Selina Textbook Solutions: Work Energy and Power | Physics Class 10 ICSE PDF Download

 Page 1


Class X Chapter 2 – Work, Energy and Power  Physics 
_____________________________________________________________________________ 
 
 
                     
EXERCISE- 2 (A) 
 
Question 1: 
Define work. Is work a scalar or a vector? 
Solution 1: 
Work is said to be done only when the force applied on a body makes the body move. It is a 
scalar quantity. 
 
Question 2: 
How is the work done by a force measured when (i) force is in direction of displacement, (ii) 
force is at an angle to the direction of displacement? 
Solution 2: 
(i) When force is in direction of displacement, then work done , W = F × S 
(ii) When force is at an angle  to the direction of displacement, then work done,  
W = F S cos
? 
 
 
Question 3: 
A force F acts on a body and displaces it by a distance S in a direction at an angle ? with the 
direction of force. (a) Write the expression for the work done by the force. (b) what should be 
the angle between the force and displacement to get the (i) zero work (ii) maximum work? 
Solution 3: 
(a) When force is at an angle to the direction of displacement, then work done, W= F S cos
 
(b)   
(i) For zero work done, the angle between force and displacement should be 90
o
 as cos 
90
o
=0 
             W =FScos90
o
= FSx0=0 
(ii) For maximum work done, the angle between force and displacement should be 0
o
 as 
cos0
o
=1 
            Hence, W=FScos 0
o
=FS 
 
 
Question 4: 
A body is acted upon by a force. State two condition when the work done is zero. 
 
Page 2


Class X Chapter 2 – Work, Energy and Power  Physics 
_____________________________________________________________________________ 
 
 
                     
EXERCISE- 2 (A) 
 
Question 1: 
Define work. Is work a scalar or a vector? 
Solution 1: 
Work is said to be done only when the force applied on a body makes the body move. It is a 
scalar quantity. 
 
Question 2: 
How is the work done by a force measured when (i) force is in direction of displacement, (ii) 
force is at an angle to the direction of displacement? 
Solution 2: 
(i) When force is in direction of displacement, then work done , W = F × S 
(ii) When force is at an angle  to the direction of displacement, then work done,  
W = F S cos
? 
 
 
Question 3: 
A force F acts on a body and displaces it by a distance S in a direction at an angle ? with the 
direction of force. (a) Write the expression for the work done by the force. (b) what should be 
the angle between the force and displacement to get the (i) zero work (ii) maximum work? 
Solution 3: 
(a) When force is at an angle to the direction of displacement, then work done, W= F S cos
 
(b)   
(i) For zero work done, the angle between force and displacement should be 90
o
 as cos 
90
o
=0 
             W =FScos90
o
= FSx0=0 
(ii) For maximum work done, the angle between force and displacement should be 0
o
 as 
cos0
o
=1 
            Hence, W=FScos 0
o
=FS 
 
 
Question 4: 
A body is acted upon by a force. State two condition when the work done is zero. 
 
Class X Chapter 2 – Work, Energy and Power  Physics 
_____________________________________________________________________________ 
 
 
Solution 4: 
Two conditions when the work done is zero are: 
(i) When there is no displacement (S=0) and, 
(ii) When the displacement is normal to the direction of the force (
?
=90
o
). 
 
 
Question 5: 
State the condition when the work done by a force is (i) positive, (ii) negative. Explain with the 
help of examples. 
Solution 5: 
(i) If the displacement of the body is in the direction of force, then work done is positive. 
Hence, W= F x S 
For example: A coolie does work on the load when he raises it up against the force of 
gravity. The force exerted by coolie (=mg) and displacement, both are in upward direction. 
(ii) If the displacement of the body is in the direction opposite to the force, then work done is 
negative. 
Hence, W = - F × S 
For example: When a body moves on a surface, the force of friction between the body and the 
surface is in direction opposite to the motion of the body and so the work done by the force of 
friction is negative. 
 
 
Question 6: 
A body is moved in a direction opposite to the direction of force acting on it. State whether the 
work is done by the force or work is done against the force. 
Solution 6: 
Work is done against the force. 
 
