PHYSICS GRADE 9

CHAPTER # 3

Q # 1. A body is moving with a uniform speed, will its velocity be uniform?

Ans: Its velocity may or may not be uniform because: -

if a body moves in straight line with uniform speed, then its velocity will be uniform because

its direction is not changing. If the body moves in a circular path with uniform

speed, its velocity will not be uniform because its direction is changing at every point.

Q # 2. A body is moving with a uniform velocity. What will be its acceleration?

Ans: We know that acceleration is the rate of change of velocity. As the velocity is

uniform, it means there is no change in velocity. Hence acceleration will be zero.

Q # 3. Can a body moving with certain velocity in the direction of east can have

acceleration in the direction of west?

Ans: Yes, a body moving with certain velocity in a direction of east can have acceleration in

the direction of west.

Example:

A car is moving towards east. If brakes are applied then its velocity will decrease and

a negative acceleration will produced. Negative acceleration will have the direction

apposite to the direction of velocity. So, in this case, direction of acceleration will be

towards west.

Q # 4. A stone of mass 2Kg is thrown in the downward direction from the roof of a

building. It reaches the surface of the earth in 2 seconds. How long a 4Kg stone will

take to reach the earth?

Ans: When two bodies lighter or heavier are dropped form a height, they reach the earth

at same time if we remove air resistance. So a body of 4kg will take the same time

as a body of 2kg.

Q # 5. Does speedometer of a car measures its velocity?

Ans: No, the speedometer of a car only measures the speed of car because it cannot

indicate the direction of car.

PHYSICS GRADE 9

CHAPTER # 4

Q # 1. Why rolling friction is less as compare to sliding friction?

Ans: It is due to area of contact. As we know, if the area of connection between two

bodies is larger the friction will be greater and if the area of connection is smaller

the friction will be less. When a body roles than the area of connection is so small as

compare to the area of sliding body. So, rolling friction is less than sliding friction.

Q # 2. Suppose you are running and want to stop at once. Surely you will have to produce

negative acceleration in your speed. Can you tell from where does the necessary

force come?

Ans: The force of friction between the earth and the feet will provide necessary force

which enables us to stop.

Q # 3. If a moving body has no acceleration, does it mean that no force is acting on it?

Ans: According to second law of motion: -

F = ma

as a = 0

then F = m(0)

so F = 0

It means that if a = 0 then there is zero force acting on object.

Q # 4. When you are standing on the ground how much upward force is exerted on you by

the earth? Why does this force of earth not lift you up?

Ans: The person standing on the earth exerts a force equal to his weight, this is the action

force. According to Newton‟s third law of motion the earth also exerts a force on

the person as a reaction. As these two forces are equal in magnitude but opposite in

direction. So, they cancel each other. That is why the reaction force does not lift

the person in the upward direction.

PHYSICS GRADE 9

PHYSICS GRADE 9

CHAPTER # 5

Q # 1. If vectors A and B are added, under what condition their resultant magnitude is A +

B and under what condition their resultant is zero?

Ans: When two vectors „A‟ and „B‟ are added, their resultant will be A + B. If both the

vectors are equal to each other and the angle between them is zero and are added

by vectors are equal to each other i.e. the angle between them is zero and are added

by “head to tail rule”. The resultant of two vectors will be zero when both the

vectors are anti parallel to each other and their magnitude is same. In this case

angles between these two vectors are 180 .

Q # 2. Can a force acting perpendicularly on a body cancel a force which is acting

horizontally on the same body?

Ans: No, because a force acting perpendicularly on a body will produce motion along y-

axis. Whereas a force acting horizontally will produce motion along x-axis.

Therefore these forces cannot cancel the effect of each other.

Q # 3. Which are those vectors whose resultant is zero?

Ans: The two vectors which have the same magnitude but they are opposite to each

other, their resultant will be zero. Suppose a vector A and its negative vector A

are added their resultant will be zero: -

A + ( A ) = A - A = 0

Q # 4. Under what condition the sum of three vectors will be zero?

Ans: If three vectors of equal magnitudes and different directions are added by head to

tail rule as shown in figure: - Their resultant will be zero if head of third vector “A”

concedes with the tail of the first vector by forming a closed figure.

PHYSICS GRADE 9

Q # 5. Will a vector be zero if one of its component is zero?

