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Q1. Objects of density greater than liquid will
Objects with density greater than that of the liquid will sink. This is because the weight of the object is more than the weight of the liquid it displaces, so the buoyant force is not enough to support the object. The downward gravitational force exceeds the upward buoyant force, causing the object to go down.
Q2. An iron nail sinks because
An iron nail sinks because its weight (downward force due to gravity) is greater than the upthrust (buoyant force) exerted by water. The nail is denser than water, so it displaces a volume of water whose weight is less than the nail’s own weight. The net downward force makes it sink.
Q3. When cork and iron nail of equal mass are placed on water
Even if cork and iron nail have equal mass, they have very different volumes. Cork has much lower density, so it displaces enough water to create a buoyant force greater than its weight, making it float. Iron has higher density, so it sinks. Mass alone does not determine floating; density does.
Q4. The different behaviour of cork and nail is due to difference in
Cork floats and iron nail sinks because of their different densities. Density is mass per unit volume. Cork is less dense than water, while iron is more dense. Volume and shape affect the density but the fundamental property causing the different behaviour is density.
Q5. The force responsible for sinking of objects is
The force responsible for sinking is gravity. Gravity pulls objects downward with a force equal to their weight. If this downward force is greater than the upward buoyant force, the object sinks. Buoyancy is the upward force that opposes sinking, and pressure and thrust are related but not the direct cause of sinking.
Q6. 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. A denser fluid exerts a greater buoyant force. The length, colour, and container shape do not affect the buoyant force.
Q7. As the stone is lowered into water, the net force on the string
As the stone is lowered into water, the buoyant force starts acting upward. This reduces the tension in the string. The net force on the string (tension) decreases because the water supports part of the stone’s weight. The decrease continues until the stone is fully immersed.
Q8. When a stone is suspended in air, the elongation of the string is due to
When a stone is suspended in air, the elongation of the string is due to the weight of the stone pulling the string downward. Buoyancy in air is negligible, pressure does not stretch the string, and air resistance is not relevant for a stationary object. The force causing the stretch is the stone’s weight.
Q9. The reading of a spring balance decreases in water because
The spring balance reading decreases in water because the water exerts an upward buoyant force on the object. This upward force partially supports the object, reducing the effective weight measured by the balance. The mass of the object does not change, and gravity does not reduce.
Q10. The upward force exerted by water on an object is called
The upward force exerted by water (or any fluid) on an object immersed in it is called upthrust or buoyant force. It is the force that makes objects feel lighter in water. Thrust is the perpendicular force, pressure is force per unit area, and weight is the downward gravitational force.
Q11. The experiment with cork and nail shows importance of
The experiment with cork and nail demonstrates that floating and sinking depend on density, not weight or mass. Even with different masses, cork floats because it is less dense than water, and iron sinks because it is denser. Density is the key factor in determining whether an object floats or sinks.
Q12. If the density of an object is greater than that of a liquid, the object will
If an object’s density is greater than the liquid’s density, it will sink. The object’s weight is greater than the weight of the liquid it displaces, so the buoyant force is insufficient to support it. This is why objects like stones and iron sink in water.
Q13. Buoyant force always acts
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 direction is what makes objects feel lighter and allows them to float.
Q14. A bottle rises when released in water because
When a bottle is released in water, it rises because the upward buoyant force is greater than its weight. This net upward force pushes the bottle to the surface. Water pressure does not decrease, weight does not increase, and gravity is still acting.
Q15. Objects float when
Objects float when the buoyant force is greater than their weight. This net upward force causes the object to rise to the surface. If the buoyant force equals the weight, the object remains suspended. If it is less, the object sinks. Weight is never zero for real objects.
Q16. The upthrust on cork is ______ its weight
The upthrust (buoyant force) on cork is greater than its weight. This is why cork rises to the surface and floats. Once it is floating at the surface, the upthrust equals the weight, but initially, it must be greater to bring it up.
Q17. Buoyancy acts opposite to
Buoyancy acts opposite to gravity. While gravity pulls objects downward, buoyancy pushes them upward. This opposition is what makes objects feel lighter in fluids and allows floating. Tension, thrust, and pressure are not the forces that buoyancy directly opposes.
