Selina ICSE Physics Class 8 Textbook Solutions

Selina Concise Physics Class 8 ICSE Solutions – Force and Pressure

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APlusTopper.com provides step by step solutions for Selina Concise ICSE Solutions for Class 8 Physics Chapter 3 Force and Pressure. You can download the Selina Concise Physics ICSE Solutions for Class 8 with Free PDF download option. Selina Publishers Concise Physics for Class 8 ICSE Solutions all questions are solved and explained by expert teachers as per ICSE board guidelines.

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Selina Concise ICSE Solutions For Class 8 Physics Chapter – 3 – Force and Pressure

• FORCE : “Is the cause which changes the state of a body (rest or
  state of motion) or it changes the size or shape of a body”.

Weight of a body → The force with which a body is attracted towards the centre of earth mg = force of gravity.

• A force does not change the mass of the body that is why mass of a
  body on earth and moon in same but weight → force exerted on
  body is different.
•  Force cannot be seen but it is felt.
• → represents force, length of arrow gives magnitude and arrow points the direction.
•  S.I. unit of force is Newton (N).
• Newton: “Is that much force, which when acting on a body of mass 1 kg produces in it (increases) a speed of 1 M s^-1 in the direction of its motion.
• 1 kgf = 9.8 N = 10 N (nearly)
• RIGID body: “When a force is applied on a body and inter-spacing between its constituent particles do not change is called RIGID body” force can cause only the motion in it.
•  NON-RIGID body: “When force applied changes inter-spacing.” Force causes both change in its size (shape) and the motion in body.
• TURNING EFFECT: “When force is applied on a pivoted (at a point) body, it can turn it and turning of body about point of rotation is called TURNING EFFECT.” or Moment of force.
  This is measured as:
  TURNING EFFECT = MOMENT OF FORCE
  Force x perpendicular distance from point of rotation.
  
  Moment of force = F x OP
  S.I. unit of moment of force=N × m=Nm
• THRUST: “Force acting normally on a surface.” Smaller the area of surface, larger is thrust.
• PRESSURE : “Thrust per unit area.p = Thrust/area = F/A S.I unit of area A pressure is Nm^-2 or pascal (Pa)
•  If Thrust is measured in kgf and area in Cm², then pressure is expressed as kgf Cm^-2.
  ATMOSPHERIC Pressure: 1 atm = 76 cm of mercury column 1 atm = 1.013 x 10⁵ Pa
•  FACTORS AFFECTING THE PRESSURE : P = F/A
  (i) Area: Greater the area, lesser is pressure and lesser area, greater is pressure.
  (ii) Magnitude of thrust acting: greater thrust, greater pressure.
• Factors Affecting LIQUID PRESSURE = hdg
  (i) High of liquid column: increases with height
  (ii) Density of liquid: increases with density of liquid.
  (iii) Gravity constant.
• ATMOSPHERIC PRESSURE : “Pressure exerted by the air of atmosphere around us.”

STANDARD ATMOSPHERIC PRESSURE 1 Atm = 76 cm of Hg column = 1.013 x 10⁵ Pa

Test yourself

A. Objective Questions

1. Write true or false for each statement

(a) The S.I. unit of force is kgf.
Answer. False.
The S.I. unit of force is newton.

(b) A force always produces both the linear and turning motions.
Answer. False.

(c) Moment of force = force × perpendicular distance of force –  from the pivoted point.
Answer. True.

(d) Less force is needed when applied at a farther distance from the pivoted point.
Answer. True.

(e) For a given thrust, pressure is more on a surface of large j area.
Answer. False.
For a given thrust, pressure is less on a surface of large area.

(f) The pressure on a surface increases with an increase in the thrust on the surface.
Answer. True.

(g) A man exerts same pressure on the ground whether he is standing or he is lying.
Answer. False.
A man exerts different pressure on the ground whether he is  standing or he is lying.

(h) It is easier to hammer a blunt nail into a piece of wood than a sharply pointed nail.
Answer. False.
It is not easier to hammer a blunt nail into a piece of wood than a sharply pointed nail.

(i) The S.I. unit of pressure is pascal.
Answer. True.

(j) Water in a lake exerts pressure only at its bottom.
Answer. False.

(k) A liquid exerts pressure in all directions.
Answer. True.

(l) Gases exert pressure in all directions.
Answer. True.

(m) The atmospheric pressure is nearly 10⁵ Pa.
Answer. True.

(n) Higher we go, greater is the air pressure.
Answer. False.

2. Fill in the blanks

(a) 1 kgf = 10 N (nearly).
(b) Moment of force = force × distance of force from the point of turning
(c) In a door, handle is provided farthest from the hinges.
(d) The unit of thrust is newton .
(e) Thrust is the normal force acting on a surface.
(f) Pressure is the thrust acting on a surface of unit area.
(g) The unit of pressure is pascal
(h) Pressure is reduced if area of surface increases.
(i) Pressure in a liquid increases with the depth.
(j) The atmospheric pressure on earth surface is nearly 10⁵ Pa.