Ans: No, a vector cannot be zero if one of its component is zero. For example:-

If NFx 2

NFy 0

then 22

yx FFF

NF

F

24

)0()2( 22

And if, NFx 0

NFy 2

then 22

yx FFF

NF

F

24

)2()0( 22

Q # 6. If one component of a vector A is zero along the direction of another vector B, then

in what direction the two vectors will be?

Ans: Let a vector A is acting along X-axis so, it‟s Y-component will be zero and a vector

B is acting along Y-axis. So, it‟s X-component will be zero. If these two vectors

are placed perpendicular to each other then one component of a vector will be zero

along the direction of another vector.

Q # 7. Under what condition the distance and displacement between two points will be

equal?

Ans: The distance and displacement between two points will be equal if these two points

are at the same line.

Q # 8. Can scalar and vector quantities be added by the same method? If no, describe the

method of addition of two vectors which are in different directions.

Ans: No, vectors and scalars cannot be added by the same rule. Vectors are added by a

rule called “Head to Tail Rule”. (Remaining portion on last page).

PHYSICS GRADE 9

Q # 9. If you ride a bicycle and go to bazaar which is 5km away from your house to buy

some grocery and return back, then how much distance you have covered? How

much is your total displacement?

Ans: Total distance covered by the person will be 5km + 5km = 10km. As displacement

is the shortest distance between initial point and final point, so, as the person comes

back to the initial point. Therefore it‟s displacement will be zero.

CHAPTER # 6 (EQUILIBRIUM)

Q # 1. Will a body be in equilibrium, under the action of a single force?

Ans: A body will be in equilibrium if a = 0, but when a force is acting on a body, it will

produce accelerating in the body. Therefore, a body will not be in equilibrium under

the action of a single force.

PHYSICS GRADE 9

Q # 2. Can a body be in equilibrium, if it is revolving clockwise under the action of a single

torque?

Ans: No, because for a body in equilibrium clockwise torque should be equal to

anticlockwise torque. Therefore, a body revolving clockwise under the action of a

single torque will possess an angular acceleration. So it is not in equilibrium.

Q # 3. Give an example of a case when the resultant force is zero but resultant torque is not

zero.

Ans: When couple is applied on a rigid body,

then resultant force acting on it is zero,

while the resultant torque is not zero as

shown in figure because OAOB , so

torque produced by „F2‟ will be greater.

Q # 4. Can a moving body be in equilibrium?

Ans: Yes, when it is moving with uniform velocity. Because when a body moves with

uniform velocity, its acceleration is zero which is the requirement for a body in

equilibrium.

Q # 5. The height of a racing car is kept small. Why?

Ans: To keep an object in stable equilibrium, its center of gravity is kept lowered. Due to

this reason, the height of a racing car is kept small. This reduces the effect of air

resistance on the car.

CHAPTER # 7 (CIRCULAR MOTION AND GRAVITATION)

Q # 1. Why the outer edge of a road is kept higher than the inner edge at a turn?

Ans: While taking turns on a road, a car moves in a circular path. To keep the car

moving along the circular path, centripetal force is required. The force of friction

PHYSICS GRADE 9

between the tyres and road provides the necessary centripetal force and keeps the

car moving along curved path.

Q # 2. What is effect of following on gravitational acceleration.

i) mass of freely falling body ii) Distance of freely body from center of

gravity

Ans: i) As all bodies, either lighter or heavier, fall freely on earth with the same

gravitational acceleration therefore mass of a body has no effect on gravitational

acceleration.

ii) As we know 2

2

)( hR

gRgh

This shown that the value of gravitational acceleration decreases with the increase of

altitude from the center of earth.

Q # 3. Is there any difference between the values of „g‟ at the equator and at the poles?

Ans: Earth is not completely spherical rather it is bulging at the equator. So the distance

between the surface and center of earth at equator is larger as compared to poles.

Value of „g‟ decreases by increasing the distance. Hence, the value of „g‟ is less at

equator as compared to poles.

CHAPTER # 11 (HEAT)

Q # 1. Is it good to place an immersion heater in a water vessel at the surface of water or

at the bottom?

Ans: It is better to place an immersion heater at the bottom of the vessel because after

getting heat, water becomes light and moves upward to the surface. The relatively

cold water of the surface then moves downward due to convection. In this way,

water will be uniformly heated.

Q # 2. Why tea in a cup becomes cold earlier as compared to a teapot?

PHYSICS GRADE 9

Ans: The mouth of a teapot is narrow as compared to a cup. Therefore, in a teapot

steam cannot escape easily, so it keeps the tea hot for a longer time whereas the

steam easily escapes from a cup and the tea becomes cold earlier.