Q18. The force exerted by water on an immersed object acts in the
The force exerted by water on an immersed object (buoyant force) acts in the upward direction. This is due to the pressure difference between the bottom and top of the object—pressure is greater at the bottom, creating a net upward force. It does not act circularly, downward, or horizontally.
Q19. Iron nail sinks because its density is
An iron nail sinks because its density is greater than that of water. Iron has a density of about 7,800 kg/m³, while water has a density of 1,000 kg/m³. This higher density means the nail’s weight exceeds the buoyant force, causing it to sink.
Q20. Floating and sinking mainly depend on
Floating and sinking mainly depend on density. If an object is less dense than the fluid, it floats; if it is more dense, it sinks. Size, colour, and shape are not the main factors—though shape affects volume, the fundamental property is density.
Q21. To immerse a floating object, one must
To immerse a floating object, a downward external force must be applied. This is because the buoyant force is greater than the weight of the object. The downward force must overcome the buoyant force to push the object below the surface. Buoyancy cannot be increased, gravity cannot be reduced, and density cannot be changed easily.
Q22. Buoyant force is a
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 object’s surface, creating the force. It is not a non-contact force like gravity, nor is it nuclear or magnetic.
Q23. Objects of density less than liquid will
Objects with density less than the liquid will float. The weight of the object is less than the weight of the liquid it displaces, so the buoyant force is greater. This causes the object to rise to the surface and float.
Q24. All objects experience buoyant force when they are
All objects experience buoyant force when they are immersed in a fluid (liquid or gas). In a vacuum, there is no fluid to exert the force. On the ground, the object is not immersed. In air, buoyancy is present but very weak. The condition for buoyant force is immersion in a fluid.
Q25. Once the stone is fully immersed, the buoyant force
Once the stone is fully immersed, the buoyant force remains constant because the volume of fluid displaced is fixed. The buoyant force depends on the volume displaced, which does not change once the object is fully submerged. Going deeper does not change the displaced volume.
Q26. The net force on a floating object is
The net force on a floating object is zero because the upward buoyant force equals the downward weight. These two forces balance each other, resulting in no net force. This is why the object remains at rest on the surface. If there were a net force, the object would move.
Q27. A floating object is in equilibrium because
A floating object is in equilibrium because the buoyant force equals its weight. When these forces are equal and opposite, the net force is zero, and the object remains at rest. Forces being unequal would cause motion, and density and weight are not zero.
Q28. When an iron nail is placed on water, it
When an iron nail is placed on water, it sinks because iron is denser than water. The nail’s weight is greater than the buoyant force. It does not dissolve (iron is not soluble in water), does not remain suspended (it goes down), and does not float.
Q29. The force pushing the nail upward in water is
The force pushing the nail upward in water is the buoyant force. This force is exerted by the water and acts opposite to the weight of the nail. Pressure is the cause of the buoyant force, but the upward force itself is buoyancy. Weight is downward, and tension would pull upward only if attached to a string.
Q30. Cork floats on water because
Cork floats on water because its density is less than that of water. This means the weight of the cork is less than the weight of the water it displaces, so the buoyant force is greater and pushes it up. Mass is not zero, shape is not the reason, and volume alone does not cause floating.
Q31. A stone experiences buoyant force when
A stone experiences buoyant force when it is immersed in water (or any fluid). On the ground or in air, the buoyant force is negligible. In a vacuum, there is no fluid to exert the force. Immersion in a fluid is the condition for experiencing buoyancy.
Q32. The upward force acting on the stone in water is due to
The upward force (buoyant force) acting on a stone in water is due to the water. Water exerts pressure on the stone, and the pressure difference creates a net upward force. Earth exerts gravity downward, the string provides tension, and air provides negligible buoyancy.
Q33. The force due to gravitational attraction of the earth acts on an object in the
The force due to gravitational attraction of the Earth (weight) acts in the downward direction, towards the centre of the Earth. This is the force that pulls objects down. It does not act upward, horizontally, or randomly.
