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Q1. Density is independent of
Density is a characteristic property of a material and depends on the nature of the substance (material). It does not depend on the shape, size, or mass of the object. For a given material under fixed conditions, density remains constant regardless of its shape. For example, a small piece of iron and a large piece of iron have the same density.
Q2. Pressure is maximum when
Pressure is defined as force per unit area (P = F/A). For a given force, pressure is inversely proportional to area. If the area is smaller, the pressure is greater. This is why sharp objects like knives and needles have small areas to produce high pressure. If area increases, pressure decreases.
Q3. Why does a steel needle float on water if placed gently?
A steel needle can float on water if placed gently because of surface tension. The water surface behaves like a stretched elastic membrane due to the cohesive forces between water molecules. This surface tension can support the needle’s weight if placed carefully. The needle is denser than water, but surface tension prevents it from sinking.
Q4. A balloon rises in air because
A balloon rises when the buoyant force of air is greater than its weight. The balloon is filled with a gas (like helium or hot air) that is less dense than the surrounding air. This creates a net upward force, causing the balloon to rise. Gravity is still present but is overcome by the buoyant force.
Q5. Air also exerts buoyant force because
Air exerts buoyant force because it is a fluid. Any fluid—liquid or gas—exerts an upward buoyant force on objects immersed in it. Air has mass and volume, but it is the fact that it is a fluid that allows it to exert buoyant force. The principle applies to all fluids.
Q6. Pressure exerted by a fluid acts
Pressure exerted by a fluid acts in all directions. This is a fundamental property of fluids. At any point in a fluid, the pressure is the same in all directions. This is why you feel pressure from all sides when you are underwater.
Q7. Buoyant force on an object depends on
The buoyant force on an object depends on the volume of fluid displaced and the density of the fluid. According to Archimedes’ principle, the buoyant force equals the weight of the displaced fluid. Colour, shape, and weight alone do not determine the buoyant force.
Q8. When weight equals buoyant force, the object
When the weight of an object equals the buoyant force, the object remains suspended in the fluid. The net force is zero, so the object neither rises nor sinks. It stays at rest at that depth. If the object is at the surface, it floats; if it is fully immersed, it remains suspended.
Q9. A submarine sinks by
A submarine sinks by increasing its overall density. It takes in water into its ballast tanks, which increases its mass without changing its volume significantly. This increases the average density, making it greater than the surrounding water, so the submarine sinks. To rise, it expels water.
Q10. Which material would float best?
A material with low density would float best because it displaces a volume of fluid whose weight is greater than its own weight. The lower the density, the greater the buoyant force relative to its weight. This is why cork (low density) floats well.
Q11. Which has the least density?
Among the options, air has the least density. Air density is about 1.2 kg/m³, while water is 1000 kg/m³, iron is 7800 kg/m³, and mercury is 13,600 kg/m³. This is why balloons filled with air do not float in air—they are denser than air.
Q12. The buoyant force in air is called
The buoyant force in air is called upthrust. Just as water exerts an upward buoyant force, air also exerts a buoyant force on objects immersed in it. This is why objects appear slightly lighter in air compared to vacuum. Upthrust is another name for buoyant force.
Q13. Why does a rubber ball float?
A rubber ball floats because its overall density is less than that of water. The air trapped inside the ball reduces its average density. This makes the weight of the ball less than the buoyant force, allowing it to float. Low pressure and high gravity are not reasons.
Q14. Why do buildings have wide foundations?
Buildings have wide foundations to reduce the pressure they exert on the ground. Pressure = Force/Area. By increasing the area of the foundation, the weight of the building is spread over a larger area, reducing pressure. This prevents the building from sinking into the ground.
Q15. Which instrument measures milk purity?
A lactometer is used to measure the purity of milk. It works on the principle of buoyancy. The lactometer floats in milk, and the depth to which it sinks indicates the density of the milk. Pure milk has a higher density than diluted milk, so the lactometer floats higher in pure milk.
Q16. Which fluid exerts pressure?
Both liquids and gases exert pressure. These are collectively called fluids. Solids also exert pressure, but they do not flow like fluids. Fluids exert pressure on the walls of their containers and on objects immersed in them in all directions.
