Goyal Brothers Textbook Solutions: Force

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Force 
 
Exercise – 1 
Question 1. 
(a) Define the following : 
1. Rigid body 
2. point of action of force 
3. line of action of force 
4. principle of transmissibility of force. 
(b) Name four bodies which can be called rigid bodies for practical purposes. 
Answer: 
(a) (i) Rigid Body : “A body which does not get deformed under the action of a force or a 
number of forces is called rigid body, 
(ii) Point Of Action Of Force : “The point on the rigid body where a force acts is called 
the point of action of force.” 
 
(iii) Line Of Action Of Force : “An imaginary line passing through the point of action of 
force and drawn in the same direction in which the force acts is called line of action of 
force. LM is the line of action of force. 
(iv) Principle Of Transmissibility Of Force : “It states that the point of action of force 
can be transmitted anywhere along the line of action of the force without causing any 
material difference in the movement of the body.” 
(b) Wood, iron rod, stone and glass are rigid bodies. 
Question 2. 
Under what condition (s) a body describes a motion of : 
(a) translation 
(b) rotation. 
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Force 
 
Exercise – 1 
Question 1. 
(a) Define the following : 
1. Rigid body 
2. point of action of force 
3. line of action of force 
4. principle of transmissibility of force. 
(b) Name four bodies which can be called rigid bodies for practical purposes. 
Answer: 
(a) (i) Rigid Body : “A body which does not get deformed under the action of a force or a 
number of forces is called rigid body, 
(ii) Point Of Action Of Force : “The point on the rigid body where a force acts is called 
the point of action of force.” 
 
(iii) Line Of Action Of Force : “An imaginary line passing through the point of action of 
force and drawn in the same direction in which the force acts is called line of action of 
force. LM is the line of action of force. 
(iv) Principle Of Transmissibility Of Force : “It states that the point of action of force 
can be transmitted anywhere along the line of action of the force without causing any 
material difference in the movement of the body.” 
(b) Wood, iron rod, stone and glass are rigid bodies. 
Question 2. 
Under what condition (s) a body describes a motion of : 
(a) translation 
(b) rotation. 
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Answer: 
Conditions for 
(a) Translational motion : Motion of rigid body in straight line and in the direction of 
applied force. 
(b) Rotational motion : Rigid body should rotate about axis (or pivot). 
Question 3. 
(a) What do you understand by the term moment of force 
(b) State two factors which determine the moment of force. 
Answer: 
(a) Moment of force : “The turning effect of force acting on a body about an axis is 
called the moment of force.” 
(b) Factors affecting the moment of force : 
1. The magnitude of the force applied 
2. The distance of line of action of the force from the axis of rotation. 
Question 4. 
State one way of 
(a) reducing moment of force 
(b) Increasing moment of force for a given force acting on a body capable of turning 
around a fixed point. 
Answer: 
(a) Moment of force can be reduced by reducing the distance between force and the 
turning point. 
(b) Moment of force can be increased by increasing the distance between force and the 
turning point. 
Question 5. 
State the law of moments. 
Answer: 
Law of moments : In equilibrium sum of anticlockwise moments is equal to sum of 
clock-wise moments. 
Sum of anticlock wise moments = sum of clockwise moments. 
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Force 
 
Exercise – 1 
Question 1. 
(a) Define the following : 
1. Rigid body 
2. point of action of force 
3. line of action of force 
4. principle of transmissibility of force. 
(b) Name four bodies which can be called rigid bodies for practical purposes. 
Answer: 
(a) (i) Rigid Body : “A body which does not get deformed under the action of a force or a 
number of forces is called rigid body, 
(ii) Point Of Action Of Force : “The point on the rigid body where a force acts is called 
the point of action of force.” 
 
(iii) Line Of Action Of Force : “An imaginary line passing through the point of action of 
force and drawn in the same direction in which the force acts is called line of action of 
force. LM is the line of action of force. 
(iv) Principle Of Transmissibility Of Force : “It states that the point of action of force 
can be transmitted anywhere along the line of action of the force without causing any 
material difference in the movement of the body.” 
(b) Wood, iron rod, stone and glass are rigid bodies. 
Question 2. 
Under what condition (s) a body describes a motion of : 
(a) translation 
(b) rotation. 
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Answer: 
Conditions for 
(a) Translational motion : Motion of rigid body in straight line and in the direction of 
applied force. 
(b) Rotational motion : Rigid body should rotate about axis (or pivot). 
Question 3. 
(a) What do you understand by the term moment of force 
(b) State two factors which determine the moment of force. 
Answer: 
(a) Moment of force : “The turning effect of force acting on a body about an axis is 
called the moment of force.” 
(b) Factors affecting the moment of force : 
1. The magnitude of the force applied 
2. The distance of line of action of the force from the axis of rotation. 
Question 4. 
State one way of 
(a) reducing moment of force 
(b) Increasing moment of force for a given force acting on a body capable of turning 
around a fixed point. 
Answer: 
(a) Moment of force can be reduced by reducing the distance between force and the 
turning point. 
(b) Moment of force can be increased by increasing the distance between force and the 
turning point. 
Question 5. 
State the law of moments. 
Answer: 
Law of moments : In equilibrium sum of anticlockwise moments is equal to sum of 
clock-wise moments. 
Sum of anticlock wise moments = sum of clockwise moments. 
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Question 6. 
(a) What do you understand by the terms (i) positive moments (ii) negative moments 
(b) State the absolute units of moment of force in (i) CGS system (ii) S.I. system. 
Answer: 
(a) 
1. Positive moments : If the force produces anticlock wise motion in the rigid body 
about turning point, it is positive moment. 
 
