02 November 2015

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/cmis 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/cmis 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

More Questions are solved Here:  http://jsuniltutorial.weebly.com/

Sure shoot MCQ for class 9 science term-2 ClicK Here

IX Thrust and Pressure, Archimedes’ Principle, Relative Density
CBSE Class 9 - Science - Chapter 10: Flotation: Notes and Quest
MCQ: Flotation: Thrust, Pressure, Buoyancy and Density
Thrust and Pressure, Archimedes’ Principle, Relative Density key point
Notes : Flotation: Thrust, Pressure, Buoyancy and Density
Physics Flotation Term-II Class IX  Buoyant force Detail Study
9th Physics Solved Numerical Floating bodies

04 April 2015

Solved Electricity numerical for class 10

1. Question: Two bulbs have ratings 100 W, 220 V and 60 W, 220 V respectively. Which one has a greater resistance?

Answer: P=VI=  V2/R For the same V, R is inversely proportional to P. 

Therefore, the bulb 60 W, 220 V has a greater resistance.

2. Question: A torch bulb has a resistance of Ω when cold. It draws a current of 0.2 A from a source of 2 V and glows. Calculate
(i) the resistance of the bulb when glowing and
(ii) explain the reason for the difference in resistance.

(i) When the bulb glows:

V = I R ---- Ohm's law R = V/I = 2/.2 =10 Ω

(ii) Resistance of the filament of the bulb increases with increase in temperature. Hence when it glows its resistances is greater than when it is cold.

3. Question: Calculate the resistance of 1 km long copper wire of radius 1 mm. (Resistivity of copper = 1.72 x 1 0-8

Answer: L = 1 km = 1000 m
R = 1 mm = 1 x 1 0-3
p = 1.72 x 1 0-8  W m

Area of cross section = p r2  = 3.14 x 1 0-3 x 1 0-3 =  3.14 x 1 0-6

R = pl/A = (1.72 x 1 0-8  x 1000 ) / 3.14 x 1 0-6  = 5.5 W

4. Question: When a potential difference of 2 V is applied across the ends of a wire of 5 m length, a current of 1 A is found to flow through it. Calculate: 
(i) the resistance per unit length of the wire                   
(ii) the resistance of 2 m length of this wire 
(iii) the resistance across the ends of the wire if it is doubled on itself.

Answer: (i) V = I R ----- Ohm's law R=V/I=2/1= 2 Ohm

Resistance per unit length: 2/5= 0.4 Ohm/m

(ii) Resistance of 2 m length of the wire = 0.4 x 2=0.8 ohm

(iii) When the wire is doubled on itself:

(a) the area of cross-section is doubled. If A is the original C.S. area, now it is 2 A.

(b) The length becomes half i.e.L/2 

Resistance of this wire =R' = p (l/2)/(2A) = 1/4(p(L/A)

But p(L/A) = 2 ohm

R' = 1/4 x 2=0.5 Ohm

5. How much work is done in moving 4 C across two point having pd. 10 v

Solution : W = VQ = 10 x 4 = 40J

6. How much energy is given to each coulomb of charge passing through a 9 v battery?

Solution:  Potential difference = Work done = Potential difference × charge

Where, Charge = 1 C and Potential difference = 6 V

Work done = 9×1 = 9 Joule.

7. 100  j of work is done in moving a charge of 5 C from one terminal of battery to another . What is the potential difference of battery?

Solution: V = W/Q =  100j/5C = 20 V

8. If 4 x 10 -3 J of work is done in moving  a particles carrying a charge   of 16 x 10 - 6 C from infinity to point P .What will be the potential at a point?

Solution: the potential at a point is work done to carry unit from one point to another  
                         = (4 x 10 -3 ) /(16 x 10 - 6 C) = 250 V

9. Calculate the current and resistance of a 100 W ,200V electric bulb.

Solution:Power,P = 100W   and     Voltage,V = 200V

Power  P  = VI
So, Current I = P/v = 100/200 = 0.5A

Resistance R = V/I = 200/0.5 = 400W.

10.Calculate the power rating of the heater coil when used on 220V supply taking 5 Amps.


Voltage ,V = 220V     and  Current ,I = 5A,

Power,P = VI = 220 × 5 = 1100W = 1.1 KW.

11.A lamp can work on a 50 volt mains taking 2 amps.What value of the resistance must be connected in series with it so that it can be operated from 200 volt mains giving the same power.

Solution: Lamp voltage ,V = 50V and  Current ,I = 2 amps.

