Selina Textbook Solutions: Energy | Physics Class 7 ICSE PDF Download

 Page 1


Energy 
Points to Remember 
?  Work is said to be done if the applied force on the body moves it. If no motion 
takes place, no work is said to be done. 
?  The amount of work done depends on two factors : (i) on the magnitude of the 
force applied (greater the force applied, greater is the work done), and (ii) on the 
distance moved in the direction of force (greater the distance moved, greater is 
the work done). 
?  The work done by a force on a body is equal to the product of the force and the 
distance moved by the body in the direction of force, i.e. Work done = Force × 
distance moved in the direction of force Or W = F × d 
?  The S.I. unit of work is joule (J), where 1 joule (J) = 1 newton (N) × 1 metre (m) 
?  The energy of a body is its capacity (or ability) to do work. The energy of a body 
in a state is equal to the work done on the body to bring it to that state. 
?  The S.I. unit of energy is joule (J). 
?  Kinetic energy of a body is the energy possessed by it due to its motion. It is the 
energy stored when work is done to bring the body in motion. 
?  Kinetic energy of a moving body depends on two factors : 
(i) on the mass of the body (greater the mass of the body, greater is its kinetic 
energy), and (iii) on the speed of the body (more the speed of the body, higher is 
its kinetic energy). 
?  The potential energy changes into the kinetic energy when it is put to use. 
?  In transformation of energy, the total sum of useful and non-useful energy 
obtained after conversion is equal to the energy converted, i.e. the total energy 
remains conserved. 
?  According to the conservation of mechanical energy, if friction is neglected, the 
total sum of potential energy and kinetic energy remains constant. Examples are : 
motion of roller coaster, free vertical fall of a body etc. 
?  The electricity obtained from the energy possessed by the flowing water is called 
the hydro-electricity. 
Test Yourself 
A. Objective Questions  
1. Write true or false for each statement 
(a) A man going up has potential energy and kinetic energy both. 
Answer. True. 
Page 2


Energy 
Points to Remember 
?  Work is said to be done if the applied force on the body moves it. If no motion 
takes place, no work is said to be done. 
?  The amount of work done depends on two factors : (i) on the magnitude of the 
force applied (greater the force applied, greater is the work done), and (ii) on the 
distance moved in the direction of force (greater the distance moved, greater is 
the work done). 
?  The work done by a force on a body is equal to the product of the force and the 
distance moved by the body in the direction of force, i.e. Work done = Force × 
distance moved in the direction of force Or W = F × d 
?  The S.I. unit of work is joule (J), where 1 joule (J) = 1 newton (N) × 1 metre (m) 
?  The energy of a body is its capacity (or ability) to do work. The energy of a body 
in a state is equal to the work done on the body to bring it to that state. 
?  The S.I. unit of energy is joule (J). 
?  Kinetic energy of a body is the energy possessed by it due to its motion. It is the 
energy stored when work is done to bring the body in motion. 
?  Kinetic energy of a moving body depends on two factors : 
(i) on the mass of the body (greater the mass of the body, greater is its kinetic 
energy), and (iii) on the speed of the body (more the speed of the body, higher is 
its kinetic energy). 
?  The potential energy changes into the kinetic energy when it is put to use. 
?  In transformation of energy, the total sum of useful and non-useful energy 
obtained after conversion is equal to the energy converted, i.e. the total energy 
remains conserved. 
?  According to the conservation of mechanical energy, if friction is neglected, the 
total sum of potential energy and kinetic energy remains constant. Examples are : 
motion of roller coaster, free vertical fall of a body etc. 
?  The electricity obtained from the energy possessed by the flowing water is called 
the hydro-electricity. 
Test Yourself 
A. Objective Questions  
1. Write true or false for each statement 
(a) A man going up has potential energy and kinetic energy both. 
Answer. True. 
(b) A gum bottle lying on a table has no energy. 
Answer. False. 
Correct — A gum bottle lying on a table has energy. 
(c) In an electric fan, electrical energy changes into the mechanical energy. 
Answer. True. 
(d) Potential energy changes into kinetic energy when it is put to use. 
Answer. True. 
(e) One form of energy cannot be converted into another form. 
Answer. False. 
Correct — One form of energy can be converted into the other form. 
(f) There is always some loss of energy in conversion from one form of energy to 
another form, so the total energy is not conserved. 
Answer. False. 
Correct — There is always some loss of energy in conversion from one form of energy 
to the other form, so the total energy is conserved. 
(g) The energy of flowing water can be converted into electric energy (electricity). 
Answer. True. 
2. Fill in the blanks 
(a) An electric fan converts electrical energy into mechanical energy. 
(b) Cooking gas converts chemical energy into heat energy. 
(c) Energy possessed by a compressed spring is potential energy. 
(d) The ability to do work is called energy 
(e) The energy possessed by a body due to its position is called potential energy. 
(f) The energy possessed by a body due to its motion is called kinetic energy. 
(g) Green plants convert light energy into chemical energy. 
(h) The S.I.unit of energy is joule 
(i) An object falling freely from the roof of a multistorey building has potential 
energy and kinetic energy when halfway down the building. 
 