Q17. Buoyant force becomes zero when
Buoyant force becomes zero when an object is in a vacuum because there is no fluid to exert the upward force. In air, buoyant force exists but is weak. The buoyant force is still present whether the object is sinking or floating. Only in the absence of a fluid does the buoyant force become zero.
Q18. Why do lifeboats have hollow structures?
Lifeboats have hollow structures to reduce their average density. By containing air spaces, the overall density of the boat becomes less than water, allowing it to float. The mass is not reduced significantly; the volume increases, which reduces the density.
Q19. A solid sinks in water but floats in oil. This means
If a solid sinks in water but floats in oil, it means the solid’s density is between the densities of water and oil. Oil has lower density than water. So the solid is denser than oil but less dense than water. This is why it sinks in water (which is denser) and floats in oil.
Q20. Snow shoes prevent people from sinking into snow because they
Snow shoes have a large area of contact with the snow. This spreads the person’s weight over a larger area, reducing the pressure on the snow. Lower pressure prevents the person from sinking into the snow. The weight remains the same, but pressure is reduced.
Q21. Which has greater pressure on ground?
A man standing exerts greater pressure on the ground than when lying down. When standing, the area of contact with the ground is smaller (only the feet), so the pressure (force/area) is higher. When lying down, the area is larger, so the pressure is lower.
Q22. Which substance has relative density less than 1?
Ice has a relative density less than 1 (about 0.92). This means it is less dense than water, which is why ice floats on water. Mercury has a relative density of 13.6, iron is about 7.8, and gold is about 19.3. All of these are greater than 1.
Q23. Why do we feel less weight in water?
We feel less weight in water because the water exerts an upward buoyant force on our body. This upward force supports part of our weight, reducing the effective weight felt by the body. The actual weight and gravity remain the same; only the apparent weight decreases.
Q24. Buoyant force is maximum in
Buoyant force is maximum in mercury because mercury is a very dense fluid (density = 13,600 kg/m³). The buoyant force depends on the density of the fluid. Since mercury has the highest density among the options, it exerts the greatest buoyant force on an immersed object.
Q25. Relative density compares density with
Relative density (specific gravity) is the ratio of the density of a substance to the density of water. Water is used as the reference because its density is 1000 kg/m³ at 4°C. This allows easy comparison of the densities of different substances.
Q26. Pressure is defined as
Pressure is defined as the force acting perpendicular to a surface divided by the area over which it acts. P = F/A. Force × area is not pressure, mass ÷ volume is density, and area ÷ force is the reciprocal of pressure.
Q27. An object floats when its average density is
An object floats when its average density is less than water. This means the object’s weight is less than the weight of the water it displaces. The buoyant force is greater, causing the object to rise and float. If density equals water, it remains suspended; if greater, it sinks.
Q28. Which factor does NOT affect pressure exerted by a body?
The pressure exerted by a body depends on the force and the area over which it acts. Acceleration due to gravity affects weight (force), but the shape of the object does not directly affect pressure. Only the force and the contact area matter. So shape is the factor that does NOT affect pressure.
Q29. Why does ice float on water?
Ice floats on water because it has less density than liquid water. The density of ice is about 920 kg/m³, while water is 1000 kg/m³. The lower density means the buoyant force is greater than the weight, causing ice to float. This is why icebergs float on oceans.
Q30. A body immersed deeper in a fluid experiences
As a body is pushed deeper into a fluid, it displaces more fluid until it is fully immersed. The buoyant force depends on the volume of fluid displaced, so it increases until full immersion. Once fully immersed, the buoyant force becomes constant regardless of further depth.
Q31. Buoyancy explains
Buoyancy explains why objects float or sink in fluids. The principle of buoyancy, discovered by Archimedes, tells us that the upward force on an object depends on the weight of the fluid displaced. This determines whether the object floats, sinks, or remains suspended.
Q32. A submarine floats by
A submarine floats by decreasing its overall density. It expels water from its ballast tanks, reducing its mass while volume remains the same. This decreases the average density, making it less than the surrounding water, so the submarine rises and floats.
Q33. A camel can walk easily on sand because
A camel can walk easily on sand because its wide feet increase the area of contact with the sand. This reduces the pressure on the sand, preventing it from sinking. The camel’s weight is spread over a larger area, so the pressure is less. Thin legs would increase pressure and cause sinking.
Q34. Which object experiences the greatest buoyant force in water?
