Selina Textbook Solutions: Force | Physics Class 10 ICSE PDF Download

 Page 1


Class X Chapter 1 – Force Physics 
______________________________________________________________________________ 
                                 
 
EXERCISE- 1 (A) 
 
Question 1: 
What are contact forces? Give two Examples. 
Solution 1: 
The forces which act on bodies when they are in physical contact, are called contact forces. 
For e.g. frictional force and force exerted on two bodies during collision. 
 
 
Question 2: 
What are non – contact forces? Give two example. 
Solution 2: 
The forces experienced by bodies even without being physically touched, are called the non-
contact forces. For e.g. Gravitational force and Electrostatic force. 
 
 
Question 3: 
Classify the following amongst contact and non-contact forces. 
(a) Frictional force 
(b) normal reaction force, 
(c) force of tension in a string 
(d) gravitation force 
(e) electrostatic force 
(f) magnetic force 
Solution 3: 
Contact force: (a) frictional force (b) normal reaction force (c) force of tension in a string 
Non-contact force: (d) gravitational force (e) electric force (f) magnetic force 
 
 
Question 4: 
Give one example in each case where: 
(a) the force is of contact and 
(b) Force is at a distance.  
Solution 4: 
(a) Force exerted on two bodies during collision. 
(b) Magnetic force between magnetic poles. 
 
 
 
 
Page 2


Class X Chapter 1 – Force Physics 
______________________________________________________________________________ 
                                 
 
EXERCISE- 1 (A) 
 
Question 1: 
What are contact forces? Give two Examples. 
Solution 1: 
The forces which act on bodies when they are in physical contact, are called contact forces. 
For e.g. frictional force and force exerted on two bodies during collision. 
 
 
Question 2: 
What are non – contact forces? Give two example. 
Solution 2: 
The forces experienced by bodies even without being physically touched, are called the non-
contact forces. For e.g. Gravitational force and Electrostatic force. 
 
 
Question 3: 
Classify the following amongst contact and non-contact forces. 
(a) Frictional force 
(b) normal reaction force, 
(c) force of tension in a string 
(d) gravitation force 
(e) electrostatic force 
(f) magnetic force 
Solution 3: 
Contact force: (a) frictional force (b) normal reaction force (c) force of tension in a string 
Non-contact force: (d) gravitational force (e) electric force (f) magnetic force 
 
 
Question 4: 
Give one example in each case where: 
(a) the force is of contact and 
(b) Force is at a distance.  
Solution 4: 
(a) Force exerted on two bodies during collision. 
(b) Magnetic force between magnetic poles. 
 
 
 
 
Class X Chapter 1 – Force Physics 
______________________________________________________________________________ 
 
 
 
Question 5: 
(a) A ball is hanging by a thread from the ceiling of the roof. Draw a neat labelled diagram 
showing the forces acting on the ball and the string. 
(b) A spring is compressed against a rigid wall. Draw a neat and labelled diagram showing 
the forces acting on the spring. 
Solution 5: 
 
 
 
 
 
Question 6: 
State one factor on which the magnitude of a non – contact force depends. How does it depend 
on the factor stated by you?  
Solution 6: 
The magnitude of a non-contact force depends on distance of separation of two bodies. 
Magnitude of force decreases as the distance increases. 
 
 
Page 3


Class X Chapter 1 – Force Physics 
______________________________________________________________________________ 
                                 
 
EXERCISE- 1 (A) 
 
Question 1: 
What are contact forces? Give two Examples. 
Solution 1: 
The forces which act on bodies when they are in physical contact, are called contact forces. 
For e.g. frictional force and force exerted on two bodies during collision. 
 
 
Question 2: 
What are non – contact forces? Give two example. 
Solution 2: 
The forces experienced by bodies even without being physically touched, are called the non-
contact forces. For e.g. Gravitational force and Electrostatic force. 
 
 
Question 3: 
Classify the following amongst contact and non-contact forces. 
(a) Frictional force 
(b) normal reaction force, 
(c) force of tension in a string 
(d) gravitation force 
(e) electrostatic force 
(f) magnetic force 
Solution 3: 
Contact force: (a) frictional force (b) normal reaction force (c) force of tension in a string 
Non-contact force: (d) gravitational force (e) electric force (f) magnetic force 
 
 
Question 4: 
Give one example in each case where: 
(a) the force is of contact and 
(b) Force is at a distance.  
Solution 4: 
(a) Force exerted on two bodies during collision. 
(b) Magnetic force between magnetic poles. 
 
