# Assertion and Reason Questions for Class 11 Physics Chapter 9 Mechanical Properties of Solids

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Assertion and Reason Questions for Class 11 Physics Chapter 9 Mechanical Properties of Solids

Directions:
(a) If both assertion and reason are true and the reason is the correct explanation of the assertion.
(b) If both assertion and reason are true but reason is not the correct explanation of the assertion.
(c) If assertion is true but reason is false.
(d) If the assertion and reason both are false.

Q.1. Assertion : Steel is more elastic than rubber.
Reason : Under given deforming force, steel is deformed less than rubber.

Answer Answer: (a) Elasticity is a measure of tendency of the body to regain its original configuration. As steel is deformed less than rubber therefore steel is more elastic than rubber.

Q.2. Assertion : Glassy solids have sharp melting point.
Reason : The bonds between the atoms of glassy solids get broken at the same temperature.

Answer Answer: (d) In a glassy solid (i.e., amorphous solid) the various bonds between the atoms or ions or molecules of a solid are not equally strong. Different bonds are broken at different temperatures. Hence there is no sharp melting point for a glassy solid.

Q.3. Assertion : Bulk modulus of elasticity (K) represents incompressibility of the material.
Reason : Bulk modulus of elasticity is proportional to change in pressure.

Q.4. Assertion : A hollow shaft is found to be stronger than a solid shaft made of same material.
Reason : The torque required to produce a given twist in hollow cylinder is greater than that required to twist a solid cylinder of same size and material.

Q.5. Assertion : The stretching of a coil is determined by its shear modulus.
Reason : Shear modulus change only shape of a body keeping its dimensions unchanged.

Answer Answer: (a) Because, the stretching of coil simply changes its shape without any change in the length of the wire used in coil. Due to which shear modulus of elasticity is involved.

Q.6. Assertion : Young’s modulus for a perfectly plastic body is zero.
Reason : For a perfectly plastic body, restoring force is zero.

Q.7. Assertion : Strain is a unitless quantity.
Reason : Strain is equivalent to force.

Answer Answer: (c) Strain is the ratio of change in dimensions of the body to the original dimensions. Because this is a ratio, therefore it is dimensionless quantity.

Q.8. Assertion : Identical springs of steel and copper are equally stretched. More work will be done on the steel spring.
Reason : Steel is more elastic than copper.

Q.9. Assertion : The bridges declared unsafe after a long use.
Reason : Elastic strength of bridges losses with time.

Answer Answer: (a) A bridge during its use undergoes alternating strains for a large number of times each day, depending upon the movement of vehicles on it when a bridge is used for long time, it losses its elastic strength. Due to which the amount of strain in the bridge for a given stress will become large and ultimately, the bridge may collapse. This may not happen, if the bridges are declared unsafe after long use.

Q.10. Assertion : Two identical solid balls, one of ivory and the other of wet-clay are dropped from the same height on the floor. Both the balls will rise to same height after bouncing.
Reason : Ivory and wet-clay have same elasticity.

Answer Answer: (d) Ivory is more elastic than wet-clay. Hence the ball of ivory will rise to a greater height. In fact the ball of wet-clay will not rise at all, it will be somewhat flattened permanently.

Q.11. Assertion : Stress is the internal force per unit area of a body.
Reason : Rubber is less elastic than steel.

Answer Answer: (b) Stress is defined as internal force (restoring force) per unit area of a body. Also, rubber is less elastic than steel, because restoring force is less for rubber than steel.

Q.12. Assertion : Spring balances show correct readings even after they had been used for a long time interval.
Reason : On using for long time, spring balances do not losses its elastic strength.

Answer Answer: (d) Because, the stretching of coil simply changes its shape without any change in the length of the wire used in coil. Due to which shear modulus of elasticity is involved.
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