 
 
Page 3


Energy 
Points to Remember 
?  Work is said to be done if the applied force on the body moves it. If no motion 
takes place, no work is said to be done. 
?  The amount of work done depends on two factors : (i) on the magnitude of the 
force applied (greater the force applied, greater is the work done), and (ii) on the 
distance moved in the direction of force (greater the distance moved, greater is 
the work done). 
?  The work done by a force on a body is equal to the product of the force and the 
distance moved by the body in the direction of force, i.e. Work done = Force × 
distance moved in the direction of force Or W = F × d 
?  The S.I. unit of work is joule (J), where 1 joule (J) = 1 newton (N) × 1 metre (m) 
?  The energy of a body is its capacity (or ability) to do work. The energy of a body 
in a state is equal to the work done on the body to bring it to that state. 
?  The S.I. unit of energy is joule (J). 
?  Kinetic energy of a body is the energy possessed by it due to its motion. It is the 
energy stored when work is done to bring the body in motion. 
?  Kinetic energy of a moving body depends on two factors : 
(i) on the mass of the body (greater the mass of the body, greater is its kinetic 
energy), and (iii) on the speed of the body (more the speed of the body, higher is 
its kinetic energy). 
?  The potential energy changes into the kinetic energy when it is put to use. 
?  In transformation of energy, the total sum of useful and non-useful energy 
obtained after conversion is equal to the energy converted, i.e. the total energy 
remains conserved. 
?  According to the conservation of mechanical energy, if friction is neglected, the 
total sum of potential energy and kinetic energy remains constant. Examples are : 
motion of roller coaster, free vertical fall of a body etc. 
?  The electricity obtained from the energy possessed by the flowing water is called 
the hydro-electricity. 
Test Yourself 
A. Objective Questions  
1. Write true or false for each statement 
(a) A man going up has potential energy and kinetic energy both. 
Answer. True. 
(b) A gum bottle lying on a table has no energy. 
Answer. False. 
Correct — A gum bottle lying on a table has energy. 
(c) In an electric fan, electrical energy changes into the mechanical energy. 
Answer. True. 
(d) Potential energy changes into kinetic energy when it is put to use. 
Answer. True. 
(e) One form of energy cannot be converted into another form. 
Answer. False. 
Correct — One form of energy can be converted into the other form. 
(f) There is always some loss of energy in conversion from one form of energy to 
another form, so the total energy is not conserved. 
Answer. False. 
Correct — There is always some loss of energy in conversion from one form of energy 
to the other form, so the total energy is conserved. 
(g) The energy of flowing water can be converted into electric energy (electricity). 
Answer. True. 
2. Fill in the blanks 
(a) An electric fan converts electrical energy into mechanical energy. 
(b) Cooking gas converts chemical energy into heat energy. 
(c) Energy possessed by a compressed spring is potential energy. 
(d) The ability to do work is called energy 
(e) The energy possessed by a body due to its position is called potential energy. 
(f) The energy possessed by a body due to its motion is called kinetic energy. 
(g) Green plants convert light energy into chemical energy. 
(h) The S.I.unit of energy is joule 
(i) An object falling freely from the roof of a multistorey building has potential 
energy and kinetic energy when halfway down the building. 
 