 
 
 
Class X Chapter 1 – Force Physics 
______________________________________________________________________________ 
 
 
 
Question 5: 
(a) A ball is hanging by a thread from the ceiling of the roof. Draw a neat labelled diagram 
showing the forces acting on the ball and the string. 
(b) A spring is compressed against a rigid wall. Draw a neat and labelled diagram showing 
the forces acting on the spring. 
Solution 5: 
 
 
 
 
 
Question 6: 
State one factor on which the magnitude of a non – contact force depends. How does it depend 
on the factor stated by you?  
Solution 6: 
The magnitude of a non-contact force depends on distance of separation of two bodies. 
Magnitude of force decreases as the distance increases. 
 
 
Class X Chapter 1 – Force Physics 
______________________________________________________________________________ 
 
 
 
Question 7: 
The separation between two masses is reduced to half. How is the magnitude of gravitational 
force between them affected?  
Solution 7: 
The magnitude of gravitational force between two masses will become four times as 
gravitational force varies inversely as the square of distance of separation. 
 
Question 8: 
Define the term ‘force’? 
Solution 8: 
A Force is that physical cause which changes(or tends or change) either the size or shape or the 
state of rest or motion of the body. 
 
 
Question 9: 
State the effects of a force applied on (i) a non- rigid, and (ii) a rigid body. How does the effect 
of the force differ in the two cases? 
Solution 9: 
A force when applied on a non-rigid body changes the inter-spacing between its constituent 
particles and therefore causes a change in its dimensions and can also produce motion in it. 
  
On the other hand, a force when applied on a rigid body, does not change the inter-spacing 
between its constituent particles and therefore it does not change the dimensions of the body 
but causes motion in it. 
 
 
Question 10: 
Give one example in each of the following cases: 
(a) A force stops a moving body 
(b) A force moves a stationary body 
(c) A force changes the size of a body 
(d) A force changes the shape of a body 
Solution 10: 
(a) A fielder on the ground stops a moving ball by applying a force with his hands. 
(b) The pull exerted by horse makes a cart moves. 
(c) In a cycle pump, when the piston is lowered, the air is compressed to occupy a less volume. 
(d) On pressing a piece of rubber, its shape changes. 
 
 
Page 4


Class X Chapter 1 – Force Physics 
______________________________________________________________________________ 
                                 
 
EXERCISE- 1 (A) 
 
Question 1: 
What are contact forces? Give two Examples. 
Solution 1: 
The forces which act on bodies when they are in physical contact, are called contact forces. 
For e.g. frictional force and force exerted on two bodies during collision. 
 
 
Question 2: 
What are non – contact forces? Give two example. 
Solution 2: 
The forces experienced by bodies even without being physically touched, are called the non-
contact forces. For e.g. Gravitational force and Electrostatic force. 
 
 
Question 3: 
Classify the following amongst contact and non-contact forces. 
(a) Frictional force 
(b) normal reaction force, 
(c) force of tension in a string 
(d) gravitation force 
(e) electrostatic force 
(f) magnetic force 
Solution 3: 
Contact force: (a) frictional force (b) normal reaction force (c) force of tension in a string 
Non-contact force: (d) gravitational force (e) electric force (f) magnetic force 
 
 
Question 4: 
Give one example in each case where: 
(a) the force is of contact and 
(b) Force is at a distance.  
Solution 4: 
(a) Force exerted on two bodies during collision. 
(b) Magnetic force between magnetic poles. 
 
 
 
 
Class X Chapter 1 – Force Physics 
______________________________________________________________________________ 
 
 
 
Question 5: 
(a) A ball is hanging by a thread from the ceiling of the roof. Draw a neat labelled diagram 
showing the forces acting on the ball and the string. 
(b) A spring is compressed against a rigid wall. Draw a neat and labelled diagram showing 
the forces acting on the spring. 
Solution 5: 
 
 
 
 
 
Question 6: 
State one factor on which the magnitude of a non – contact force depends. How does it depend 
on the factor stated by you?  
Solution 6: 
The magnitude of a non-contact force depends on distance of separation of two bodies. 
Magnitude of force decreases as the distance increases. 
 
 
Class X Chapter 1 – Force Physics 
______________________________________________________________________________ 
 
 
 
Question 7: 
The separation between two masses is reduced to half. How is the magnitude of gravitational 
force between them affected?  
Solution 7: 
The magnitude of gravitational force between two masses will become four times as 
gravitational force varies inversely as the square of distance of separation. 
 
Question 8: 
Define the term ‘force’? 
Solution 8: 
A Force is that physical cause which changes(or tends or change) either the size or shape or the 
state of rest or motion of the body. 
 