 
Question 7: 
When a body moves in a circular path, how much work is done by the body? Give reason. 
Solution 7: 
When a body moves in a circular path, no work is done since the force on the body is directed 
towards the centre of circular path (the body is acted upon by the centripetal force), while the 
displacement at all instants is along the tangent to the circular path, i.e., normal to the direction 
of force 
 
Page 3


Class X Chapter 2 – Work, Energy and Power  Physics 
_____________________________________________________________________________ 
 
 
                     
EXERCISE- 2 (A) 
 
Question 1: 
Define work. Is work a scalar or a vector? 
Solution 1: 
Work is said to be done only when the force applied on a body makes the body move. It is a 
scalar quantity. 
 
Question 2: 
How is the work done by a force measured when (i) force is in direction of displacement, (ii) 
force is at an angle to the direction of displacement? 
Solution 2: 
(i) When force is in direction of displacement, then work done , W = F × S 
(ii) When force is at an angle  to the direction of displacement, then work done,  
W = F S cos
? 
 
 
Question 3: 
A force F acts on a body and displaces it by a distance S in a direction at an angle ? with the 
direction of force. (a) Write the expression for the work done by the force. (b) what should be 
the angle between the force and displacement to get the (i) zero work (ii) maximum work? 
Solution 3: 
(a) When force is at an angle to the direction of displacement, then work done, W= F S cos
 
(b)   
(i) For zero work done, the angle between force and displacement should be 90
o
 as cos 
90
o
=0 
             W =FScos90
o
= FSx0=0 
(ii) For maximum work done, the angle between force and displacement should be 0
o
 as 
cos0
o
=1 
            Hence, W=FScos 0
o
=FS 
 
 
Question 4: 
A body is acted upon by a force. State two condition when the work done is zero. 
 
Class X Chapter 2 – Work, Energy and Power  Physics 
_____________________________________________________________________________ 
 
 
Solution 4: 
Two conditions when the work done is zero are: 
(i) When there is no displacement (S=0) and, 
(ii) When the displacement is normal to the direction of the force (
?
=90
o
). 
 
 
Question 5: 
State the condition when the work done by a force is (i) positive, (ii) negative. Explain with the 
help of examples. 
Solution 5: 
(i) If the displacement of the body is in the direction of force, then work done is positive. 
Hence, W= F x S 
For example: A coolie does work on the load when he raises it up against the force of 
gravity. The force exerted by coolie (=mg) and displacement, both are in upward direction. 
(ii) If the displacement of the body is in the direction opposite to the force, then work done is 
negative. 
Hence, W = - F × S 
For example: When a body moves on a surface, the force of friction between the body and the 
surface is in direction opposite to the motion of the body and so the work done by the force of 
friction is negative. 
 
 
Question 6: 
A body is moved in a direction opposite to the direction of force acting on it. State whether the 
work is done by the force or work is done against the force. 
Solution 6: 
Work is done against the force. 
 
 
Question 7: 
When a body moves in a circular path, how much work is done by the body? Give reason. 
Solution 7: 
When a body moves in a circular path, no work is done since the force on the body is directed 
towards the centre of circular path (the body is acted upon by the centripetal force), while the 
displacement at all instants is along the tangent to the circular path, i.e., normal to the direction 
of force 
 
Class X Chapter 2 – Work, Energy and Power  Physics 
_____________________________________________________________________________ 
 
 
 
Question 8: 
A satellite revolves around the earth in a circular orbit. What is the work done by the force of 
gravity? Give reason. 
Solution 8: 
Work done by the force of gravity (which provides the centripetal force) is zero as the force of 
gravity acting on the satellite is normal to the displacement of the satellite. 
 
 
Question 9: 
In which of the following cases, is work being done? 
(i) A man pushing a wall. 
(ii) a coolie standing with a load of 12 kgf on his head.  
(iii) A boy climbing up a staircase. 
Solution 9: 
Work is done only in case of a boy climbing up a stair case. 
 
 
Question 10: 
A coolie carrying a load on his head and moving on a frictionless horizontal platform does no 
work. Explain the reason. 
Solution 10: 
When a coolie carrying some load on his head moves, no work is done by him against the 
force of gravity because the displacement of load being horizontal, is normal to the direction 
of force of gravity. 
 
 
Question 11: 
The work done by a fielder when he takes a catch in a cricket match, is negative Explain. 
Solution 11: 
Force applied by the fielder on the ball is in opposite direction of displacement of ball. So, work 
done by the fielder on the ball is negative. 
 
