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Q1. The force of buoyancy is also called
The force of buoyancy is also called upthrust. It is the upward force exerted by a fluid on an object immersed in it. This force acts in the upward direction, opposite to the weight of the object. Thrust is the force perpendicular to a surface, weight is the downward force due to gravity, and pressure is force per unit area. So upthrust is the correct term for the buoyant force.
Q2. When an object is released in water, it comes up due to
When an object is released in water and it comes up, it is due to the buoyant force. The buoyant force acts upward and is greater than the weight of the object. This net upward force causes the object to rise to the surface. Pressure is the force per unit area, gravity pulls objects downward, and friction opposes motion. So the upward movement is due to buoyancy.
Q3. Decrease in elongation of the string indicates the presence of
When a stone is suspended from a spring balance and then dipped in water, the elongation of the spring decreases. This decrease indicates that an upward force (buoyant force or upthrust) is acting on the stone, reducing the effective weight. Gravity still acts downward, but the upward force partially cancels it, resulting in a smaller reading on the spring balance.
Q4. Objects of lower density than water will
Objects with lower density than water will float. This is because the weight of the water displaced by the object is greater than the weight of the object itself. The buoyant force is therefore greater, causing the object to float. Examples include cork, wood, and plastic. Dissolving is a chemical process, sinking occurs for denser objects, and breaking is not relevant.
Q5. Buoyancy helps explain
Buoyancy is the phenomenon that explains why some objects float and others sink in fluids. It is based on the difference between the weight of the object and the buoyant force exerted by the fluid. Friction, gravity, and pressure are separate concepts, though buoyancy is related to pressure differences in fluids.
Q6. Objects of greater density than water will
Objects with greater density than water will sink because the weight of the water displaced is less than the weight of the object. The buoyant force is insufficient to support the object’s weight, so it sinks. Iron, lead, and stones are denser than water and sink. Expanding is a property of heating, evaporation is a phase change, and floating occurs for less dense objects.
Q7. Density is defined as
Density is defined as mass per unit volume. The formula is ρ = m/V, where ρ (rho) is density, m is mass, and V is volume. Volume per unit mass is the reciprocal of density (specific volume). Force per unit area is pressure, and weight per unit area is not a standard physical quantity.
Q8. Density depends on
Density depends on mass and volume. It is the amount of mass contained in a unit volume. For example, a material with more mass in the same volume has higher density. Weight and force depend on gravity, area and thrust relate to pressure, and pressure and force are different concepts. So mass and volume are the correct factors.
Q9. An iron nail sinks in water because
An iron nail sinks because its weight (downward force) is greater than the buoyant force (upthrust) exerted by the water. The nail has higher density than water, so it displaces less water than its own weight. As a result, the upward force is insufficient to support the nail, and it sinks. The density of water being low is the reason, but the direct cause is that the downward force (weight) exceeds the upthrust.
Q10. When an iron nail is placed on the surface of water, it
An iron nail placed on the surface of water sinks because iron is denser than water. The weight of the nail is greater than the buoyant force. Even if placed gently on the surface, the surface tension may briefly support it, but it will eventually sink. Dissolving occurs for soluble substances, breaking is not relevant, and floating occurs for less dense objects.
Q11. The reason a nail sinks is
A nail sinks because it has high density compared to water. Density is the mass per unit volume. Since the nail’s density is greater than water, its weight is greater than the weight of the water it displaces. The buoyant force is less than the weight, causing it to sink. Low pressure, low volume, and low mass are not the primary reasons for sinking.
Q12. The upward force acting on a stone in water is due to
The upward force (buoyant force) acting on a stone in water is exerted by the water. The water exerts pressure on all surfaces of the stone, and the pressure is greater at the bottom than at the top. This pressure difference results in a net upward force. The string provides tension, air provides negligible buoyancy, and Earth provides the downward gravitational force.
Q13. A floating object rises to the surface because
A floating object rises to the surface because the buoyant force acting on it is greater than its weight. This net upward force pushes the object up until it reaches the surface. At the surface, the buoyant force equals the weight, and the object floats. Weight does not increase, gravity does not disappear, and density does not become zero.
Q14. The buoyant force on a body increases when
As a body is pushed deeper into a fluid, more of its volume is submerged, displacing more fluid. The buoyant force depends on the volume of fluid displaced. As more fluid is displaced, the buoyant force increases. Once fully immersed, the buoyant force becomes constant regardless of further depth. Decreasing mass or volume would decrease the buoyant force, and taking it out would remove the buoyant force.
Q15. A body floats if its average density is
A body floats if its average density is less than the density of the liquid. This means the weight of the body is less than the weight of the liquid it displaces, so the buoyant force is greater than the weight. If density is equal, the body will remain suspended. If density is greater, it will sink. The average density includes any hollow spaces (like in a ship) that make the overall density less than water.