 
 
3. Match the following columns 
 
 
4. Select the correct alternatives  
(a) When we rub our hands 
1.  kinetic energy changes into potential energy 
2.  mechanical energy changes into heat energy 
3.  potential energy changes into kinetic energy 
4.  heat energy changes into mechanical energy. 
(b) A ball rolling on the ground possesses 
1.  kinetic energy 
2.  potential energy 
3.  no energy 
4.  heat energy 
(c) The energy stored in an electric cell is 
1.  chemical energy 
Page 4


Energy 
Points to Remember 
?  Work is said to be done if the applied force on the body moves it. If no motion 
takes place, no work is said to be done. 
?  The amount of work done depends on two factors : (i) on the magnitude of the 
force applied (greater the force applied, greater is the work done), and (ii) on the 
distance moved in the direction of force (greater the distance moved, greater is 
the work done). 
?  The work done by a force on a body is equal to the product of the force and the 
distance moved by the body in the direction of force, i.e. Work done = Force × 
distance moved in the direction of force Or W = F × d 
?  The S.I. unit of work is joule (J), where 1 joule (J) = 1 newton (N) × 1 metre (m) 
?  The energy of a body is its capacity (or ability) to do work. The energy of a body 
in a state is equal to the work done on the body to bring it to that state. 
?  The S.I. unit of energy is joule (J). 
?  Kinetic energy of a body is the energy possessed by it due to its motion. It is the 
energy stored when work is done to bring the body in motion. 
?  Kinetic energy of a moving body depends on two factors : 
(i) on the mass of the body (greater the mass of the body, greater is its kinetic 
energy), and (iii) on the speed of the body (more the speed of the body, higher is 
its kinetic energy). 
?  The potential energy changes into the kinetic energy when it is put to use. 
?  In transformation of energy, the total sum of useful and non-useful energy 
obtained after conversion is equal to the energy converted, i.e. the total energy 
remains conserved. 
?  According to the conservation of mechanical energy, if friction is neglected, the 
total sum of potential energy and kinetic energy remains constant. Examples are : 
motion of roller coaster, free vertical fall of a body etc. 
?  The electricity obtained from the energy possessed by the flowing water is called 
the hydro-electricity. 
Test Yourself 
A. Objective Questions  
1. Write true or false for each statement 
(a) A man going up has potential energy and kinetic energy both. 
Answer. True. 
(b) A gum bottle lying on a table has no energy. 
Answer. False. 
Correct — A gum bottle lying on a table has energy. 
(c) In an electric fan, electrical energy changes into the mechanical energy. 
Answer. True. 
(d) Potential energy changes into kinetic energy when it is put to use. 
Answer. True. 
(e) One form of energy cannot be converted into another form. 
Answer. False. 
Correct — One form of energy can be converted into the other form. 
(f) There is always some loss of energy in conversion from one form of energy to 
another form, so the total energy is not conserved. 
Answer. False. 
Correct — There is always some loss of energy in conversion from one form of energy 
to the other form, so the total energy is conserved. 
(g) The energy of flowing water can be converted into electric energy (electricity). 
Answer. True. 
2. Fill in the blanks 
(a) An electric fan converts electrical energy into mechanical energy. 
(b) Cooking gas converts chemical energy into heat energy. 
(c) Energy possessed by a compressed spring is potential energy. 
(d) The ability to do work is called energy 
(e) The energy possessed by a body due to its position is called potential energy. 
(f) The energy possessed by a body due to its motion is called kinetic energy. 
(g) Green plants convert light energy into chemical energy. 
(h) The S.I.unit of energy is joule 
(i) An object falling freely from the roof of a multistorey building has potential 
energy and kinetic energy when halfway down the building. 
 