 
Question 12: 
Give an example when work done by the force of gravity acting on a body is zero even though 
the body gets displaces from its initial position. 
Page 4


Class X Chapter 2 – Work, Energy and Power  Physics 
_____________________________________________________________________________ 
 
 
                     
EXERCISE- 2 (A) 
 
Question 1: 
Define work. Is work a scalar or a vector? 
Solution 1: 
Work is said to be done only when the force applied on a body makes the body move. It is a 
scalar quantity. 
 
Question 2: 
How is the work done by a force measured when (i) force is in direction of displacement, (ii) 
force is at an angle to the direction of displacement? 
Solution 2: 
(i) When force is in direction of displacement, then work done , W = F × S 
(ii) When force is at an angle  to the direction of displacement, then work done,  
W = F S cos
? 
 
 
Question 3: 
A force F acts on a body and displaces it by a distance S in a direction at an angle ? with the 
direction of force. (a) Write the expression for the work done by the force. (b) what should be 
the angle between the force and displacement to get the (i) zero work (ii) maximum work? 
Solution 3: 
(a) When force is at an angle to the direction of displacement, then work done, W= F S cos
 
(b)   
(i) For zero work done, the angle between force and displacement should be 90
o
 as cos 
90
o
=0 
             W =FScos90
o
= FSx0=0 
(ii) For maximum work done, the angle between force and displacement should be 0
o
 as 
cos0
o
=1 
            Hence, W=FScos 0
o
=FS 
 
 
Question 4: 
A body is acted upon by a force. State two condition when the work done is zero. 
 
Class X Chapter 2 – Work, Energy and Power  Physics 
_____________________________________________________________________________ 
 
 
Solution 4: 
Two conditions when the work done is zero are: 
(i) When there is no displacement (S=0) and, 
(ii) When the displacement is normal to the direction of the force (
?
=90
o
). 
 
 
Question 5: 
State the condition when the work done by a force is (i) positive, (ii) negative. Explain with the 
help of examples. 
Solution 5: 
(i) If the displacement of the body is in the direction of force, then work done is positive. 
Hence, W= F x S 
For example: A coolie does work on the load when he raises it up against the force of 
gravity. The force exerted by coolie (=mg) and displacement, both are in upward direction. 
(ii) If the displacement of the body is in the direction opposite to the force, then work done is 
negative. 
Hence, W = - F × S 
For example: When a body moves on a surface, the force of friction between the body and the 
surface is in direction opposite to the motion of the body and so the work done by the force of 
friction is negative. 
 
 
Question 6: 
A body is moved in a direction opposite to the direction of force acting on it. State whether the 
work is done by the force or work is done against the force. 
Solution 6: 
Work is done against the force. 
 
 
Question 7: 
When a body moves in a circular path, how much work is done by the body? Give reason. 
Solution 7: 
When a body moves in a circular path, no work is done since the force on the body is directed 
towards the centre of circular path (the body is acted upon by the centripetal force), while the 
displacement at all instants is along the tangent to the circular path, i.e., normal to the direction 
of force 
 
Class X Chapter 2 – Work, Energy and Power  Physics 
_____________________________________________________________________________ 
 
 
 
Question 8: 
A satellite revolves around the earth in a circular orbit. What is the work done by the force of 
gravity? Give reason. 
Solution 8: 
Work done by the force of gravity (which provides the centripetal force) is zero as the force of 
gravity acting on the satellite is normal to the displacement of the satellite. 
 
 
Question 9: 
In which of the following cases, is work being done? 
(i) A man pushing a wall. 
(ii) a coolie standing with a load of 12 kgf on his head.  
(iii) A boy climbing up a staircase. 
Solution 9: 
Work is done only in case of a boy climbing up a stair case. 
 
 
Question 10: 
A coolie carrying a load on his head and moving on a frictionless horizontal platform does no 
work. Explain the reason. 
Solution 10: 
When a coolie carrying some load on his head moves, no work is done by him against the 
force of gravity because the displacement of load being horizontal, is normal to the direction 
of force of gravity. 
 
 
Question 11: 
The work done by a fielder when he takes a catch in a cricket match, is negative Explain. 
Solution 11: 
Force applied by the fielder on the ball is in opposite direction of displacement of ball. So, work 
done by the fielder on the ball is negative. 
 
