Mechanical Behaviour Questions and Answers – Stiffness

This set of Mechanical Behaviour Multiple Choice Questions & Answers (MCQs) focuses on “Stiffness”.

1. What is the inverse of stiffness?
a) Hardness
b) Stress
c) Flexibility
d) Toughness

Explanation: The inverse of stiffness is known as flexibility. The higher the stiffness of the material, the lower is its flexibility. Flexibility is measured with unit m/N.

2. A material’s resistance to elastic deflection is known as ___
a) stiffness
b) toughness
c) hardness
d) elasticity

Explanation: Stiffness is the property of a material to resist toward elastic deflection. While toughness of a material is its resistance to fracture. Hardness is defined as material resistance to indentation.

3. What is the SI unit of stiffness?
a) N/m
b) N/m
c) Nm2
d) N

Explanation: Stiffness is the resistance to deflection. It is the ratio of force (unit N) to the extension (m). Hence its unit is N/m.

4. How can stiffness of material be improved?
a) By increasing the cross-sectional area
b) By decreasing the cross-sectional area
c) By increasing temperature
d) By increasing the length of spring

Explanation: Materials with the large cross sectional area are stiffer than the ones with less area. As the thickness of the bar increases, it becomes harder to deflect it. Longer springs tend to have less stiffness.

5. Plasticizers in polymers ___ toughness and ____ stiffness.
a) increase, increase
b) increase, decrease
c) decrease, increase
d) decrease, decrease

Explanation: Plasticizers are liquids having low vapour pressure and low molecular weights. These reduce secondary intermolecular bonding. Thus increase toughness but decrease stiffness.

6. Stiffness and strength of polymer ___ with an increase in the degree of crystallinity.
a) increases
b) decreases
c) remains constant
d) varies showing no relation to the degree of crystallinity

Explanation: Higher degree of crystallinity gives a stiffer polymer. In semi crystalline polymer, strong intermolecular forces are found. These make polymer stiffer but also brittle.

7. Increasing the stiffness of material diminishes its impact strength.
a) True
b) False

Explanation: Stiffer materials are generally dense. This causes in their reduced impact strength.

8. A stiff material is always of high strength.
a) True
b) False

Explanation: A stiffer material is not always strong. For example, steel has a lower stiffness than cast iron. On excess bending, steel deflects more while cast iron breaks.

9. Which statement is true regarding stiffer?
a) A longer spring is stiffer
b) A spring with thin wire is stiffer
c) Copper wire spring is stiffer than steel wire spring
d) A spring with a smaller diameter of coils is stiffer

Explanation: With an increase in the diameter of coils, stiffness of spring decreases. A shorter spring and with thick wire is stiffer. Copper wire spring is less stiff than steel wire.

10. With an increase in temperature, elastic modulus of metals ____ and stiffness ____
a) increases, increases
b) increases, decreases
c) decreases, increases
d) decreases, decreases

Explanation: With an increase in temperature, stretching of atomic bonds becomes easy. So, Young’s modulus decreases. The stiffness of the material is K = AE/L where A is cross sectional area, E is elastic modulus and L is length.

Sanfoundry Global Education & Learning Series – Mechanical Behaviour & Testing of Materials.

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