Q16. When an object sinks, the buoyant force is
When an object sinks, the buoyant force acting on it is less than its weight. This means the downward force (weight) is greater than the upward force (buoyant force), causing the object to sink. If the buoyant force were equal to weight, the object would be suspended or float. If it were greater, the object would rise.
Q17. 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. The buoyant force is therefore greater than the weight, causing it to float. The volume being small does not cause floating, and cork certainly has mass and weight. The key factor is lower density.
Q18. 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 the buoyant force. This force is due to the pressure difference between the bottom and top of the object. Pressure is force per unit area, friction opposes motion, and weight is the downward gravitational force. So the correct answer is buoyant force.
Q19. Buoyant force is a
Buoyant force is a contact force because it acts only when the object is in contact with the fluid. The force arises from the pressure exerted by the fluid molecules on the surface of the object. It is not a non-contact force like gravity or magnetism, and it is not electric or magnetic. The fluid must be in contact with the object for the buoyant force to act.
Q20. An iron ship floats because
An iron ship floats because its shape allows it to displace a large volume of water. Although iron is denser than water, the ship is hollow, so its average density is less than water. The shape creates a large volume, displacing enough water to produce a buoyant force greater than the ship’s weight. This is why a solid iron block sinks but a ship made of iron floats.
Q21. The density of iron compared to water is
The density of iron 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 is why solid iron sinks in water. However, an iron ship can float because its overall density (including the air inside) is less than water.
Q22. Objects sink in a liquid when
Objects sink in a liquid when their weight is greater than the buoyant force. This means the downward force due to gravity exceeds the upward force exerted by the liquid. If the buoyant force were greater, the object would rise. Density being less would cause floating, and pressure is not zero in a liquid.
Q23. The upthrust acting on cork is ______ its weight
The upthrust (buoyant force) acting on cork is greater than its weight. This net upward force is what causes the cork to rise to the surface and float. Once the cork is floating, the buoyant force equals the weight, but initially, it is greater to bring it up. If it were less, the cork would sink.
Q24. Buoyancy acts against
Buoyancy acts against 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. Friction opposes motion, pressure is the force per unit area, and thrust is the perpendicular force. So buoyancy directly opposes gravity.
Q25. 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. The shape and colour of the object do not directly affect the buoyant force, and the mass of the object affects its weight, not the buoyant force.
Q26. To keep a floating bottle immersed in water, an external force must act
To keep a floating bottle fully immersed in water, an external downward force must be applied. This is because the buoyant force acts upward and is greater than the weight of the bottle. To overcome this upward force and keep the bottle submerged, a downward force is needed. If you release it, the bottle will rise to the surface.
Q27. Objects float in a liquid when
Objects float in a liquid when the buoyant force is greater than their weight. This net upward force causes the object to rise to the surface. Once floating, the buoyant force equals the weight. If the weight is greater, the object sinks. Density being high would cause sinking, and weight is never zero for real objects.
Q28. If the density of an object is greater than that of a liquid, the object will
If the density of an object is greater than that of a liquid, the object will sink. This is because the weight of the object is greater than the weight of the liquid it displaces, so the buoyant force is insufficient to support it. Expanding, floating, and evaporating are not related to density comparison.
Q29. A floating object is in equilibrium because
A floating object is in equilibrium because the upward buoyant force equals the downward weight. This means there is no net force acting on the object, so it remains at rest. If the forces were unbalanced, the object would move. Pressure is not zero, and density is not zero for real objects.
Q30. When the stone is dipped in water, the elongation of the string
When a stone suspended from a string is dipped into water, the elongation of the string decreases. This is because the water exerts an upward buoyant force on the stone, reducing the tension in the string. The effective weight of the stone becomes less, so the string stretches less. This demonstrates the presence of buoyant force.
Q31. The spring balance reading decreases in water because
The spring balance reading decreases when an object is immersed in water because the water exerts an upward buoyant force on the object. This upward force partially supports the weight of the object, so the balance reads a lower value. The actual weight and mass of the object do not change; only the effective weight is reduced.
Q32. The downward force acting on an iron nail is due to
The downward force acting on an iron nail is its weight, which is due to the gravitational attraction of the Earth. Gravity pulls the nail downward towards the centre of the Earth. Buoyancy is the upward force, pressure is force per unit area, and air resistance opposes motion. So the downward force is gravity.
Q33. All objects immersed in fluids experience
All objects immersed in fluids experience a buoyant force (upthrust) exerted by the fluid. This is a universal phenomenon—every object in a fluid experiences an upward force due to pressure differences. Pressure is also experienced, but the question asks for the specific force related to immersion. Gravity is present too, but buoyant force is the unique force experienced due to immersion.