3. Match the following

4. Select the correct alternative

(a) SI. unit of moment of force ¡s

1. N
2.  N cm
3.  kgfm
4.  N m

(b) To obtain a given moment of force for turning a body, the force needed can be decreased by

1. applying the force at the pivoted point
2.  applying the force very close to the pivoted point
3.  applying the force farthest from the pivoted point
4.  none of the above

(c) The unit of thrust is

1.  kgf
2.  kg
3.  g
4. m s^-1

(d) The unit of pressure is

1.  N × m
2.  kgf
3.  N m^-2
4.  kgf m²

(e) The pressure and thrust are related as

1.  Pressure = Thrust
2.  Pressure = Thrust x Area
3.  Pressure = Thrust / Area,
4.  Pressure = Area / Thrust

(f) A body weighing 5 kgf, placed on a surface of area 0.1 m2, exerts a thrust on the surface equal to

1.  50 kgf
2.  5 kgf
3.  50 kgf  m^-2
4.  5 kgf  m^-2

P.Q. A body weighing 5 kgf, placed on a surface of area 0.1 m2, exerts a pressure on the surface equal to

1.  50 kgf
2.  5 kgf
3.  50 kgf m^-2
4.  5 kgf m^-2

(g) The feet of lizards act like

1.  moving pads
2.  drilling pads
3.  suction pads
4.  none of the above

(h) Pressure exerted by a liquid is due to its

1.  weight
2.  mass
3. volume
4.  area

(i) Pressure inside a liquid increases with :

1.  increase in depth
2.  decrease in depth
3.  decrease in density
4.  none of the above

(j) The atmospheric pressure at sea level is nearly

1.  10 Pa
2.  100,000 Pa
3.  100 Pa
4.  10,000 Pa

(k) Nose bleeding may occur at a high altitude because

1.  the atmospheric pressure decreases
2.  the oxygen content of atmosphere decreases
3.  the atmospheric pressure increasess
4.  there are strong air currents at the high altitude

Selina Concise Physics Class 8 ICSE Solutions – Energy

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APlusTopper.com provides step by step solutions for Selina Concise ICSE Solutions for Class 8 Physics Chapter 4 Energy. You can download the Selina Concise Physics ICSE Solutions for Class 8 with Free PDF download option. Selina Publishers Concise Physics for Class 8 ICSE Solutions all questions are solved and explained by expert teachers as per ICSE board guidelines.

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Selina Concise ICSE Solutions For Class 8 Physics Chapter – 4 – Energy

• When we are pushing a wall we are not doing any work as the position of wall is not change i.e. wall has not moved in the direction of force.
• WORK: “is said to be done if on applying force on a body, the body moves (or changes it position) from it place in the direction of force. W = F × d
  Or
  “Work is said to be done by a force applied on a body, if it changes its size or shape.”
•  FACTORS AFFECTING THE AMOUNT OF WORK DONE : W = F × d
  (i) Magnitude of force applied.
  (ii) Distance moved by the body in the direction of force.
  UNIT OF WORK : W = F × d
  s.i  unit W= 1N × 1m = Nm = joule (J)
  1 kgf = 9.8 N is force on 1 kg ∴ F = mg
  Work done – 1 kgf × m = 9.8 N m = 9.8 J = 10 J nearly
• A cooli standing with a box on his head, does no work as distance moved is zero. ~
•  A cooli with a box on his head and walking is doing no work as force is acting vertically downward and direction of motion is at right angle.
•  ENERGY: “is capacity of doing work.”
  Or
  “The work done on a body in changing its state is called energy.”
  s.1. unit of energy = S.I. unit of work = (J)
•  JOULE: “A body is said to possess a energy of one joule. If a force of 1 Newton moves the body by a distance of 1 metre in the direction of force.”
• MECHANICAL ENERGY: “The energy possessed by a body due to its state of rest or state of motion is called mechanical energy.
•  Potential energy and kinetic energy are mechanical energies.
•  POTENTIAL Energy (P.E.) : “Is energy possessed by body due to its state of rest or position.” P.E. = mgh
•  KINETIC Energy (K.E.) : “Is energy possessed by body due to its motion.”
  K.E. = 1/2 M V²
• GRAVITATIONAL POTENTIAL ENERGY: “When a stone or water is raised (lifted) from ground to a height, work is done against the force of gravity. This work is stored in the stone or water in the form of GRAVITATIONAL POTENTIAL ENERGY.”
• A stretched bow, due to change in position possesses potential energy. When stretched bow is relreased the arrow comes in motion and due to motion possesses the kinetic energy and hits the body on which it strikes.
•  When a body at a hight, it possess P.E. = mgh. When it falls, height decreases and speed increases
  ∴ its P.E. decreases and K.E. increases.
• Powder : “Rate of doing work”. P = W/t
  

Test your self

A.Objective Questions

1. Write true or false for each statement

(a) A coolie does no work against the force of gravity while carrying a luggage on a road.
Answer. True.

