CBSE Class 9 Physics Hydrostatics MCQs Set 11

Answer: (d) N

Explanation:
1. Thrust is a force acting perpendicular to a surface, so its unit is the newton (N).
2. N m⁻² is the unit of pressure.

Teacher's Note:
1. Thrust and force have the same unit.
2. Pressure = thrust / area.

Answer: (a) viscosity

Explanation:
1. A superfluid (such as helium below 2.17 K) flows without any internal friction.

Teacher's Note:
1. Viscosity is the internal friction of a fluid.
2. It still has density and shows buoyancy.

Answer: (c) remain unchanged

Explanation:
1. A floating ice cube displaces water equal in weight to the ice itself.
2. When it melts, the water produced has that same weight and so the same volume as the water displaced, so the level does not change.

Teacher's Note:
1. Floating means weight of ice = weight of displaced water.
2. This would not be true for ice held below the surface.

Answer: (c) increase

Explanation:
1. The water pushes the hand up with the buoyant force, and by Newton's third law the hand pushes the water down with an equal force.
2. This downward force is added to the weight of the beaker and water, so the reading increases.

Teacher's Note:
1. The reaction to the buoyant force acts on the water.
2. The increase equals the weight of the water displaced by the hand.

Answer: (a) in all directions

Explanation:
1. By Pascal's law, pressure applied to an enclosed liquid is transmitted equally in all directions.

Teacher's Note:
1. Liquids transmit pressure equally in all directions.
2. This principle is used in hydraulic machines.

Answer: (d) Its shape, which lets it displace water weighing as much as the ship

Explanation:
1. A ship is built hollow so that its average density is less than that of water.
2. It sinks only until the weight of water displaced equals its own weight; a needle displaces far less than its weight.

Teacher's Note:
1. Floating depends on the weight of the water displaced, not on the metal alone.
2. A steel needle can rest on the surface only because of surface tension when placed carefully.

Answer: (a) a storm

Explanation:
1. A rapid fall in pressure means that air from neighbouring regions of higher pressure rushes in, giving rise to a storm.

Teacher's Note:
1. Steady high pressure usually means fair weather.
2. A barometer is used to watch pressure changes.

Answer: (c) 89%

Explanation:
1. Fraction immersed = \( \dfrac{\text{density of ice}}{\text{density of sea water}} = \dfrac{0.92}{1.03} = 0.893 \).
2. So about 89% is below the surface.

Teacher's Note:
1. Mass of iceberg = mass of water displaced.
2. Only about 11% shows above water.

Answer: (d) the flow of air around the wings provides the lift

Explanation:
1. The density of air is far too small for buoyancy to support an aeroplane.
2. The shape of the wings makes the air flow faster over the top than below, giving a net upward force called lift.

Teacher's Note:
1. Buoyancy supports balloons, not aeroplanes.
2. The lift depends on the speed of the plane.