 
 
3. Match the following columns 
 
 
4. Select the correct alternatives  
(a) When we rub our hands 
1.  kinetic energy changes into potential energy 
2.  mechanical energy changes into heat energy 
3.  potential energy changes into kinetic energy 
4.  heat energy changes into mechanical energy. 
(b) A ball rolling on the ground possesses 
1.  kinetic energy 
2.  potential energy 
3.  no energy 
4.  heat energy 
(c) The energy stored in an electric cell is 
1.  chemical energy 
2.  electrical energy 
3.  heat energy 
4.  mechanical energy. 
(d) When a bulb lights up on passing current, the change of energy is 
1.  from electrical energy to heat energy 
2.  from electrical energy to light energy 
3.  from electrical energy to heat and light energy 
4.  from electrical energy to mechanical energy. 
(e) The correct statement is 
1.  Both work and energy have the same units 
2.  Potential energy of a body is due to its motion 
3.  Kinetic energy of a body is due to its position or state 
4.  Kinetic energy can change into potential energy, but potential energy cannot 
change into kinetic energy. 
(f) According to law of conservation of energy, energy changes from one form to 
another form, but the total energy of that system 
1.  increases 
2.  decreases 
3.  alternates 
4.  remains the same 
B. Short/Long Answer Questions 
Question 1. 
Define the term energy. 
Answer: 
Energy is the capacity of doing work. 
Question 2. 
State the unit of energy and define it. 
Answer: 
The energy is measured in the same unit as work. Therefore the S.I. unit of energy is 
joule (symbol J). 
A body is said to possess an energy of one joule if a force of 1 newton moves the body 
by a distance of 1 metre in the direction of force. 
Another unit of energy is calorie (symbol cal) where 1 cal = 4.2 J. A bigger unit is kilo-
calorie (symbol kcal) where 1 kcal = 1000 cal. 
Page 5


Energy 
Points to Remember 
?  Work is said to be done if the applied force on the body moves it. If no motion 
takes place, no work is said to be done. 
?  The amount of work done depends on two factors : (i) on the magnitude of the 
force applied (greater the force applied, greater is the work done), and (ii) on the 
distance moved in the direction of force (greater the distance moved, greater is 
the work done). 
?  The work done by a force on a body is equal to the product of the force and the 
distance moved by the body in the direction of force, i.e. Work done = Force × 
distance moved in the direction of force Or W = F × d 
?  The S.I. unit of work is joule (J), where 1 joule (J) = 1 newton (N) × 1 metre (m) 
?  The energy of a body is its capacity (or ability) to do work. The energy of a body 
in a state is equal to the work done on the body to bring it to that state. 
?  The S.I. unit of energy is joule (J). 
?  Kinetic energy of a body is the energy possessed by it due to its motion. It is the 
energy stored when work is done to bring the body in motion. 
?  Kinetic energy of a moving body depends on two factors : 
(i) on the mass of the body (greater the mass of the body, greater is its kinetic 
energy), and (iii) on the speed of the body (more the speed of the body, higher is 
its kinetic energy). 
?  The potential energy changes into the kinetic energy when it is put to use. 
?  In transformation of energy, the total sum of useful and non-useful energy 
obtained after conversion is equal to the energy converted, i.e. the total energy 
remains conserved. 
?  According to the conservation of mechanical energy, if friction is neglected, the 
total sum of potential energy and kinetic energy remains constant. Examples are : 
motion of roller coaster, free vertical fall of a body etc. 
?  The electricity obtained from the energy possessed by the flowing water is called 
the hydro-electricity. 
Test Yourself 
A. Objective Questions  
1. Write true or false for each statement 
(a) A man going up has potential energy and kinetic energy both. 
Answer. True. 
(b) A gum bottle lying on a table has no energy. 
Answer. False. 
Correct — A gum bottle lying on a table has energy. 
(c) In an electric fan, electrical energy changes into the mechanical energy. 
Answer. True. 
(d) Potential energy changes into kinetic energy when it is put to use. 
Answer. True. 
(e) One form of energy cannot be converted into another form. 
Answer. False. 
Correct — One form of energy can be converted into the other form. 
(f) There is always some loss of energy in conversion from one form of energy to 
another form, so the total energy is not conserved. 
Answer. False. 
Correct — There is always some loss of energy in conversion from one form of energy 
to the other form, so the total energy is conserved. 
(g) The energy of flowing water can be converted into electric energy (electricity). 
Answer. True. 
2. Fill in the blanks 
(a) An electric fan converts electrical energy into mechanical energy. 
(b) Cooking gas converts chemical energy into heat energy. 
(c) Energy possessed by a compressed spring is potential energy. 
(d) The ability to do work is called energy 
(e) The energy possessed by a body due to its position is called potential energy. 
(f) The energy possessed by a body due to its motion is called kinetic energy. 
(g) Green plants convert light energy into chemical energy. 
(h) The S.I.unit of energy is joule 
(i) An object falling freely from the roof of a multistorey building has potential 
energy and kinetic energy when halfway down the building. 
 