(b) The energy stored in water of a dam is the kinetic energy.
Answer. False.
The energy stored in water of a dam is the potential energy.

(c) The energy of a flying kite is kinetic energy.
Answer. True.

(d) Work done by a boy depends on the time in which he does work.
Answer. False.

(e) Power spent by a body depends on the time for which it does work.
Answer. True.

2. Fill in the blanks

(a) Work is said to be done by a forte only when the body moves.
(b) Work done = Force × distance moved in direction of force.
(c) The energy of a body is its capacity to do work.
(d) The S.I. unit of energy is joule.
(e) The potential energy of a body is due to its state of rest or position and kinetic energy of body is due to its state of motion.
(f) Gravitational potential energy U = mass × force of gravity on unit mass × height.
(g) Kinetic energy = 1/2 × mass × (speed)²
(h) Power P=work done/time taken.
(i) The S . i.  unit of power is watt
(j) I H.P. = 746 W

3. Match the following

4. Select the correct alternative

(a) The S.I. unit of work is

1. second
2. metre
3. joule
4. newton

Answer:
joule

(b) No work is done by a force if the body

1. moves in direction of force
2. does not move
3. moves in opposite direction
4. none of the these

Answer:
does not move

(c) Two coolies A and B do some work in time 1 minute and 2 minute respectively. The power spent is

1. same by both coolies
2. is more by coolie A than by B
3. is less by coolie A than by B
4. nothing can be said.

Answer:
is more by coolie A than by B

(d) The expression of power P is

1. P = mgh
2. P = P = 1/2 Mv²
3. P = F × d
4. P = F × d/t

Answer:
P = F × d/t

(e) I H.P. ¡s equal to

1. 1 W
2.  1 J
3.  746 J
4.  746 W

Answer:
 746 W

(f) When a boy doubles his speed, his kinetic energy becomes

1. half
2. double
3. four times
4. no change

Answer:
four times

(g) A boy lifts a luggage from height 2 m to 4 m. The potential energy will become

1. half
2. double
3. one-third
4. one-fourth

Answer:
double

Selina Concise Physics Class 8 ICSE Solutions – Heat Transfer

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APlusTopper.com provides step by step solutions for Selina Concise ICSE Solutions for Class 8 Physics Chapter 6 Heat Transfer. You can download the Selina Concise Physics ICSE Solutions for Class 8 with Free PDF download option. Selina Publishers Concise Physics for Class 8 ICSE Solutions all questions are solved and explained by expert teachers as per ICSE board guidelines.

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Selina Concise ICSE Solutions for Class 8 Physics Chapter 6 Heat Transfer

•  Heat is a form of energy. When two bodies are in contact heat flows from body at higher temperature to body at lower temperature till the lower temperature of both is same.
•  When a body is heated, its molecules move faster about their means position and kinetic energy increases and with fall in temperature their K.E. decreases.
•  When a substance is heated
  (i) It expands i.e. a change in size takes place
  (ii) Change in temperature takes place.
  (iii) Change in state takes place.
• CHANGE OF STATE : “The process of change from one state to another at a constant temperature is called change of state.”
• Solid on heating changes into LIQUID. LIQUID on absorbing heat changes to VAPOURS some SOLIDS on heating DIRECTLY change in vapours called SUBLIMATION. Substance is called SUBLIMATE.
  SOLIDIFICATION on cooling when a vapours change into SOLID. GAS OR VAPOURS on cooling \(\xrightarrow { Condensation }\) changes to LIQUID also called LIQUIFACTION.
•  MELTING: Change of solid into liquid at constant temperature. FUSION ⇒ FREEZING is change of LIQUID into SOLID at constant temperature and change of solid into liquid at a constant _ temperature is called FUSION.
•  EVAPORATION: “Change liquid to gas at ALLTEMPERATURES” It is surface phenomenon. “
• VAPOURIZATION : “Change of liquid into vapours at fixed temperature”.
•  METING POINT: “Is the temperature at which a solid starts melting and remains constant till the whole of solid melts.”
  M.P. is same as freezing point.
  M.P. of ice is 0°C or freezing point, of water is 0°C.
•  BOILING POINT: “Is the temperature of a liquid at which it start, boiling i.e. change into vapours or gaseous state.”
  B .P. of pure water is 100°C.
• ABSOLUTE ZERO: “The temperature at which molecular motion completely ceases.”
•  FACTORS EFFECTING THE RATE OF EVAPORATION :
  (i) Temperature: Increases with increase in temperature
  (ii) S.A.: Increases with increase in S.A.
  (iii) BLOWING AIR—Renewal of air increases evaporation.
  (iv) NATURE—Some liquids like spirit, alcohol, petrol evaporate easily.
•  EVAPOURATION → produces coolness, BOILING produces Hotness.
•  LINEAR EXPANSION: When a solid rod (metal) is heated change in length takes place, which depends upon
  (i) original length (L₀)
  (ii) Increase in temperature
  (iii) Material of rod.
  Let L0 be the original length at 0°C, when heated to T°C final length becomes L
  Increase in length (L_t – L₀) a L₀ (T – 0)
  Or
  Coefficient of linear expension a which depends upon material of rod.
  L_t– L₀ = L₀ α T
  α = L_t – L₀ / L₀ T = increase in length / original length × Rise in temperature
•  When a metal plate is heated, change in area takes place and the expansion is called SUPERFICIAL expansion.
•  When a solid of volume v₀ is heated change in volume called cubical expansion takes place.
• α : β : γ = 1 : 2 : 3