 
Question 12: 
Give an example when work done by the force of gravity acting on a body is zero even though 
the body gets displaces from its initial position. 
Class X Chapter 2 – Work, Energy and Power  Physics 
_____________________________________________________________________________ 
 
 
Solution 12: 
When a coolie carries a load while moving on a ground, the displacement is in the horizontal 
direction while the force of gravity acts vertically downward. So the work done by the force of 
gravity is zero. 
 
 
Question 13: 
What are the S.I. and C.G.S units of work?  How are they related? Establish the relationship.  
Solution 13: 
S.I unit of work is Joule. 
C.G.S unit of work is erg. 
Relation between joule and erg : 
1 joule = 1N x 1m 
But 1N = 10
5 
dyne 
And 1m=100 cm = 10
2 
cm 
Hence, 1 joule = 10
5
dyne × 10
2
cm 
= 10
7
dyne × cm =10
7
erg 
Thus, 1 Joule = 10
7
 erg 
 
 
Question 14: 
State and define the S.I. unit of work. 
Solution 14: 
S.I unit of work is Joule. 
1 joule of work is said to be done when a force of 1 newton displaces a body through a distance 
of 1 meter in its own direction. 
 
 
 
Question 15: 
Express joule in terms of erg. 
Solution 15: 
Relation between joule and erg : 
1 joule = 1N × 1m 
But 1N = 10
5 
dyne 
And 1m =100 cm = 10
2 
cm 
Hence, 1 joule = 10
5 
dyne × 10
2
cm 
= 10
7
dyne x cm = 10
7
erg 
Page 5


Class X Chapter 2 – Work, Energy and Power  Physics 
_____________________________________________________________________________ 
 
 
                     
EXERCISE- 2 (A) 
 
Question 1: 
Define work. Is work a scalar or a vector? 
Solution 1: 
Work is said to be done only when the force applied on a body makes the body move. It is a 
scalar quantity. 
 
Question 2: 
How is the work done by a force measured when (i) force is in direction of displacement, (ii) 
force is at an angle to the direction of displacement? 
Solution 2: 
(i) When force is in direction of displacement, then work done , W = F × S 
(ii) When force is at an angle  to the direction of displacement, then work done,  
W = F S cos
? 
 
 
Question 3: 
A force F acts on a body and displaces it by a distance S in a direction at an angle ? with the 
direction of force. (a) Write the expression for the work done by the force. (b) what should be 
the angle between the force and displacement to get the (i) zero work (ii) maximum work? 
Solution 3: 
(a) When force is at an angle to the direction of displacement, then work done, W= F S cos
 
(b)   
(i) For zero work done, the angle between force and displacement should be 90
o
 as cos 
90
o
=0 
             W =FScos90
o
= FSx0=0 
(ii) For maximum work done, the angle between force and displacement should be 0
o
 as 
cos0
o
=1 
            Hence, W=FScos 0
o
=FS 
 
 
Question 4: 
A body is acted upon by a force. State two condition when the work done is zero. 
 
Class X Chapter 2 – Work, Energy and Power  Physics 
_____________________________________________________________________________ 
 
 
Solution 4: 
Two conditions when the work done is zero are: 
(i) When there is no displacement (S=0) and, 
(ii) When the displacement is normal to the direction of the force (
?
=90
o
). 
 
 
Question 5: 
State the condition when the work done by a force is (i) positive, (ii) negative. Explain with the 
help of examples. 
Solution 5: 
(i) If the displacement of the body is in the direction of force, then work done is positive. 
Hence, W= F x S 
For example: A coolie does work on the load when he raises it up against the force of 
gravity. The force exerted by coolie (=mg) and displacement, both are in upward direction. 
(ii) If the displacement of the body is in the direction opposite to the force, then work done is 
negative. 
Hence, W = - F × S 
For example: When a body moves on a surface, the force of friction between the body and the 
surface is in direction opposite to the motion of the body and so the work done by the force of 
friction is negative. 
 
 
Question 6: 
A body is moved in a direction opposite to the direction of force acting on it. State whether the 
work is done by the force or work is done against the force. 
Solution 6: 
Work is done against the force. 
 
