Class 9 Physics thrust Pressure and Buoyancy (Gravitation) NCERT Solutions

Thrust and Pressure, Pressure in Fluids, Buoyancy, Why objects floats or sink when placed on the surface of water?, Archimedes’ Principle, Relative Density

Why is it difficult to hold a school bag having a strap made of a thin and strong string?

It is difficult to hold a school bag having a strap made of a thin and strong string because it apply the large pressure on the shoulders due to very small contact surface area . we know that the pressure is inversely proportional to the surface area on which the force acts

What do you mean by buoyancy?

The tendency of a liquid to exert an upward force on an object placed in it is called buoyancy. 

When a body is completely or partially immersed in a fluid, then the upward thrust acting on the body is called force of buoyancy or buoyant force.

It increases as we push the object deeper into water. The magnitude of this buoyant force depends on the density of the fluid.

Why does an object float or sink when placed on the surface of water?

An object floats or sinks when placed on the surface of water because of the buoyant force acting on the object.

Why the cutting edge of a knife should be as sharp as possible? 

The cutting edge of a knife should as sharp to create greater pressure even for a very small applied force for quick action.

You find your mass to be 42 kg on a weighing machine. Is your mass more or less than 42 kg?


When we weight our body, a downward ward gravitational force acts on it. Hence our actual mass is less than 42 kg

You have a bag of cotton and an iron bar, each indicating a mass of 100 kg when measured on a weighing machine. In reality, one is heavier than other. Can you say which one is heavier and why?

In reality, a bag of an iron bar is heavier than a bag of cotton.Each indicating a mass of 100 kg when measured on a weighing machine as more gravitational force act on a bag of cotton due to its larger the surface area.

In what direction does the buoyant force on an object immersed in a liquid act?

Ans: The buoyant force acts on an object in the vertically upward direction through the center of gravity of the displaced liquid. 

Why does a block of plastic released under water come up to the surface of water?

A block of plastic released under water come up to the surface of water because the up thrust or the buoyant force exerted by the water is greater than the downward gravitational force. 

The volume of 50 gm of a substance is 20 cm³. If the density of water is 1 g/cm³, will the substance float or sink?

Given the mass of the substance = 50 g     Volume of the substance = 20 cm³

Density of substance = Mass /Volume = (50 /20) g/cm³ = 2.5 g/cm³

Since the density of the substance 2.5 g/cm³ is greater than the density of water (1 g/cm³), so the substance will sink in the water.

The volume of a 50 gm sealed packet is 350 cm³. Will the packet float or sink in water if the density of water is 1 g/cm³ ? What will be the mass of the water displaced by this packet  ?

Mass of packet = 500 g               

Volume of packet = 350 cm³

Density of packet = Mass ÷ Volume 
                                = (500 / 350) g/cm³ = 1.43 g/cm³

The sealed packet will sink in the water as its density 1.43 g/cm³ is more than that of water.

The volume of water that will be displaced by the packet = 350 cm³

And mass of this displaced water = Volume x Density = 350 x 1 = 350 gm.


State Archimedes’ principle. Give any two applications of Archimedes principle?

Archimedes’ principle states that when a body is immersed partially or fully in a fluid, it experiences an upward force that is equal to the weight of the fluid displaced by it.

The two applications of Archimedes principle are given below:

(i) Floating of ships and submarines (ii) The lactometers to test purity of milk

How do a submarine sink and float on water?

A submarine can both sink and float on water because of the ballast tank . 

To make a submarine sink into the water, these tanks are filled with water. This increases the weight of the submarine and it sinks.

To make a submarine float on the water surface, these tanks are emptied by pumping out water from them. This decreases the weight of the submarine and it floats

To make a submarine float on the water surface, these tanks are emptied by pumping out water from them. This decreases the weight of the submarine and it floats

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IX Thrust and Pressure, Archimedes’ Principle, Relative Density
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