2. Negative moment : If the force produces clock wise motion in the body about 
turning point, it is negative moment. 
 
(b) Absolute units of moment of force in 
1. CGS system is dyne-cm. 
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Force 
 
Exercise – 1 
Question 1. 
(a) Define the following : 
1. Rigid body 
2. point of action of force 
3. line of action of force 
4. principle of transmissibility of force. 
(b) Name four bodies which can be called rigid bodies for practical purposes. 
Answer: 
(a) (i) Rigid Body : “A body which does not get deformed under the action of a force or a 
number of forces is called rigid body, 
(ii) Point Of Action Of Force : “The point on the rigid body where a force acts is called 
the point of action of force.” 
 
(iii) Line Of Action Of Force : “An imaginary line passing through the point of action of 
force and drawn in the same direction in which the force acts is called line of action of 
force. LM is the line of action of force. 
(iv) Principle Of Transmissibility Of Force : “It states that the point of action of force 
can be transmitted anywhere along the line of action of the force without causing any 
material difference in the movement of the body.” 
(b) Wood, iron rod, stone and glass are rigid bodies. 
Question 2. 
Under what condition (s) a body describes a motion of : 
(a) translation 
(b) rotation. 
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Answer: 
Conditions for 
(a) Translational motion : Motion of rigid body in straight line and in the direction of 
applied force. 
(b) Rotational motion : Rigid body should rotate about axis (or pivot). 
Question 3. 
(a) What do you understand by the term moment of force 
(b) State two factors which determine the moment of force. 
Answer: 
(a) Moment of force : “The turning effect of force acting on a body about an axis is 
called the moment of force.” 
(b) Factors affecting the moment of force : 
1. The magnitude of the force applied 
2. The distance of line of action of the force from the axis of rotation. 
Question 4. 
State one way of 
(a) reducing moment of force 
(b) Increasing moment of force for a given force acting on a body capable of turning 
around a fixed point. 
Answer: 
(a) Moment of force can be reduced by reducing the distance between force and the 
turning point. 
(b) Moment of force can be increased by increasing the distance between force and the 
turning point. 
Question 5. 
State the law of moments. 
Answer: 
Law of moments : In equilibrium sum of anticlockwise moments is equal to sum of 
clock-wise moments. 
Sum of anticlock wise moments = sum of clockwise moments. 
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Question 6. 
(a) What do you understand by the terms (i) positive moments (ii) negative moments 
(b) State the absolute units of moment of force in (i) CGS system (ii) S.I. system. 
Answer: 
(a) 
1. Positive moments : If the force produces anticlock wise motion in the rigid body 
about turning point, it is positive moment. 
 
2. Negative moment : If the force produces clock wise motion in the body about 
turning point, it is negative moment. 
 
(b) Absolute units of moment of force in 
1. CGS system is dyne-cm. 
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2. S.I system is newton-metre (N-m) 
Question 7. 
(a) What do you understand by the following terms ? (i) couple, (ii) arm of couple, (iii) 
moment of couple 
(b) State the units of the moment of couple in (i) CGS system, (ii) S.I. system. 
(c) Give four examples of couple in everyday life. 
(d) State the mathematical expression for the moment of a couple. 
Answer: 
(a) 
1. Couple : “Two forces acting on a rigid body are equal in magnitude opposite 
parallel and not along the same straight line constitute a couple.” 
2. Arm of the couple : “Perpendicular distance between two forces is called arm of 
the couple.” i.e. AB is arm of couple. 
3. Moment of the couple : “Is the product of any one of the force and the arm of 
couple. i.e. F × AB is moment of couple. 
 
(b) 
1. In CGS system unit of moment of couple-dyne-cm 
2. In S.I. system unit of moment of couple-N-m. 
(c) Examples of couple in everyday life : 
1. Steering wheel of a car. 
2. Turning of a screw driver. 
3. Opening and closing of water tap. 
4. Winding the watch. 
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Force 
 
Exercise – 1 
Question 1. 
(a) Define the following : 
1. Rigid body 
2. point of action of force 
3. line of action of force 
4. principle of transmissibility of force. 
(b) Name four bodies which can be called rigid bodies for practical purposes. 
Answer: 
(a) (i) Rigid Body : “A body which does not get deformed under the action of a force or a 
number of forces is called rigid body, 
(ii) Point Of Action Of Force : “The point on the rigid body where a force acts is called 
the point of action of force.” 
 