Resistance of the lamp = V/I   = 50 / 2     = 25 Ω

Resistance connected in series with lamp = r.
Supply voltage = 200 volt. and  Circuit current I = 2 A

Total resistance Rt= V/I  = 200/2    = 100Ω
                         Rt = R + r          =>    100 = 25 + r      =>  r = 75Ω 

12. Calculate the work done in moving a charge of 5 coulombs from a point at a potential of 210 volts to another point at 240 volts

Solution: Potential diffrence  = 210 ­ - 240 = ­30 V 

So, W.= V x  Q = ­30V  x  5C = ­150 Joules

13. How many electrons pass through a lamp in one minute if the current be 220 mA?


I  = 220 mA = 0.22 A 

I = Q/T 

0.22 = Q/60 

Q= 0.22 x 60 = 13.2 C 

No of electron carry 1 C charge = 6 x 10 18 

No of electron carry 13.2 C charge = 6 x 10 18 x 13.2 C = 79.2 x 10 18

14.Calculate the current supplied by a cell if the amount of charge passing through the cell in 4 seconds is 12 C ?


I = Q/t = 12/4 = 3A

15. A 2 Volt cell is connected to a 1 Ω resistor. How many electrons come out of the negative terminal of the cell in 2 minutes?

Solution: V = IR =>  I = V/R = 2/1 = 2 A

I = Q/t  =>  Q = It = 2 x 2 x 20 = 80 C

No of electron carry 1 C charge = 6 x 10 18 

No of electron carry  80 C  charge = 6 x 10 18 x 80 C = 108  x 10 18   = 1. 08  x 10 20

16. (a) How much current will an electric bulb draw from a 220 V source, if the resistance of the bulb filament is 1200 Ω?

(b) How much current will an electric heater coil draw from a 220 V source, if the resistance of the heater coil is 100 Ω?


(a) We are given V = 220 V; R = 1200 Ω.
we have the current I = V/R  = 220 V/1200 Ω = 0.18 A.
(b) We are given, V = 220 V, R = 100 Ω.
 we have the current I = V/R =  220 V/100 Ω = 2.2 A.

17. The potential difference between the terminals of an electric heater is 60 V when it draws a current of 4 A from the source. What current will the heater draw if the potential difference is increased to 120 V?


We are given, potential difference V = 60 V, current I = 4 A.

According to Ohm’s law, R = V/I = 60/4 =15Ω

When the potential difference is increased to 120 V

the current is given by current = V/R = 120V/15 = 8A

The current through the heater becomes 8 A.

18. A 4 Ω resistance wire is doubled on it. Calculate the new resistance of the wire.


We are given, R = 4 Ω.

When a wire is doubled on it, its length would become half and area of cross-section would double. T
So,  a wire of length l and area of cross-section A becomes of length l/2 and area of cross section 2A. we have R = ρ(l/A)

R1 = ρ((l/A) / 2A)   where R1 is the new resistance.

Therefore, R1/R = ρ((l/A)/2A) / ρ(l/A) = 1/4

Or, R1 = R/4 = 4Ω/4 = 1Ω

The new resistance of the wire is 1 Ω.

19 . 3.A circuit is made of 0.4 Ω wire,a 150Ω bulb and a 120Ω rheostat connected inseries.Determine the total resistance of the resistance of the circuit. 

Solution: Resistance of the wire = 0.4Ω              
Resistance of bulb = 150Ω
Resistance of rheostat = 120Ω
In series,  Total resistance ,R = 0.4 + 150 +120 = 270.4Ω

20. A current of 0.2 Ampere flows through a conductor of resistance 4.5 Ω. Calculate the potential difference at the ends of the conductor.


The potential difference at the ends of the conductor. =  V = IR = 0.2  x  4.5  = 0.9 V

21. A lamp has a resistance of 96 ohms. How much current flows through the lamp when it is connected to 120 volts?

Solution: I = V/R = 120/96 = 1.25 A [V = IR]

The current through the lamp equals 1.25 A.'

22. The manufacturer specifies that a certain lamp will allow 0.8 ampere of current when 120 volts is applied to it. RRWhat is the resistance of the lamp?

Solution: V = IR So, R = V/I = 120/0.8 = 150 W

23. How much voltage is required to cause 1.6 amperes in a device that has 30 ohms of resistance?
Given: V = IR = 1.6 x 30 = 48 V

24. How much power is dissipated when 0.2 ampere of current flows through a 100-ohm resistor?
Ans: P = V I = IR x I = I2 R = 0.2 x 0.2 x 100 = 4 W

25, How much energy is converted by a device that draws 1.5 amperes from a 12-volt battery for 2 hours?

W = Pt, P = V I So, W = VIt = 12 x 1.5 x 2 = 36 Wh

Fore Fiction and Pressure for VIII Physics-Notes and Evaluation


What is force :

A force is a push or pulls acting on an object which changes or tends to change the state of the object.