Q # 3. Why the diameter of iron rim round a tonga wheel is made slightly smaller.

Ans: It is due to Thermal Expansion. When someone wants to use an iron ring on the

wooden wheel of a tonga, then he mounts the iron rim on the wheel by heating. On

cooling, iron rim comes to its original position and tightly fits on the wheel.

Q # 4. Why desert nights are colder whereas the days are hot.

Ans: Specific heat of sand is very low, therefore it becomes cooler and hotter soon. Due

to this, sand becomes hotter soon due to heat of the sun and the day remains hot.

During night, sand becomes colder soon that is why the desert nights are cooler than

the days.

PHYSICS GRADE 9

PHYSICS SCIENTIFIC REASONS

Q # 1. Why unit of acceleration is m/s2?

Ans: As we know

t

va

unit of velocity = m/s

unit of time = second

2

1

/

s

m

ss

ma

s

sma

Q # 2. Write conditions for equations of motion.

Ans: The relation between any four quantities out of following five quantities that quantities

are distance, time, Vi, Vf, and acceleration. Conditions for equations of motion are

followings.

a. Motion will be taken along a straight line

b. Acceleration will be uniform.

c. Magnitude of velocity & acceleration will be considered although they are

vector quantity.

d. Initial velocity & all the other quantities which are in direction of initial

velocity are taken as positive and the quantities which are opposite of Vi are

taken as negative.

Q # 3. What is gravitational acceleration?

Ans: The acceleration of body due to gravity is called gravitational acceleration. Its value is

10m/s2. When body moves upward it is “negative” and on downward it is “positive”.

Q # 4. Give difference b/w distance and displacement.

Ans:

PHYSICS GRADE 9

Distance Displacement

The total length covered by a body is

called distance. In figure distance

BCAB

Shortest directed distance from one

point to other is called displacement.

In figure displacement AC

Q.# 5. If mass of the body is doubled while keeping the force constant. What will be the

acceleration of the body?

Ans: As we know

ma

1

This shows that acceleration is inversely proportional to mass. So by keeping the force

constant, If the mass is doubled then acceleration will be one half.

Q # 6. Can Newton‟s first law be deduced from 2nd law?

Ans: As by second law

0

0

0

a

ma

Fput

maF

as 0m

As acceleration become zero when F = 0, so we can say in the absence of external force a

body at rest or moves with uniform velocity. This is the first law of motion.

Q # 7. What will be the centripetal force if mass of the body become double?

Ans: As we know,

cc

c

Fr

mv

r

mvF

mmput

r

mvF

222

2

22

2

This shows that centripetal force become double.

Q # 8. What will be Fc if the velocity is doubled / halved?

PHYSICS GRADE 9

Ans: As we know

r

mvFc

2

put vv 2

cc

c

Fr

mvF

r

vm

r

vmF

44

4.)2(

2

22

put 2

vv

cc

c

Fr

mvF

r

mv

r

vm

F

4

1

4

1

4

2

2

2

2

Q # 9. What will be the velocity of satellite if the radius is doubled?

Ans: As we know

R

GMV

put RR 2

2

2

1

2

VV

R

GMV

R

GMV

Q # 10. Mass of coin and stone is 10g and 100g respectively. They are dropped from

same point. What will be their acceleration? Also find their weight.

Ans:

Mass of coin = 10g = 0.01Kg

Mass of stone = 100g = 0.1Kg

Both bodies coin and stone will have same acceleration i.e. 10m/s2.

Weight of coin = mg = 0.01 x 10 = 0.1N

PHYSICS GRADE 9

Weight of stone = mg = 0.1 x 10 = 1N

Q # 11. What will be the K.E. of the body if the velocity is doubled?

Ans: As we know

2

2

1.. mvEK

put vv 2

..4.

..4.

2

14.

4.2

1.

)2(2

1.

2

2

2

EKEK

EKEK

mvEK

vmEK

vmEK

This shows that by increasing velocity to double K.E. will become four times.

Q # 12. On which factors M.A. of lever depends?

Ans: As we know:

M.A = Effort arm

Load arm

This equation shows that

M.A Effort arm

M.A 1 .

Load arm

Above relation shows that M.A depends upon two factors.

PHYSICS GRADE 9

i. effort arm

ii. Load arm

By increasing effort arm M.A will increase and by increasing load arm M.A will decrease.

Q # 13. On which factors M.A of wheel and axle depends?

