**Properties of Surfaces and Solids Interview Questions and Answers:**

**1. Differentiate between Lamina and Body.**

**Ans.**

The lamina has only area but no mass. The body has volume and mass.

**2. Define Centroid.**

**Ans.** The centre of area of a plane figure is known as Centroid.

**3. How does Centroid differ from Centre of Gravity?**

**Ans.** The centre of area of a plane figure is known as Centroid. The point through which the whole weight of a two dimensional body is assumed to act is known as Centre of Gravity.

**4. Define First moment of an element.**

**Ans.** The first moment of an element about the origin is defined as the product of its position vector with the element itself. The element may belong to any physical quantity such as continuous length, area, volume or mass.

**5. Define Composite Solid.**

**Ans.** A composite solid is defined as the combination of two or more basic solids like cylinder, cone, sphere etc.

**6. State the Pappus – Guldinus theorems.**

**Ans.**

**Theorem 1:** The area of the surface generated by revolving a plane curve about a nonintersecting axis in its plane is equal to the product of the length of the curve and the distance travelled by the centroid ‘G’ of the curve during generation.

**Theorem 2:** The volume of a solid generated by revolving a plane of area about a non-intersecting axis in its plane is equal to the product of the area and the length of path the centroid during generation.

**7. Differentiate between inertia and moment of inertia.**

**Ans.** The property of matter by virtue of which it resists any change it its state of rest or of uniform motion is called Inertia.

The rotational inertia is termed as moment of inertia.

**8. Define Moment of Inertia.**

**Ans.** Moment of Inertia about an axis is the algebric sum of the products of the elements of mass and square of the distance of the respective element of mass from the axis.

**9. State the ‘Parallel axis theorem’ of Moment of Inertia.**

**Ans.** It states that, “If the moment of inertia of a plane area about an axis parallel to the centroidal axis is equal to the addition of moment of inertia about the centroidal axis with the product of the area and square of the perpendicular distance between the two parallel axis”.

**10. State the ‘Perpendicular Axis Theorem’.**

**Ans.** The moment of inertia of an area about any axis perpendicular to the area and passing through the centroid is equal to sum of the moments of inertias about two mutually perpendicular axis passing through the centroid.

**11. Define ‘Radius of Gyration’.**

**Ans.** It is the distance from an axis of reference where the entire area is assumed to be concentrated such that the moment of inertia is not changed about the given axis.

**12. Define ‘Section Modulus’. **

**Ans.** It is defined as the ratio between the moment of inertia and distance of the extreme fibre.

**13. Define ‘Product of Inertia’.**

**Ans.** It is defined as the product of the area and the distance of the area with respect to the x and y axes.

**14. Define Principal axis and Principal moment of inertia.**

**Ans.** The axis about which the product of inertia is zero is known as Principal axis. The moment of inertia about the principal axis is called principal moment of inertia.

**15. What are ‘Major and Minor Principal axes’?**

**Ans.** In a given area, there are two principal axes at a point. These two axes will be mutually perpendicular to each other. The moment of inertia about one of the axis will be maximum and the other will be minimum. These axes are called major principal axis and minor principal axes respectively.

**16. Define ‘Mass moment of Inertia’.**

**Ans.** The mass moment of inertia of an element of mass is defined as the product of the mass of the element and the square of the distance of the element from the axis.

**17. Define ‘Density’.**

**Ans.**The density of a body is the ratio between mass of the body to the volume of the body.

**18. Define Radius of gyration of a body.**

**Ans.**The radius of gyration of a body of mass ‘m’ is defined as the square root of the moment of inertia of mass divided by the mass.

**19. Define ‘Polar Moment of Inertia’.**

**Ans.**The second moment of area about a pole ‘O’ is called the polar moment of inertia.