/cm³ is also a common unit but is not the SI unit. kg/m² is not a density unit, and N/m² is the unit of pressure (pascal).
Q23. The force pulling an object downward is
The force pulling an object downward is gravity. This force is the weight of the object. Pressure, thrust, and buoyant force are not downward forces—buoyant force is actually upward. So gravity is the correct answer.
Q24. Buoyancy acts in which direction?
Buoyancy acts in the vertically upward direction. It is the upward force exerted by a fluid on an object immersed in it. This force opposes the downward weight of the object. It does not act horizontally or along the surface.
Q25. Density of a substance under fixed conditions
Density of a substance under fixed conditions (temperature and pressure) remains constant. It is an intrinsic property of the material and does not depend on shape, size, or force. This constancy makes density useful as a characteristic property.
Q26. When an object is immersed in water, the apparent weight
When an object is immersed in water, its apparent weight decreases because the water exerts an upward buoyant force. This upward force partially supports the object, reducing the effective weight. The actual weight remains the same, but the apparent weight is less.
Q27. Density of gold is approximately
The density of gold is approximately 19,300 kg/m³. This makes gold one of the densest common metals. 1000 kg/m³ is water, 2700 kg/m³ is aluminium, and 7800 kg/m³ is iron.
Q28. Relative density has
Relative density 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 pure number. It is a dimensionless quantity.
Q29. An iron nail sinks in water because
An iron nail sinks because its density is greater than water. This means the weight of the nail exceeds the buoyant force. The volume being small is not the direct cause; the density being greater is the reason. The buoyant force is actually less, not greater.
Q30. If density of water is 1000 kg m⁻³, density of silver is
The relative density of silver is 10.8. To find density, we multiply relative density by the density of water: 10.8 × 1000 = 10,800 kg/m³. So the density of silver is 10,800 kg/m³.
Q31. An object floats when
An object floats when its weight is less than the buoyant force. This net upward force causes the object to rise to the surface. Once floating, the buoyant force equals the weight. Weight is never zero for real objects.
Q32. An iron sheet sinks because
An iron sheet sinks because the density of iron is greater than water. Even though the sheet is flat, it is still denser than water, so the buoyant force is less than its weight. Buoyancy is not zero; it is just less than the weight.
Q33. Fluids include
Fluids include both liquids and gases. A fluid is any substance that can flow and take the shape of its container. This includes liquids like water and gases like air. Solids are not fluids because they have fixed shapes.
Q34. A body experiences buoyant force when it is
A body experiences buoyant force when it is fully or partially immersed in a fluid. In air, buoyancy exists but is weak. In a vacuum, there is no fluid. On land, the body is not immersed. So immersion in a fluid is the condition for buoyant force.
Q35. Density is defined as
Density is defined as mass divided by volume (ρ = m/V). Volume divided by mass is specific volume, weight × volume is not a standard quantity, and mass × volume is not density.
Q36. Objects of density greater than a liquid will
Objects with density greater than the liquid will sink. The weight of the object is greater than the buoyant force. Expanding, floating, and evaporating are not related to this condition.
Q37. When buoyant force equals weight, the object
When the buoyant force equals the weight, the object floats at rest. The net force is zero, so the object remains stationary at the surface. It does not sink or break. This is the condition for floating equilibrium.
Q38. The force due to gravity acting on an object is called
The force due to gravity acting on an object is called its weight. It is given by W = mg, where m is mass and g is the acceleration due to gravity. Pressure is force per unit area, density is mass per unit volume, and thrust is the perpendicular force.
Q39. Density is a characteristic property because it
Density is a characteristic property because different substances have different densities under fixed conditions. This makes density useful for identifying materials. It does not change easily, and it is not the same for all substances.
Q40. An object remains immersed only when
An object remains immersed only when an external downward force is applied. This is because the buoyant force pushes the object upward. To keep it submerged, a downward force must overcome the buoyant force. If no external force is applied, the object will rise to the surface.
Q41. A person feels lighter while swimming because
A person feels lighter while swimming because the water exerts an upward buoyant force on the body. This force supports part of the person’s weight, making them feel lighter. Gravity does not decrease, and mass does not change.
Q42. A weighing machine shows slightly less mass because of
A weighing machine shows slightly less mass because of the buoyant force of air. The air exerts an upward force on the object, reducing its effective weight. This effect is usually very small but can be significant for large, low-density objects.
Q43. The upward force on a bottle in water is due to
The upward force on a bottle in water (buoyant force) is due to the pressure difference between the top and bottom of the bottle. The pressure is greater at the bottom, creating a net upward force. Gravity acts downward, tension is from a string, and magnetic force is not relevant.
Q44. Buoyant force increases when
Buoyant force increases when fluid density increases. The buoyant force equals the weight of the displaced fluid, which depends on the fluid’s density. A denser fluid exerts a greater buoyant force. Mass and area changes do not directly increase the buoyant force.
Q45. Archimedes was a
Archimedes was a Greek scientist who lived in Syracuse, Sicily (then part of Magna Graecia), from about 287 to 212 BC. He was one of the greatest mathematicians and physicists of antiquity.
Q46. Weight measured in water is
Weight measured in water is less than the actual weight because the water exerts an upward buoyant force. This reduces the effective weight. The object appears lighter in water. The actual weight is the weight in air.
Q47. Between a cotton bag and an iron bar of equal mass, the heavier in reality is
If a cotton bag and an iron bar have the same mass, they are equal in reality. However, due to the buoyant force of air, the cotton bag (larger volume) experiences more buoyancy, so a weighing machine shows it as slightly lighter. But in reality, both have the same mass.
Q48. Relative density is the ratio of
Relative density (specific gravity) is defined as the ratio of the density of a substance to the density of water. It tells how many times a substance is denser than water. It is a dimensionless quantity.
Q49. The decrease in elongation of a spring balance in water indicates
A decrease in elongation of a spring balance in water indicates the presence of an upward force (buoyant force). This force reduces the effective weight, so the spring stretches less. It does not mean mass is lost or density has increased.
Q50. A cork floats on water because
Cork floats on water because its density is less than water. The weight of the cork is less than the weight of the water it displaces, so the buoyant force is greater. This net upward force causes the cork to float. Gravity is still present, and pressure is not the reason.