Ans: As we know: -

M.A = R/r

(R = radius of wheel)

(r = radius of axle)

This equations shows that

rAM

RAM

1.

.

Above relation shows that M.A depends upon two factors.

i. Radius of wheel

ii. Radius of axle

By increasing radius of wheel M.A will increase and by increasing radius of axle M.A will

decrease.

Q # 14. One which factors M.A of inclined plane depends.

Ans: As we know:-

hAM

.

( = length of plane)

(h = height to be raised by weight)

This relation shows that M.A depends upon two factors

i. length of plane.

ii. height

Greater the height of plane, smaller will be M.A and larger the length of plane, greater will

be M.A.

In terms of angle ‘ ’

As we know:-

PHYSICS GRADE 9

Sin

AM1

.

This equation shows that by increasing value of , M.A will decrease and vice versa, as

shown in figure.

fig (i) has more M.A

Q # 15. On which factor M.A of screw jack depends?

Ans: As we know

hAM

2.

This equation shows that

hAM

AM

.

.

Above relation show that M.A depends upon two factors

i. Length of Tommy bar

ii. Pitch of boit “h”

By increasing length of Tommy bar M.A will increase and by increasing pitch of bolt M.A

will decrease.

Q # 16. We enjoy sweetened fluffy ice ball in summer. How they are made?

Ans: It is due to regulation. By increasing pressure on crushed ice its melting point

decreases and some of the pieces of ice melt. When we remove pressure the pieces of

ice refreezes again and join together to form an ice ball.

Q # 17. At which temperature Surface tension of a liquid will be maximum?

PHYSICS GRADE 9

Ans: The surface tension of a liquid is due to the intermolecular forces. Increasing the

liquid‟s temperature intermolecular forces will decrease and by decreasing these

forces, surface tension will decrease. So we can say surface tension of liquid is

greater at lower temperature as compared to higher temperatures.

Q # 18. Why “Baba fazlo” blacken the bottom of “degchi” used in kitchen?

Ans: It is due to black body radiation. When radiation strikes the black bottom of

“degchi” it will absorb almost all the radiations in such a way larger hotness will be

absorbed by the object inside the “degchi”.

Q # 19. The insects can walk over the surface of water. Why?

Ans: It is due to surface tension. The molecules of the surface are stretched due to

intermolecular forces which are acting only for one side on molecules. That‟s why

these molecules become in state of tension which helps insects to move.

Q # 20. Why mercury is preferred is in thermometer?

Ans: It is due to two factors

i. Greater boiling point

ii. It does not stick with the walls of container.

Due to greater boiling point thermometer can measure a large temperature. And as

it do not stick with the walls of container so it show uniform expansion.

Q # 21. If you start walking forward on a wooden log floating on the surface of water,

the log starts moving in the backward direction. What is its reason?

Ans: A person walking forward on a wooden log floating on the surface of water, he

pushed the log in backward direction. According to Newton‟s third law of motion

the log pushed the person in forward direction as a reaction. So, it is due to action

and reaction forces.

Q # 22. An athlete always selects a sand track for long jump?

Ans: It is due to second law of motion when an athlete jumps in the sand his body does

not stop abruptly it means lesser deceleration is produced due to sand. So body is

not stopped suddenly as a result he is not hurt seriously.

PHYSICS GRADE 9

Q # 23. Horse is able to pull Tonga although action and reaction of horse and Tonga are

equal?

Ans: It is due to 3rd law of motion when horse applies force on Tonga then a s a reaction

Tonga also applies equal and opposite force on the horse. But when horse applies the

force on the earth with his feet the force of friction is produced. As a reaction the

earth applied force and Tonga moved forward.

Q # 24. A person with a bucket of water in one hand bends in opposite directions?

Ans: It is due to centre of gravity when a person walks then its weight passes through his

base and is in equilibrium. But when he picks a bucket of water in one hand then his

centre of gravity is shifted towards the bucket. Which doesn‟t pass through his foot

hence he bends him self in opposite direction to maintain his equilibrium.

Q # 25. An acrobat always has pole in his hands?

Ans: It is due to equilibrium an acrobat walking on a rope is in unstable equilibrium.

When he slightly looses his equilibrium he may fall. But he always has long pole in

his hand he shifts the pole in opposite direction of disturbing side to make the weight

passing through the points of suspension.

Q # 26. Edge of knife is made sharp?