 
Question 7: 
When a body moves in a circular path, how much work is done by the body? Give reason. 
Solution 7: 
When a body moves in a circular path, no work is done since the force on the body is directed 
towards the centre of circular path (the body is acted upon by the centripetal force), while the 
displacement at all instants is along the tangent to the circular path, i.e., normal to the direction 
of force 
 
Class X Chapter 2 – Work, Energy and Power  Physics 
_____________________________________________________________________________ 
 
 
 
Question 8: 
A satellite revolves around the earth in a circular orbit. What is the work done by the force of 
gravity? Give reason. 
Solution 8: 
Work done by the force of gravity (which provides the centripetal force) is zero as the force of 
gravity acting on the satellite is normal to the displacement of the satellite. 
 
 
Question 9: 
In which of the following cases, is work being done? 
(i) A man pushing a wall. 
(ii) a coolie standing with a load of 12 kgf on his head.  
(iii) A boy climbing up a staircase. 
Solution 9: 
Work is done only in case of a boy climbing up a stair case. 
 
 
Question 10: 
A coolie carrying a load on his head and moving on a frictionless horizontal platform does no 
work. Explain the reason. 
Solution 10: 
When a coolie carrying some load on his head moves, no work is done by him against the 
force of gravity because the displacement of load being horizontal, is normal to the direction 
of force of gravity. 
 
 
Question 11: 
The work done by a fielder when he takes a catch in a cricket match, is negative Explain. 
Solution 11: 
Force applied by the fielder on the ball is in opposite direction of displacement of ball. So, work 
done by the fielder on the ball is negative. 
 
 
Question 12: 
Give an example when work done by the force of gravity acting on a body is zero even though 
the body gets displaces from its initial position. 
Class X Chapter 2 – Work, Energy and Power  Physics 
_____________________________________________________________________________ 
 
 
Solution 12: 
When a coolie carries a load while moving on a ground, the displacement is in the horizontal 
direction while the force of gravity acts vertically downward. So the work done by the force of 
gravity is zero. 
 
 
Question 13: 
What are the S.I. and C.G.S units of work?  How are they related? Establish the relationship.  
Solution 13: 
S.I unit of work is Joule. 
C.G.S unit of work is erg. 
Relation between joule and erg : 
1 joule = 1N x 1m 
But 1N = 10
5 
dyne 
And 1m=100 cm = 10
2 
cm 
Hence, 1 joule = 10
5
dyne × 10
2
cm 
= 10
7
dyne × cm =10
7
erg 
Thus, 1 Joule = 10
7
 erg 
 
 
Question 14: 
State and define the S.I. unit of work. 
Solution 14: 
S.I unit of work is Joule. 
1 joule of work is said to be done when a force of 1 newton displaces a body through a distance 
of 1 meter in its own direction. 
 
 
 
Question 15: 
Express joule in terms of erg. 
Solution 15: 
Relation between joule and erg : 
1 joule = 1N × 1m 
But 1N = 10
5 
dyne 
And 1m =100 cm = 10
2 
cm 
Hence, 1 joule = 10
5 
dyne × 10
2
cm 
= 10
7
dyne x cm = 10
7
erg 
Class X Chapter 2 – Work, Energy and Power  Physics 
_____________________________________________________________________________ 
 
 
Thus, 1 Joule = 10
7
 erg 
 
 
 
Question 17: 
A boy of mass m climbs up a staircase of vertical height h. 
(a) What is the work done by the boy against the force of gravity?  
(b) What would have been the work done if he uses a lift in climbing the same vertical height? 
Solution 17: 
Let a boy of mass m climb up through a vertical height h either through staircase of using a lift. 
The force of gravity on the boy is F = mg acting vertically downwards and the displacement in 
the direction opposite to force (i.e., vertical) is S = -h. Therefore the work done by the force of 
gravity on the boy is 
W = FS = - mgh 
or, the work W = mgh is done by the boy against the force of gravity. 
 
 
Question 18: 
Define the term energy and state its S.I. unit.  
Solution 18: 
The energy of a body is its capacity to do work. Its S.I unit is Joule (J). 
 
 
Question 19: 
What physical quantity does the electron volt (eV) measure? How is it related to the S.I. unit of 
that quality? 
 
Question 16: 
A body of mass m falls down through a height h. Obtain an expression for the work done by the 
force of gravity. 
Solution 16: 
Let a body of mass m fall down through a vertical height h either directly or through an inclined 
plane e.g. a hill, slope or staircase. The force of gravity on the body is F = mg acting vertically 
downwards and the displacement in the direction of force (i.e., vertical) is S = h.  
Therefore the work done by the force of gravity is W = FS = mgh