A fully immersed large object experiences the greatest buoyant force because it displaces the largest volume of water. The buoyant force depends on the volume of fluid displaced. A larger volume means more displaced fluid and therefore a greater buoyant force.
Q35. Which factor does buoyant force NOT depend on?
Buoyant force does NOT depend on the mass of the object. It depends only on the density of the fluid, the volume of fluid displaced, and the acceleration due to gravity (which is included in the weight of the displaced fluid). The mass of the object affects its weight, not the buoyant force.
Q36. Why do ships float even though they are made of iron?
Ships float because their large volume (due to their hollow structure) makes their average density less than water. Although iron is denser than water, the ship contains a lot of air, which reduces the overall density. This allows the ship to displace enough water to create a buoyant force greater than its weight.
Q37. A helium balloon rises because
A helium balloon rises because helium has a lower density than air. The balloon’s weight is less than the weight of the air it displaces, so the buoyant force is greater. This net upward force causes the balloon to rise. Gravity is still present, but buoyancy overcomes it.
Q38. Which scientist is linked with buoyancy?
Archimedes is the scientist linked with buoyancy. He formulated Archimedes’ principle, which states that the buoyant force on an object is equal to the weight of the fluid displaced. This principle explains floating and sinking.
Q39. Which of the following fluids provides maximum buoyant force?
Mercury provides the maximum buoyant force because it is the densest fluid among the options. The buoyant force is directly proportional to the density of the fluid. Mercury has a density of 13,600 kg/m³, much higher than water, oil, or air, so it exerts the greatest buoyant force.
Q40. Which object will displace more water?
A fully immersed object displaces a volume of water equal to its own volume. A floating object displaces only a portion of its volume. A sinking object also displaces its full volume, but it is not fully immersed until it is completely submerged. Among the options, a fully immersed object displaces the most water.
Q41. A stone appears lighter in water because
A stone appears lighter in water because water exerts an upward buoyant force on the stone. This upward force partially supports the stone’s weight, reducing the effective weight felt. The mass and gravity remain unchanged, and density does not increase. The upward force from water is the reason.
Q42. Which property helps identify purity of gold?
Density helps identify the purity of gold. Pure gold has a specific density (19,300 kg/m³). If the gold is mixed with other metals, its density changes. Archimedes famously used this principle to determine the purity of a gold crown.
Q43. A body displaces maximum fluid when it is
A body displaces maximum fluid when it is fully immersed because the volume of fluid displaced equals the entire volume of the body. When floating or partially immersed, only part of the body is in the fluid, so less fluid is displaced.
Q44. Which law explains floating of bodies?
Archimedes’ principle explains the floating of bodies. It states that the buoyant force on an object equals the weight of the fluid displaced. This principle determines whether an object floats, sinks, or remains suspended in a fluid.
Q45. A diver feels maximum buoyant force when
A diver feels maximum buoyant force when fully immersed because the volume of water displaced is maximum. When partially immersed, less water is displaced, so the buoyant force is less. The maximum buoyant force occurs when the entire body is submerged.
Q46. The SI unit of pressure is
The SI unit of pressure is the pascal (Pa), named after Blaise Pascal. One pascal is equal to one newton per square metre (1 Pa = 1 N/m²). Joule is the unit of energy, newton is the unit of force, and watt is the unit of power.
Q47. A floating body displaces fluid equal to
A floating body displaces a volume of fluid whose weight is equal to its own weight. This is because the buoyant force equals the weight of the displaced fluid, and for a floating object, the buoyant force equals the object’s weight. So the weight of displaced fluid equals the weight of the object.
Q48. The density of a hollow object is
A hollow object has a lower average density than the solid form of the same material because it contains air spaces. The volume is larger for the same mass, reducing the overall density. This is why hollow objects (like ships) can float even if the material is dense.
Q49. Pressure in liquids increases with
Pressure in liquids increases with depth. The deeper you go in a liquid, the greater the pressure due to the weight of the liquid above. This is why dams are thicker at the bottom. Pressure increases with depth, not with height, area, or volume.
Q50. Why does a sharp knife cut better than a blunt one?
A sharp knife cuts better than a blunt one because it has a very small area at the edge. For the same force, a smaller area produces higher pressure (P = F/A). This high pressure allows the knife to cut through materials easily. Weight, mass, and density are not the primary reasons.