# Class 11 Physics Chapter 9 Important Questions Mechanical Properties of Solid

1 Q:- Name the property of a material which is opposite to elasticity. Ans:- The property of plasticity. 2 Q:- What is a deforming force? Ans:-

It is important for the students that all the concepts should be very clear for better marks in future. Here, we are providing important conceptual questions and answers for class 11 physics chapter 9 Mechanical Properties of Solid. In this lesson, students will learn about Mechanical Properties of Solid. This will not only help the students to know the important questions but will also help them during revision.

1 Q:- Name the property of a material which is opposite to elasticity.
Ans:- The property of plasticity.

2 Q:- What is a deforming force?
Ans:- Deforming force is a force which tries to deform a body,i.e., which cause a change in shape or size (configuration) of a body.

3 Q:- Which force is responsible for elastic behaviour of a material, the deforming force or the internal interatomic restoring force?
Ans:- The internal interatomic restoring force.

4 Q:- Are inter atomic forces developed in a deformed body attractive or repulsive?
Ans:- Interatomic forces are attractive when a body is stressed and repulsive when the body is compressed.

5 Q:- What are the factors on which modulus of elasticity of a material depends?
Ans:- The modulus of elasticity depends on the nature of a material and the manner of deformation of the body.

6 Q:- Both stress and pressure are defined as the force per unit area. Then, how do they differ?
Ans:- Stress and pressure are different as:

• (a) pressure is always normal to a surface but stress may be either normal or tangential.
• (b) pressure is always compressive but a stress may be tensile or compressive.

7 Q:- Name the object which are the nearest approach to:

(a) perfectly elastic, and

(b) perfectly plastic bodies.
Ans:-

• (a) Quartz,ivory and Phosphor – bronze are the nearest approach to perfectly elastic materials.
• (b) Putti mud plasticin and dough are the nearest approach to perfectly plastic materials.

8 Q:- Which of the three moduli of elasticity is possessed by all the three states of matter?
Ans:- The bulk modulus B (i.e.,volume elasticity) is possessed by the solids, liquids as well as gases.

9 Q:- The length of a wire is cut to have. What will be the effect on:

• (a) increase in its length, and
• (b) longitudinal strain under a given tensile force?

Ans:-

• (a) The increase in length of the wire will be reduced to one half of its previous value because ∆L is proportional to L.
• (b) longitudinal strain ∆L/L remains unchanged.

10 Q:- Is poisson's ratio elastic moduli or the elastic constant?
Ans:- Poisson's ratio is an elastic constant whose value depends on the nature of material but it is not a moduli of elasticity.

11 Q:- How can a material have more than one moduli of elasticity?
Ans:- For a given material there can be a different modulus of elasticity depending on the type of strain and stress.

12 Q:- Why is work to be done to stretch a wire? where does this work go?
Ans:- While stretching a wire, work is to be done against internal restoring forces developed in a body. The work done is stored in the body itself as its elastic potential energy (or strain energy).

13 Q:- What is an elastomer? Give an example.
Ans:- An elastomer is an elastic substance which can be subjected to large values of strain (even much more than 100%). Rubber is a very good example of elastomers.

14 Q:- What is the effect of temperature on elastic behaviour of materials? In why?
Ans:- A material becomes less elastic (i.e., values of moduli of elasticity become less) as the temperature is increased. It is because value of intra atomic distance increases slightly with increase in temperature and consequently, internal restoring forces become weak.

15 Q:- Which is more elastic, steel or rubber, and why?
Ans:- Steel is much more elastic than rubber. Moduli of elasticity is given by the ratio of stress to the strain. For longitudinal strain, young modulus,

Y=\frac{FL}{A.∆L}

A material is said to be more elastic if for same stress, the value of strain is less. A rubber cord can be stretched easily but for same stress, elongation of a steel wire is extremely small. Hence, steel is more elastic than rubber.

16 Q:- A wire is cut to half its original length.what effect would it have on the maximum load that can be supported by the wire now?
Ans:- there will be no effect and even now, the wire can support the same value of maximum load. It is because maximum load = yield strength × cross sectional area of the wire and does not depend on the length of the wire.

17 Q:- If we double the diameter of a steel rope, what will be the effect on the breaking stress, breaking force for the rope?
Ans:- If diameter (or radius) of a steel rope is doubled, its cross sectional area increases to 4 times of its original value and hence breaking force (or breaking load) will also become four times of its previous value However,breaking stress is the force per unit area and it depends only on the material of the rope. Hence, it remains unchanged.

18 Q:- What would be the effect on Young's modulus of the material of a wire if its length as well as radius are doubled? Why?
Ans:- The Young's modulus for the material of wire will remain unchanged because the value of Young's modulus is a characteristics of the material and is independent of the dimensions of the wire.

19 Q:- For small deformation of a metal wire, the ratio stress/strain remains constant. what happens to this ratio when the deformation is made large enough?
Ans:- For large deformation beyond the elastic limit of material of the wire, the ratio of stress/strain decreases because now the metal wire is not behaving as a perfectly elastic body. Rather it is showing plastic deformation.

20 Q:- Are internal restoring forces developed in a body due to elasticity property,conservative or nonconservative?
Ans:- Internal restoring forces developed due to elasticity are conservative forces so long as a stress-strain curves for loading and unloading are identical (as for a metal within elastic limit). however, the forces are non conservative in nature for a material which stress-strain curves for a loading and unloading are different as for an elastomer).

21 Q:- Read the following statement given below carefully and state with reason, if it is true or false : A piece of rubber under an ordinary stress can display even 1000% strain and yet it returns to its original length when unloaded. It means that there is much more elastic than Steel.
Ans:- No, the statement is not correct. although rubber piece can undergo a large strain and yet I regain its original dimension but as here, slope of stress-strain curve is comparatively much smaller than that for steel, rubber is less elastic .again, yield strength of rubber is much less than that of Steel.

Q 22:- A heavy wire is suspended freely with one end of wire fixed. Is it under some (a) stress, and (b) strain?
Ans:- Yes, the wire is under stress due to its own weight which is effectively acting at its centre of gravity. Accordingly, some longitudinal tensile strain too will also be present in the wire.

Q 23:- Springs used in automobiles as shockers are prepared for steel not copper. Why?
Ans:- Because moduli of elasticity of Steel are more than that of copper. As a result, for a given force, less strain will be produced in a steel spring and action of Steel spring will be better.

Q 24:- Which effectively governs the strength of elastic material -the yield strength or the ultimate strength? Give reason.
Ans:- The yield strength governs the strength effectively of an elastic material because beyond his limit, the material will start exhibiting plastic behaviour. A machine should never be deformed beyond its yield point, otherwise it may be permanently deformed.

Q 25:- Steel ropes used in suspension bridges, cranes and lifts, etc., are made of a number of thin wires braided together. Why?
Ans:- In suspension bridges, cranes and lifts depending upon load and margin of safety thick Steel ropes (radius 2-3 cm) are used. A single steel wire of such dimension will be a rigid rod. So, the rupees are always made up of a number of thin wire braided together for ease in manufacture, flexibility and strength.

Q 26:- An old spring balance does not give correct reading of weight. Why?
Ans:- An old spring balance, being used for a long time, has suffered a large number of alternatively opposite types of strains. As a result, the spring of spring balance suffers with loss of elastic property (known as elastic fatigue) and does not give correct readings of weights.