Test your self

A. Objective Questions

1. Write true or false for each statement

(a) Evaporation is rapid on a wet day.
Answer. False.

(b) Evaporation takes place only from the surface of liquid.
Answer. True.

(c) All molecules of a liquid take part in the process of evaporation.
Answer. False.

(d) Temperature of a liquid rises during boiling or vaporization
Answer. False.

(e) All molecules of a liquid take part in boiling.
Answer. True.

(f) Boiling is a rapid phenomenon.
Answer. True.

(g) All solids expand by the same amount when heated to the same rise in temperature.
Answer. False.

(h) Telephone wires are kept tight between the two poles in winter.
Answer. True.

(i) Equal volumes of different liquids expand by the different amount when they are heated to the same rise in temperature.
Answer. True.

(j) Solids expand the least and gases expand the most on being heated.
Answer. True.

(k) A mercury thermometer makes use of the property of expansion of liquids on heating.
Answer. True.

(l) Kerosene contracts on heating.
Answer. False.

2. Fill in the blanks 

(a) Boiling occurs at a fixed temperature.
(b) Evaporation takes place at all temperature.
(c) The molecules of liquid absorb heat from surroundings in evaporation.
(d) Heat is absorbed during boiling.
(e) Cooling is produced in evaporation.
(f) A longer rod expands more than a shorter rod on being heated to the same temperature.
(g) Liquids expand more than the solids.
(h) Gases expand more than the liquids.
(i) Alcohol expands more than water.
(j) Iron expands less than copper.

3. Match the following

4. Select the correct alternative 

(a) In evaporation

1. all molecules of liquid begin to escape out
2.  only the molecules at the surface escape out
3.  the temperature of liquid rises by absorbing heat from surroundings.
4.  the molecules get attracted within the liquid.

(b) The rate of evaporation of a liquid increases when :

1.  temperature of liquid falls
2.  liquid is poured in a vessel of less surface area
3.  air is blown above the surface of liquid
4.  humidity increases.

(c) During boiling or vaporization

1.  all molecules take part
2.  temperature rises
3.  no heat is absorbed
4.  the average kinetic energy of molecules increases.

(d) The boiling point of a liquid is increased by

1.  increasing the volume of liquid
2.  increasing the pressure, on liquid
3.  adding ice to the liquid
4.  decreasing pressure on liquid.

(e) Two rods A and B of the same metal, but of length 1 m and 2 m respectively, are heated from 0°C to 100°C. Then

1.  both the rods A and B elongate the same
2.  the rod A elongates more than the rod B
3.  the rod B elongates more than the rod A
4.  the rod A elongates, but the rod B contracts.

(f) Two rods A and B of the same metal, same length, but one solid and the other hollow, are heated to the same rise in temperature.
Then

1.  the solid rod A expands more than the hollow rod B
2.  the hollow rod B expands more than the solid rod A
3.  the hollow rod B contracts, but the solid rod A expands
4.  both the rods A and B expand the same.

(g) A given volume of alcohol and the same volume of water are heated from the room temperature to the same temperature then.

1.  alcohol contracts, but water expands
2.  water contracts, but alcohol expands
3.  water expands more than alcohol
4.  alcohol expands more than water.

(h) The increase in length of a metal rod depends on

1.  the initial length of the rod only
2.  the rise in temperature only
3.  the material of rod only
4.  all the above three factors.

(i) The correct statement is

1.  Iron rims are cooled before they are placed on the cart wheels.
2.  A glass stopper gets tighten on warming the neck of the bottle.
3.  Telephone wires sag in winter, but become tight in summer.
4.  A little space is left between two rails on a railway track.