Q34. The density of cork compared to water is
The density of cork is less than that of water. This is why cork floats on water. Cork has many air spaces, making it light for its volume. Its density is about 200-300 kg/m³, while water has a density of 1000 kg/m³. This lower density results in a greater buoyant force than weight, causing it to float.
Q35. The unit of density is
The unit of density is kilograms per cubic metre (kg/m³). Density is mass divided by volume, so the unit is the unit of mass (kg) divided by the unit of volume (m³). N/m² is the unit of pressure, kg is the unit of mass, and m/s² is the unit of acceleration. So kg/m³ is the correct unit.
Q36. Once a stone is fully immersed in water, the buoyant force
Once a stone is fully immersed in water, the buoyant force remains constant. This is because the buoyant force depends on the volume of fluid displaced, and for a fully immersed object, the volume displaced is constant (equal to the object’s volume). Further depth does not change the displaced volume (assuming constant fluid density), so the buoyant force remains constant.
Q37. If the density of an object is less than that of a liquid, the object will
If the density of an object is less than that of a liquid, the object 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. Dissolving is a chemical process, and sinking happens when density is greater.
Q38. Two objects of equal mass placed on water behave differently due to
Two objects of equal mass can behave differently in water due to differences in density. Density is mass per unit volume. If the objects have equal mass but different volumes, they will have different densities. The object with lower density will float, while the one with higher density may sink. Temperature and colour do not affect floating behaviour directly, and shape only matters as it affects volume.
Q39. The force that pulls objects downward is
The force that pulls objects downward is gravity. Gravity is the attractive force between objects with mass, and on Earth, it pulls everything towards the centre. Pressure is force per unit area, buoyancy is the upward force in fluids, and thrust is the perpendicular force. So gravity is the downward force.
Q40. Buoyancy is experienced in
Buoyancy is experienced 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 (like water buoyancy). In gases like air, it is also present, though weaker. For example, helium balloons rise because of buoyancy in air. So buoyancy is experienced in all fluids.
Q41. The force acting upward on immersed objects comes from
The upward force acting on immersed objects (buoyant force) comes from the fluid (liquid or gas) in which the object is immersed. The fluid exerts pressure on all surfaces, and the pressure difference results in an upward force. Earth provides gravity downward, the surface supports objects at rest, and air is one type of fluid but not the only one.
Q42. The upthrust acting on an iron nail is ______ its weight
The upthrust (buoyant force) acting on an iron nail is less than its weight. This is why the nail sinks. The nail is denser than water, so the weight of the water it displaces is less than the weight of the nail. The buoyant force is insufficient to support the nail, causing it to sink.
Q43. The force of buoyancy acts in which direction?
The force of buoyancy always acts in the 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 acts vertically upward, towards the surface of the fluid.
Q44. A cork floats but an iron nail sinks because of difference in
A cork floats and an iron nail sinks because of the difference in their densities. Cork has lower density than water, so it floats. Iron has higher density than water, so it sinks. Size, shape, and weight are related, but the fundamental reason is density. The cork’s weight is less than the buoyant force, while the nail’s weight is greater.
Q45. The net force on a stone immersed in water is
The net force on a stone immersed in water is decreased compared to when it is in air. This is because the buoyant force acts upward, partially cancelling the downward weight. The net downward force is weight minus buoyant force, which is less than the weight alone. The stone feels lighter. It does not become zero unless it floats (buoyant force equals weight).
Q46. 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. The stone’s weight pulls the string downward, stretching it. Buoyancy in air is negligible, density is a property, and pressure is not causing the elongation. The force causing the stretch is the stone’s weight.
Q47. A fully immersed object experiences
A fully immersed object experiences a constant buoyant force, provided the fluid density is constant. The buoyant force depends on the volume of fluid displaced, which is fixed once the object is fully immersed. Going deeper does not change the displaced volume, so the buoyant force remains constant. It does not increase or decrease.
Q48. Floating and sinking depend mainly on
Floating and sinking depend mainly on density. If an object’s density is less than the fluid’s density, it will float. If it is greater, it will sink. If they are equal, it will remain suspended. Temperature affects density but is not the direct cause. Shape and colour do not determine whether an object floats or sinks.
Q49. A decrease in spring balance reading in water shows
A decrease in spring balance reading when an object is immersed in water shows the presence of buoyant force. The water exerts an upward force on the object, reducing the effective weight measured by the balance. The mass of the object does not change, and the actual weight (mg) is still the same. The decrease is due to the buoyant force partially supporting the object.
Q50. The force responsible for sinking is
The force responsible for sinking is gravity. Gravity pulls the object downward with a force equal to its weight. If this downward force is greater than the upward buoyant force, the object sinks. Thrust and pressure are related but are not the force causing sinking. Buoyancy is the upward force that opposes sinking. So gravity is the correct answer.