 
Question 9: 
State the effects of a force applied on (i) a non- rigid, and (ii) a rigid body. How does the effect 
of the force differ in the two cases? 
Solution 9: 
A force when applied on a non-rigid body changes the inter-spacing between its constituent 
particles and therefore causes a change in its dimensions and can also produce motion in it. 
  
On the other hand, a force when applied on a rigid body, does not change the inter-spacing 
between its constituent particles and therefore it does not change the dimensions of the body 
but causes motion in it. 
 
 
Question 10: 
Give one example in each of the following cases: 
(a) A force stops a moving body 
(b) A force moves a stationary body 
(c) A force changes the size of a body 
(d) A force changes the shape of a body 
Solution 10: 
(a) A fielder on the ground stops a moving ball by applying a force with his hands. 
(b) The pull exerted by horse makes a cart moves. 
(c) In a cycle pump, when the piston is lowered, the air is compressed to occupy a less volume. 
(d) On pressing a piece of rubber, its shape changes. 
 
 
Class X Chapter 1 – Force Physics 
______________________________________________________________________________ 
 
 
 
Question 11: 
State Newton’s first law of motion. Why is it called the law of inertia? 
Solution 11: 
Newton's first law of motion: A body continues to be in its state of rest or of uniform motion in 
a straight line unless an external force is applied on it. It is called the law of inertia because it 
tells that every material body has a property by virtue of which it resists the change in its state 
of rest or in its state of motion. This property is called inertia. 
 
 
Question 12: 
Define the term linear momentum. State its S.I. unit. 
Solution 12: 
The product of mass and velocity of the body is called linear momentum. S.I. unit of linear 
momentum is kg m s
-1
. 
 
 
Question 13: 
(a) Write an expression for the change in momentum of a body of mass m moving with 
velocity v if (i) v « c and (ii) v ? c. 
(b) State the condition when the change in the momentum of a body depends only on the 
change in its velocity. 
Solution 13: 
(i) When v <c, . 
(ii) When v c, the change in momentum is . 
        If velocity v of the moving body is much smaller than the velocity of light c (v<c) 
 
 
Question 14: 
How is force related to the momentum of a body? 
Solution 14: 
The rate of change of momentum of a body is directly proportional to the force applied on it 
and this change in momentum takes place in the direction of the applied force, i.e., 
?? ?
??? ??? 
??? ???  
Where p denotes momentum and 
?p
 is the change in momentum in time 
?t
. 
 
 
Question 15: 
State Newton’s Second law of motion. Under what condition does it take the form F = ma? 
Solution 15: 
Page 5


Class X Chapter 1 – Force Physics 
______________________________________________________________________________ 
                                 
 
EXERCISE- 1 (A) 
 
Question 1: 
What are contact forces? Give two Examples. 
Solution 1: 
The forces which act on bodies when they are in physical contact, are called contact forces. 
For e.g. frictional force and force exerted on two bodies during collision. 
 
 
Question 2: 
What are non – contact forces? Give two example. 
Solution 2: 
The forces experienced by bodies even without being physically touched, are called the non-
contact forces. For e.g. Gravitational force and Electrostatic force. 
 
 
Question 3: 
Classify the following amongst contact and non-contact forces. 
(a) Frictional force 
(b) normal reaction force, 
(c) force of tension in a string 
(d) gravitation force 
(e) electrostatic force 
(f) magnetic force 
Solution 3: 
Contact force: (a) frictional force (b) normal reaction force (c) force of tension in a string 
Non-contact force: (d) gravitational force (e) electric force (f) magnetic force 
 
 
Question 4: 
Give one example in each case where: 
(a) the force is of contact and 
(b) Force is at a distance.  
Solution 4: 
(a) Force exerted on two bodies during collision. 
(b) Magnetic force between magnetic poles. 
 
 
 
 
Class X Chapter 1 – Force Physics 
______________________________________________________________________________ 
 
 
 
Question 5: 
(a) A ball is hanging by a thread from the ceiling of the roof. Draw a neat labelled diagram 
showing the forces acting on the ball and the string. 
(b) A spring is compressed against a rigid wall. Draw a neat and labelled diagram showing 
the forces acting on the spring. 
Solution 5: 
 
 
 
 
 
Question 6: 
State one factor on which the magnitude of a non – contact force depends. How does it depend 
on the factor stated by you?  
Solution 6: 
The magnitude of a non-contact force depends on distance of separation of two bodies. 
Magnitude of force decreases as the distance increases. 
 
 
Class X Chapter 1 – Force Physics 
______________________________________________________________________________ 
 
 
 
Question 7: 
The separation between two masses is reduced to half. How is the magnitude of gravitational 
force between them affected?  
Solution 7: 
The magnitude of gravitational force between two masses will become four times as 
gravitational force varies inversely as the square of distance of separation. 
 