In the international system of units (SI System), the unit of force is Newton (N) that is named after Sir Isaac Newton.

There are also other units like dyne, kilogram weight and pound.

Action of force and its effects: A force

a) can change the state or rest or motion

b) can change the shape of object
c) can change speed of a body
d) can change direction of motion

Contact and non-contact forces

Forces which act only when there is physical contact between two interacting objects are known as
Contact forces. 

Forces which can act without physical contact between objects, i.e. those that can act from a distance, are called non-contact forces or field forces.

What is the definition of balanced and unbalanced force?

When two equal forces acting on an object in opposite directions called balanced force. 

When something does not move the forces are balanced, e.g. Gravity pulls a table down but that table is pushed up by the floor or the ground it stands on so doesn’t’ move.  

An Unbalanced force is needed for something to change movement or change direction, e.g. a seesaw moving up and down because the forces are not equal.

Net force: - The sum of all the forces acting on a body is known as net force.

Types of forces

1. Muscular force: This is the force we can exert with our bodies by using our muscles, e.g. pull, push, kick etc. These are contact forces

2. Magnetic force : Magnets exert forces of attraction or repulsion on other magnets. An important feature of magnetic force is that it can act from a distance, and is therefore of a non-contact force.

3. Electrostatic force: The force exerted by a charged body on another charged or uncharged body is known as electrostatic force. Electrostatic force is used to separate solid pollutant particles from smoke given out from factories.

4. Gravitational force : All objects in the universe exert a force on all other objects. This is called gravitational force. The  gravitational force exerted by the Earth on all the bodies on its surface is called gravity.

5. Frictional force : The fact that the rolling ball comes to rest after some time shows that there must be a force acting on it which tends to slow it down. This force seems to be more on rough surfaces than on smooth surfaces. The force acting against the relative motion of surfaces in contact is called frictional force or friction.

6. Tension Force:Strings, ropes and chains can only pull on things!  The force of pull supplied by strings, ropes or chains is called the tension force. The tension force is always directed along the length of the thing doing the pulling (string, rope, chain).

7. Spring Force‎: is the force exerted by a compressed or stretched spring upon any object that is attached to it.

8. Applied Force:  An applied force is a force that is applied to an object by a person or another object. If a person is pushing a desk across the room, then there is an applied force acting upon the object. The applied force is the force exerted on the desk by the person.

Mass and Weight : 

The mass of an object is the amount of matter that is contained by the object; the weight of an object is the force of gravity acting upon that object.

The mass of an object (measured in kg) by Pan Balance and  will be the same no matter where in the universe that object is located. 

On the other hand, the weight of an object (measured in Newton) by Spring balance and  will vary according to where in the universe the object is.

Type of friction:

Static friction:

The block remains at rest because a force of friction, equal but opposite to the applied pull, comes into action between the surfaces. Increase the force a little. The block des not move. This means that the force  of friction has increased to balance the pulling force on the block. 

If the pulling force P is increased further, at a certain stage the block begins the move on the table. At this point the friction developed has reached the maximum value F for the two surfaces. This is called static friction.

Kinetic or sliding friction

Continue pulling the block with the spring balance, so that it slides at a steady speed. The reading on the spring balance is also steady and is slightly less than the static friction. This is a measure of the kinetic or sliding friction between the two surfaces.

Kinetic or sliding friction is less than static friction.

Rolling friction

Repeat the above procedure using an identical wooden block provided with wheels on either side. The reading on the spring balance when the block moves with a steady speed is much less than the sliding friction measured above. 

Rolling friction is less than sliding friction.

Advantages of friction

Friction plays an important role in our daily life.

1) Without friction we would slip and fall every time we attempt to walk or run. There is very little
friction on a wet polished floor. That is why it is easy to slip on such a floor.

2) Friction causes nails and screws to hold on to walls.

3) It would not be possible to light a matchstick without friction between its head and the side of the

4) Cars and buses are able to run on roads because of friction between the tyres and the road.

5) Without friction writing on paper would be impossible as the tip of the pen will slip on paper.

6) It is because of friction between the brake ‘shoes’ and wheels that bicycles and automobiles stop
when brakes are applied.

Disadvantages of friction

Friction is a nuisance in some circumstances.

1) The heat produced in the moving parts of machinery due to friction results in wear and tear of the parts.

2) Forest fires are caused due to friction between branches of threes rubbing against each other.

3) Tyres of vehicles and soles of footwear wear out because of friction.

4) Energy is wasted in overcoming the force of friction.

Minimizing friction

1) By using a suitable lubricant, like oil (for light machinery) or grease (for heavy machinery). This
helps because fluid friction is less than solid friction

2) By using wheels and ball bearings.