Q34. If the density of an object is less than that of a liquid, the object will
If an object’s density is less than that of a liquid, it will float. The object’s weight is less than the buoyant force because it displaces a weight of liquid greater than its own weight. Breaking, sinking, and dissolving are not related to this condition.
Q35. When the stone is immersed in water, the elongation of the string
When the stone is immersed in water, the elongation of the string decreases because the water exerts an upward buoyant force on the stone. This reduces the tension in the string, so it stretches less. The weight of the stone is partially supported by the water.
Q36. Another name for upthrust is
Another name for upthrust is buoyant force. Both terms refer to the upward force exerted by a fluid on an object immersed in it. Thrust force is the perpendicular force, pressure is force per unit area, and gravitational force is the downward weight.
Q37. Objects sink when
Objects sink when their weight is greater than the buoyant force. This means the downward force due to gravity exceeds the upward force from the fluid. If the buoyant force were greater, the object would rise. If they were equal, it would remain suspended.
Q38. The density of water compared to iron is
The density of water is less than that of iron. Water has a density of 1,000 kg/m³, while iron has a density of about 7,800 kg/m³. This is why iron sinks in water. If water were denser than iron, iron would float.
Q39. Buoyancy is experienced in
Buoyancy is experienced in all fluids—both liquids and gases. In liquids, it is more noticeable (like water), and in gases, it is present but weaker (like air buoyancy). So buoyancy is not limited to liquids or gases alone; it exists in all fluids.
Q40. The force responsible for floating of objects is
The force responsible for floating of objects is buoyancy. The upward buoyant force pushes objects up. If this force is greater than or equal to the weight, the object floats. Gravity is the downward force, friction opposes motion, and pressure is the force per unit area.
Q41. The weight of an object is the force due to
The weight of an object is the force due to the gravitational attraction of the Earth. It is the downward force acting on the object. Pressure, air resistance, and buoyancy are different forces. Weight is specifically the gravitational force.
Q42. Decrease in elongation indicates
A decrease in the elongation of a spring indicates the presence of an upward force (buoyant force) acting on the object. This upward force reduces the effective weight, so the spring stretches less. It does not mean gravity has increased, there is no force, or mass has been lost.
Q43. Density is defined as
Density is defined as mass per unit volume. The formula is ρ = m/V. Force per unit area is pressure, volume per unit mass is specific volume, and weight per unit area is not a standard physical quantity.
Q44. The external downward force must be at least equal to
To keep a floating bottle fully immersed, the external downward force must be at least equal to the difference between the buoyant force and the weight of the bottle. This is the net upward force that must be overcome. If the downward force is less, the bottle will rise.
Q45. The magnitude of buoyant force depends on the
The magnitude of buoyant force depends on the density of the fluid and the volume of fluid displaced. A denser fluid exerts a greater buoyant force. The shape and mass of the object affect the displaced volume but do not directly determine the buoyant force. Colour does not affect it.
Q46. The upthrust on an iron nail is ______ its weight
The upthrust (buoyant force) on an iron nail is less than its weight. This is why the nail sinks. The nail is denser than water, so it displaces less water than its own weight. The buoyant force is insufficient to support the nail.
Q47. The reason a bottle comes up after release is
A bottle comes up after release because the buoyant force dominates—it is greater than the weight of the bottle. This net upward force pushes the bottle to the surface. Density does not change, gravity does not stop, and pressure is still present.
Q48. To keep a bottle completely immersed in water, an external force must act
To keep a bottle completely immersed in water, an external downward force must be applied. The buoyant force acts upward and is greater than the bottle’s weight. The downward force must overcome this buoyant force to keep the bottle submerged.
Q49. The density of water compared to cork is
The density of water is more than that of cork. Water has a density of 1,000 kg/m³, while cork has a density of about 200-300 kg/m³. This lower density of cork is why it floats on water.
Q50. The force pulling the iron nail downward is due to
The force pulling the iron nail downward is due to gravity. This is the weight of the nail, which acts towards the centre of the Earth. Thrust is the perpendicular force, buoyancy is upward, and pressure is force per unit area. The downward force is gravity.