Selina Textbook Solutions: Energy | Physics Class 8 ICSE PDF Download

 Page 1


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 V
2
 
? 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. 
Page 2


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 V
2
 
? 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 
Page 3


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 V
2
 
? 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 
(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 
(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. 
Page 4


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 V
2
 
? 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 
(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 
(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. 
(d) The expression of power P is 
1. P = mgh 
2. P = P = 1/2 Mv
2
 
3. P = F × d 
4. P = F × d/t 
(e) I H.P. ¡s equal to 
1. 1 W 
2. 1 J 
3. 746 J 
4. 746 W 
(f) When a boy doubles his speed, his kinetic energy becomes 
1. half 
2. double 
3. four times 
4. no change 
(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 
B. Short/Long Answer Questions 
Question 1. 
Define work. 
Answer: 
WORK “When a force is applied on a body and there is displacement of the body, work 
is said to be done.” 
Question 2. 
When does a force perform work ? 
Answer: 
Work is said to be done only when the force applied on a body i. makes the body more 
(/. e. there is a displacement of the body). 
Question 3. 
State two conditions when no work is done by a force. 
Page 5


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 V
2
 
? 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 
(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 
(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. 
(d) The expression of power P is 
1. P = mgh 
2. P = P = 1/2 Mv
2
 
3. P = F × d 
4. P = F × d/t 
(e) I H.P. ¡s equal to 
1. 1 W 
2. 1 J 
3. 746 J 
4. 746 W 
(f) When a boy doubles his speed, his kinetic energy becomes 
1. half 
2. double 
3. four times 
4. no change 
(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 
B. Short/Long Answer Questions 
Question 1. 
Define work. 
Answer: 
WORK “When a force is applied on a body and there is displacement of the body, work 
is said to be done.” 
Question 2. 
When does a force perform work ? 
Answer: 
Work is said to be done only when the force applied on a body i. makes the body more 
(/. e. there is a displacement of the body). 
Question 3. 
State two conditions when no work is done by a force. 
Answer: 
Two conditions are : 
(i) There should be no displacement i.e. S = 0 
(ii) The displacement is NORMAL to the direction of FORCE 
i.e. – ? = 90° 
Question 4. 
In which of the following cases is work being done : 
(a) A boy pushing a rock 
(b) A boy climbing up the stairs 
(c) A coolie standing with a box in his head 
(d) A girl moving on the road. 
Answer: 
(b) A boy climbing up the stairs 
(d) A girl moving on the road. 
Question 5. 
A coolie is moving on a road with a luggage on his head. Does he perform work against 
the force of gravity ? Give reason for your answer. 
Answer: 
A coolie carrying a luggage on his head moving on ground does i no work against the 
force of gravity as displacement is normal to the direction of force of gravity. 
Question 6. 
The moon is revolving around the earth in a circular path. How much work is done by 
the moon ? 
Answer: 
No work is done, since displacement is NORMAL to the direction | of force on the body. 
The force is CENTRIPETAL. 
Question 7. 
Write the expression for work done by a force, 
Answer: 
Work done by applying force F is the product of force applied on the body and distance 
moved by the body in the direction of force , work done = Force x distance moved in the 
direction of force. 
W = F × d