(iii) Line Of Action Of Force : “An imaginary line passing through the point of action of 
force and drawn in the same direction in which the force acts is called line of action of 
force. LM is the line of action of force. 
(iv) Principle Of Transmissibility Of Force : “It states that the point of action of force 
can be transmitted anywhere along the line of action of the force without causing any 
material difference in the movement of the body.” 
(b) Wood, iron rod, stone and glass are rigid bodies. 
Question 2. 
Under what condition (s) a body describes a motion of : 
(a) translation 
(b) rotation. 
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Answer: 
Conditions for 
(a) Translational motion : Motion of rigid body in straight line and in the direction of 
applied force. 
(b) Rotational motion : Rigid body should rotate about axis (or pivot). 
Question 3. 
(a) What do you understand by the term moment of force 
(b) State two factors which determine the moment of force. 
Answer: 
(a) Moment of force : “The turning effect of force acting on a body about an axis is 
called the moment of force.” 
(b) Factors affecting the moment of force : 
1. The magnitude of the force applied 
2. The distance of line of action of the force from the axis of rotation. 
Question 4. 
State one way of 
(a) reducing moment of force 
(b) Increasing moment of force for a given force acting on a body capable of turning 
around a fixed point. 
Answer: 
(a) Moment of force can be reduced by reducing the distance between force and the 
turning point. 
(b) Moment of force can be increased by increasing the distance between force and the 
turning point. 
Question 5. 
State the law of moments. 
Answer: 
Law of moments : In equilibrium sum of anticlockwise moments is equal to sum of 
clock-wise moments. 
Sum of anticlock wise moments = sum of clockwise moments. 
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Question 6. 
(a) What do you understand by the terms (i) positive moments (ii) negative moments 
(b) State the absolute units of moment of force in (i) CGS system (ii) S.I. system. 
Answer: 
(a) 
1. Positive moments : If the force produces anticlock wise motion in the rigid body 
about turning point, it is positive moment. 
 
2. Negative moment : If the force produces clock wise motion in the body about 
turning point, it is negative moment. 
 
(b) Absolute units of moment of force in 
1. CGS system is dyne-cm. 
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2. S.I system is newton-metre (N-m) 
Question 7. 
(a) What do you understand by the following terms ? (i) couple, (ii) arm of couple, (iii) 
moment of couple 
(b) State the units of the moment of couple in (i) CGS system, (ii) S.I. system. 
(c) Give four examples of couple in everyday life. 
(d) State the mathematical expression for the moment of a couple. 
Answer: 
(a) 
1. Couple : “Two forces acting on a rigid body are equal in magnitude opposite 
parallel and not along the same straight line constitute a couple.” 
2. Arm of the couple : “Perpendicular distance between two forces is called arm of 
the couple.” i.e. AB is arm of couple. 
3. Moment of the couple : “Is the product of any one of the force and the arm of 
couple. i.e. F × AB is moment of couple. 
 
(b) 
1. In CGS system unit of moment of couple-dyne-cm 
2. In S.I. system unit of moment of couple-N-m. 
(c) Examples of couple in everyday life : 
1. Steering wheel of a car. 
2. Turning of a screw driver. 
3. Opening and closing of water tap. 
4. Winding the watch. 
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(d) Mathematical expression : Moment of couple = force × Arm of couple. 
Question 8. 
Explain the following : 
(a) Jack screw is provided with a long arm. 
(b) It is easier to open a door by holding it from its edge. 
(c) A small boy can balance a stout man on a see-saw. 
(d) The handles of a hand flour grinder (chakki) is provided near its edge. 
(e) It is easier to turn a steering wheel of larger diameter than a steering wheel of 
smaller diameter. 
(f) A wrench or a spanner has a long handle. 
Answer: 
(a) So that there is maximum distance between force and turning point and minimum 
force is needed. 
(b) Holding a door from edge increases the distance between force and turning point 
(hinge) and hence minimum force is needed. 
(c) By making the smaller boy sit at longer distance from axis of rotation. Hence turning 
effect of force increases. 
(d) To increase the distance between force and turning point and hence to apply 
minimum force. 
(e) In this way we increase the ? distance between force and point of rotation and 
minimum force is needed to rotate the steering. 
(f) Moment of force increases by increasing the length of handle and minimum force is 
required. 
Question 9. 
The diagram along side shows a heavy roller, with its axle at O which is to be pulled on 
to the pavement XY by applying the minimum possible force. Draw the diagram and 
mark on it the point and direction in which the force should be applied? 
 
Answer: 
Force should be applied at A near the rim to increase the moment of force by increasing 
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