Ans: It is practical application of mechanical advantage of wedge. Knife of sharp edge

cuts thing easily. The mechanical advantage of wedge is maximum. When the

thickness of wedge is small and inclined surface is greater. Hence edge of knife is

made sharp.

Q # 27. Steel is more elastic than rubber?

Ans: It is due to molecular structure of steel and rubber. The molecules of steel are more

close to each other then rubber hence more force is needed to bring some change in

the shape of molecules of steel which shows steel is more elastic than rubber.

Q # 28. When a wire is bent back and forth many times becomes warm?

PHYSICS GRADE 9

Ans: It is the phenomenon of molecular structure of the substance. When we bend a wire

by applying some force then intermolecular distance increases many times. This

function increases the K.E of the molecules. Hence the wire becomes warm.

Q # 29. Vacuum is created between the double walls of thermo flask?

Ans: It is due to conduction and convection. We know that for conducts in and

convection we need a medium for transmission of heat. As vacuum has no medium

so there is no loss of heat due to conduction and convection.

Q # 30. Ventilators are provided in the houses near ceiling?

Ans: It is due to convection. The gas of room becomes hot and rushes up. To compensate

the gap of this gas colder air from outside comes in the room through the doors and

windows. Hence inside the room remains moderate.

Q # 31. The climate of costal regions remains same throughout the year?

Ans: This is due to convection. Earth heats up during day time and the air on it is heated

also. This heated air goes up. To compensate this deficiency cold air from the sea

rushes towards the land and the temperature becomes moderate similarly at night

the temperature of land near sea falls. But the temperature of water doesn‟t falls.

Q # 32. The steam of c100 is hotter than boiling water at c100 ?

Ans: It is due to latent heat of vaporization. The specific heat capacity of water and

steam is same but water at c100 absorbs 2.26 x 1006 j/kg heat and then changes

in to steam at c100 hence steam contains more heat and produces more severs

burn.

Q # 33. A small quantity of ice can cool a large amount of water?

Ans: This is due to latent heat of fusion of ice. The latent heat of fusion of ice is 3.6 x

105j/kg. It absorbs this amount of heat and converts into water at c0 . This

amount of heat is very large. So small quantity of ice absorbs larger amount of heat

and it can cool large amount of water.

Q # 34. A moving ball stops after covering some distances

PHYSICS GRADE 9

Ans: It is due to first law of motion. Two forces act on the ball the resistance of air and

friction of ground. These forces go on reducing the velocity of ball until it comes into

rest. If we get rid of these forces the ball will continue moving in straight line with

uniform velocity forever.

Q # 35. It is not safe to jump from moving train:

Ans: This is due to first law of motion. The feet of man comes into rest when they strike

the ground while the upper part of body keeps on moving the man may thus fall in

forward direction and injure himself.

Q # 36. It is dangerous to apply brakes on the front wheel of a cycle:

Ans: This is due to first law of motion. The front wheel stops at once on applying brakes

while the rear wheel continues it motion due to inertia, so rider may be thrown off

the cycle and injured seriously.

Q # 37. When we shake the branch of tree fruit falls down:

Ans: This is due to first law of motion. The branch comes in motion when we shake it

while the fruit maintains it state of rest due to inertia and thus fruit falls down.

Q # 38. An athlete runs for some time before jumping over a long distance.

Ans: It is due to first law of motion. Inertia of the motion of his body at time of jump

helps him to jump through greater distance.

Q # 39. A man who falls on cemented place is seriously hurt:

Ans: It is due to second law of motion. When he falls down on a cemented place the

velocity of his body is decreased in short period of time hence the great opposing

force injures the man seriously. However if he tries to break his fall by landing on his

hand the time increases and he is not hurt.

PHYSICS GRADE 9

Q # 40. A cricketer draws his hand while catching a ball.

Ans: This is due to second law of motion. If time increases the magnitude of force

decrease, so by drawing his hand the cricketer increases the time of application of

force and thus is not hurt.

Q # 41. We feel blow in backward direction while firing a gun.

Ans: This is due to third law of motion. The bullet moves with great speed and as a

result of this action the gun is pushed backwards as a reaction.

Q # 42. Walking becomes difficult on the muddy and quick sand.

Ans: When we walk we push the earth in backward direction. The reaction of earth

pushes in forward direction resulting in movement the friction between ground and

feet is essential for walking. As these is less friction on the mud so our feet slip

rather being pushed forwards. The reaction of earth also becomes less and walking

becomes difficult.

Q # 43. A boat man pushes the bank with bamboo stick before taking it into river.