Use of wheels between surface moving over each other reduces friction. Ball bearings have small balls of steel between steel surfaces. Because of the balls the steel surfaces can easily moves over each other.

3) By making the rubbing surfaces smooth by polishing them.

4) Friction due to air (air resistance) or water is reduced by using streamlined shapes in aeroplanes or ships. 

A streamlined shape is narrow in front and broader at the back. Birds and aquatic animals have streamlined shapes which held them in flying or swimming.

Increasing friction

1) By making the moving surfaces rough, e.g. tyres have designs and patterns with grooves on the surface to increase resistance with the road. This prevents slipping of the tyres on a wet road.

2) Sand and gravel is strewn on slippery ground during the rainy season to increase friction. It is them easier to walk on the ground.

3) To increase friction, spikes are provided in the soles of shoes used by players and athletes.

Pressure : 

Pressure is defined as the force  acting on a unit area.  Pressure = Force/ Area  

If the force increases the pressure increases.

If the area over which the force acts decreases, the pressure increases

Unit of pressure

The SI unit of pressure is the pascal (Pa). It is the pressure exerted by a force of 1 N acting over an area of 1 m2.

Thus 1Pa = 1N/m2

Solved Problem 

Q.1: A liquid gives force of 100N over an area of 2m2. What is the pressure?
Solution: Force = 100N  Area = 2m2         Pressure = 100N / 2m2 = 50 N/m2  Pressure = 50 N/m2

Q. A liquid’s force is acting over an area of 4m2. If the pressure is 25 N/m2, what is the force?
Solution : F = P x A = 25 N/m2     x 4m2 = 100N

Everyday examples

@  When you cut an apple with a knife, the shaper the knife, the smaller is the area of contact of the knife with the apple. Thus a sharper knife exerts a greater pressure making it easier to cut the apple.

@ It is easier to pierce a piece of cardboard with a pin if the pin has a sharp point, as the area of contact is  then smaller.

@ School bags and shopping bags have broad belts or straps as handles. Narrow string-like handles cause severe pain in the hand because the weight of the bag acts on a small area and so the pressure is
considerably higher.

Q Porters wear turbans when they have to carry heavy loads on their heads, to increase the area of
contact. This reduces the pressure on the head.

Q Lorries and trucks carrying heavy loads have 8 tyres instead of four, and the tyres are broader. This
increases the area of contact with the ground, this reducing the pressure exerted on the ground.

Q War tanks move on caterpillar tracks which are broad and chain-like and cover the wheel. This 

causes large increase in the area of contact with the ground. The pressure on the round reduces so much that tanks can even move on soft wet ground without sinking.

Atmospheric Pressure

The Earth is surrounded by air and this covering of air is known as the atmosphere. 

The atmospheric air exists to height of about 1000 km. But beyond 100 km the air is very thin. The force exerted by the atmosphere on unit area is called atmospheric pressure.

Liquid Pressure

a) Water exerts pressure downwards
b) Water exerts pressure in the upward direction
c) Water exerts pressure in all directions on the sides. This is called lateral pressure.
d) Liquid pressure increases with depth.

Pressure at the bottom of the ocean is very high. Deep sea divers wear steel diving suits to prevent their bodies from begin crushed by the tremendous pressure of water exerted on all sides.

Submarines are built of hard thick sheets of metal to withstand the high pressure under water. Fro the
same reason dams are broader at the base than at the top.

Measuring liquid pressure – the manometer

An instrument called a monometer is used to measure liquid pressure.
@ A liquid exerts pressure in all directions (downward, upward and sideways).
@The pressure is the same in all directions at the same depth.
@  Pressure increases with depth.

The pressure exerted by water in the oceans increases with depth. Very deep down the pressure is enough to crush the human body. That is why deep-sea divers wear special suits, which can withstand such high pressures.

Short answer questions

1. What is meant by force?

2. What changes can a force produce to the motion of an object?

3. What is elastic force? Give two examples.

4. How is pressure related to the area over which a force acts?

5. What is the relation between pascal and N/m2?

6. If you wear pointed heels in one leg and slippers on the other leg, which leg exerts greater pressure
on the ground?

7. What is atmospheric pressure?

8. Explain how it is possible to drink a liquid by using a straw.

9. Why are dams over rivers made broader at the bottom than at the top?

10. Differentiate between mass and weight. What are the SI units?

Long answer questions

1. Explain contact and non-contact forces. Give two examples for each.

2. What are the advantages and disadvantages of friction?

3. Illustrate static, kinetic and rolling friction with an example. Which is the highest?

4. (a) How can friction be reduced? (b) How can it be increased?

Related search: 

1. Force , Frictions ad Pressure

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