 
 
3. Match the following columns 
 
 
4. Select the correct alternatives  
(a) When we rub our hands 
1.  kinetic energy changes into potential energy 
2.  mechanical energy changes into heat energy 
3.  potential energy changes into kinetic energy 
4.  heat energy changes into mechanical energy. 
(b) A ball rolling on the ground possesses 
1.  kinetic energy 
2.  potential energy 
3.  no energy 
4.  heat energy 
(c) The energy stored in an electric cell is 
1.  chemical energy 
2.  electrical energy 
3.  heat energy 
4.  mechanical energy. 
(d) When a bulb lights up on passing current, the change of energy is 
1.  from electrical energy to heat energy 
2.  from electrical energy to light energy 
3.  from electrical energy to heat and light energy 
4.  from electrical energy to mechanical energy. 
(e) The correct statement is 
1.  Both work and energy have the same units 
2.  Potential energy of a body is due to its motion 
3.  Kinetic energy of a body is due to its position or state 
4.  Kinetic energy can change into potential energy, but potential energy cannot 
change into kinetic energy. 
(f) According to law of conservation of energy, energy changes from one form to 
another form, but the total energy of that system 
1.  increases 
2.  decreases 
3.  alternates 
4.  remains the same 
B. Short/Long Answer Questions 
Question 1. 
Define the term energy. 
Answer: 
Energy is the capacity of doing work. 
Question 2. 
State the unit of energy and define it. 
Answer: 
The energy is measured in the same unit as work. Therefore the S.I. unit of energy is 
joule (symbol J). 
A body is said to possess an energy of one joule if a force of 1 newton moves the body 
by a distance of 1 metre in the direction of force. 
Another unit of energy is calorie (symbol cal) where 1 cal = 4.2 J. A bigger unit is kilo-
calorie (symbol kcal) where 1 kcal = 1000 cal. 
Question 3. 
Name five different forms of energy. 
Answer: 
The different forms of energy are : 
(i) Mechanical energy 
(ii) Heat energy 
(iii) Light energy 
(iv) Chemical energy 
(v) Sound energy 
(vi) Magnetic energy 
(vii) Electrical energy and 
(viii) Atomic energy or nuclear energy. 
Question 4. 
What are the two kinds of mechanical energy. 
Answer: 
The mechanical energy is found in two forms namely : 
(a) The potential energy, and (b) The kinetic energy. 
Question 5. 
What is potential energy ? State its unit. 
Answer: 
The energy of a body at rest is called the potential energy. It is defined as follows : 
Potential energy of a body is the energy possessed by it due to its state of rest or 
position. Actually, it is the work spent in bringing the body to that state of rest or 
position. 
It is written as P.E. or U. 
The S.I. unit of potential energy is Joules. 
Question 6. 
Give one example of a body that has potential energy, in each of the following : (i) due 
to its position, (ii) due to its state. 
Answer: 
(i) Potential energy of a body is the energy possessed by it due to its state of rest or 
position. It is the energy stored when work is done on the body to bring it to that state or 
position. 
(ii) Potential energy of a body in the raised (or lifted) position depends on two factors : 
(1) the mass of the body greater the mass of the body, greater is the potential energy of 
the body), and (2) the height of the body above the ground (greater the height of the 
body, greater is its potential energy.)