Question 8: 
Define the term ‘force’? 
Solution 8: 
A Force is that physical cause which changes(or tends or change) either the size or shape or the 
state of rest or motion of the body. 
 
 
Question 9: 
State the effects of a force applied on (i) a non- rigid, and (ii) a rigid body. How does the effect 
of the force differ in the two cases? 
Solution 9: 
A force when applied on a non-rigid body changes the inter-spacing between its constituent 
particles and therefore causes a change in its dimensions and can also produce motion in it. 
  
On the other hand, a force when applied on a rigid body, does not change the inter-spacing 
between its constituent particles and therefore it does not change the dimensions of the body 
but causes motion in it. 
 
 
Question 10: 
Give one example in each of the following cases: 
(a) A force stops a moving body 
(b) A force moves a stationary body 
(c) A force changes the size of a body 
(d) A force changes the shape of a body 
Solution 10: 
(a) A fielder on the ground stops a moving ball by applying a force with his hands. 
(b) The pull exerted by horse makes a cart moves. 
(c) In a cycle pump, when the piston is lowered, the air is compressed to occupy a less volume. 
(d) On pressing a piece of rubber, its shape changes. 
 
 
Class X Chapter 1 – Force Physics 
______________________________________________________________________________ 
 
 
 
Question 11: 
State Newton’s first law of motion. Why is it called the law of inertia? 
Solution 11: 
Newton's first law of motion: A body continues to be in its state of rest or of uniform motion in 
a straight line unless an external force is applied on it. It is called the law of inertia because it 
tells that every material body has a property by virtue of which it resists the change in its state 
of rest or in its state of motion. This property is called inertia. 
 
 
Question 12: 
Define the term linear momentum. State its S.I. unit. 
Solution 12: 
The product of mass and velocity of the body is called linear momentum. S.I. unit of linear 
momentum is kg m s
-1
. 
 
 
Question 13: 
(a) Write an expression for the change in momentum of a body of mass m moving with 
velocity v if (i) v « c and (ii) v ? c. 
(b) State the condition when the change in the momentum of a body depends only on the 
change in its velocity. 
Solution 13: 
(i) When v <c, . 
(ii) When v c, the change in momentum is . 
        If velocity v of the moving body is much smaller than the velocity of light c (v<c) 
 
 
Question 14: 
How is force related to the momentum of a body? 
Solution 14: 
The rate of change of momentum of a body is directly proportional to the force applied on it 
and this change in momentum takes place in the direction of the applied force, i.e., 
?? ?
??? ??? 
??? ???  
Where p denotes momentum and 
?p
 is the change in momentum in time 
?t
. 
 
 
Question 15: 
State Newton’s Second law of motion. Under what condition does it take the form F = ma? 
Solution 15: 
Class X Chapter 1 – Force Physics 
______________________________________________________________________________ 
 
 
 
Newton's second law of motion - The rate of change of momentum of a body is directly 
proportional to the force applied on it and this change in momentum takes place in the 
direction of the applied force, i.e., 
F? 
??? ???  
where p denotes momentum and 
?p
 is the change in momentum in time 
?t
. 
When mass m of the body is constant at velocity v (which is much smaller than the velocity of 
light c) then rate of change of momentum is: 
F =
??? ??? = m 
??? ??? = ma 
F= ma 
 
 
Question 16: 
Complete the following sentences: 
(a) Mass X change in velocity = ….. x time interval. 
(b) The mass of a body remains constant till the velocity of body is…… 
Solution 16: 
(a)  Mass X change in velocity = Force x time interval. 
(b) The mass of a body remains constant till the velocity of body is Much less than  the 
velocity of light. 
 
 
Question 17: 
Prove the force = mass x acceleration. State the condition when it holds. 
Solution 17: 
The rate of change of momentum = 
??? ??? = 
? (???? )
???                                 
(When v c or m is not constant). 
But if mass m is constant i.e., v<c 
rate of change of momentum = 
??? ??? = ?? 
??? ???                     
Here the quantity 
??? ??? = rate of change of velocity i.e., acceleration a. 
Rate of change in momentum =  
??? ??? = ?? 
??? ??? = ????                                   
Thus, by Newton's second law of motion, 
 
   ?? ? ???? 
or, F= k ma 
Where k is a constant of proportionality which can be made equal to 1 by choosing the suitable 
unit for force. 
Hence, F = ma when mass m of the body is constant at velocity v which is much smaller than 
the velocity of light.