Ans: This is due to third law of motion. The man pushes the bank and as a reaction the

boat is pushed forwards into the river.

Q # 44. A rocket can travel in space also.

Ans: It is due to 3rd law of motion. In rockets hot gases burn in the presence of oxygen.

The hot gases rush out with great speed and their reaction pushes the rocket in

upward direction. As there is no oxygen in space the rockets carry their own liquid

oxygen for combustion so they can move in space.

Q # 45. Weight of a body is different on different places:

Ans: It is due to value of „g‟ we know that W=mg and „m‟ remains constant everywhere

and „w‟ depends on value of „g‟ and the value of „g‟ depends on the distance of the

boy from the centre of the earth. This value is not same everywhere and thus value

of weight differs.

PHYSICS GRADE 9

Q # 46. Weight of body is greater at poles as compared to its weight on the equator.

Ans: Weight of the body depends on the distance of the body from the centre of the

earth. If this distance is more then the weight will be less and vice versa. The

distance of the poles from the centre of the earth is less as compared to the distance

of equator from the centre of the earth so the weight is more at poles.

Q # 47. A man carrying a heavy load on his back bends forward.

Ans: It is due to centre of gravity. Extra load displaces the centre of gravity and

equilibrium position is thus disturbed. By bending forward he maintains the specific

point of gravity and his body remains in equilibrium.

Q # 48. Can a body move with uniform velocity in circle?

Ans: No a body cannot move with uniform velocity in a circle. The speed of body remains

same but the direction of velocity changes at every point. This change in velocity

producing centripetal force and acceleration and because of this centripetal

acceleration the body cannot move in uniform velocity in a circle.

Q # 49. While turning his plane the pilot tilts the outer wing in upward direction.

Ans: It is due to centripetal force. While turning a plane move along a circular path and

requires centripetal force. The pilot tilts the outer wing in upward direction so that

the difference of air pressure between the wings may provide necessary centripetal

force.

Q # 50. At turning points the outer side of the road is raised.

Ans: It is due to centripetal force. On the turning points the vehicles move along a

circular path and require centripetal force. To provide the necessary centripetal

force to the vehicles the outer side of road is raised as compare to the inner side.

Q # 51. The passengers are nearly thrown out when the bus takes turn.

Ans: When a body moves in a circular path the direction of velocity is along the tangents

to the circle. When the bus takes turn the passengers move along the tangents and

feel as if they will be thrown of the bus.

PHYSICS GRADE 9

Q # 52. It is difficult to shut the door if we push it at a point near the hinge.

Ans: It is due to principle of lever. According to the principle of lever if the force arm is

small more force is required by applying force near the hinge the force arm

decreases and more force is required to shut the door.

Q # 53. Bob of a pendulum stops after some time.

Ans: This is due to gravity and friction of bob with air molecules. The energy of bob stops

on changing into heat energy due to friction and thus stops after some time.

Q # 54. It is easy to run a water pump with a long handle.

Ans: It is due to principle of lever. According to the principle of lever if the force arm is

large less force is required. A water pump with long handle has a great force arm so

less force is required to run it.

Q # 55. A piece of iron sinks in water while ship float.

Ans: It is due to Archimedes principle density of ship is less and it displaces more liquid,

experiences upward thrust and floats while piece of iron sinks because of high density

the density of iron is more it displaces less liquid experiences less up thrust and thus

sinks.

Q # 56. Man cannot drown in Dead Sea.

Ans: Upward thrust depends on the density of liquid (thrust = ahdg) the density of water

in dead sea is very large due to the presence of salts so the upward thrust is also

very large hence the man does not drown.

Q # 57. Ice / cork floats on the water.

Ans: It is due to Archimedes principle. Ice has less density it experiences more up thrust

and thus floats.

Q # 58. Speed of bullet is more than the speed of gun when it recoils.

PHYSICS GRADE 9

Ans: It is according to the law of conservation of momentum. Before firing the

momentum of both gun and bullet is aero. After firing the bullet moves forward

with great speed but its mass is very less. As a reaction the gun moves backward

with less velocity as its mass is greater. Their momentum is equal and opposite and

momentum of bullet and gun is zero again.

Q # 59. A balloon filled with hydrogen gas goes up and bursts

Ans: It is due to the Archimedes principle. Density of balloon is less. It displaces more

air, experiences more upward thrust and rises up. At greater height the air pressure

decreases, the pressure of hydrogen in balloon becomes more than the pressure of air

and bursts.