Write 7 units of fundamental quantity in S.I units, convert 72 km/sec into m/sec, define unit vector and component vector, and differentiate between speed and velocity.

$$
72 \times 1000 = 72000 \frac{\text{m}}{\text{sec}}
$$
The converted value is $\boxed{72000 \text{ m/sec}}$.

3. Define unit vector. A unit vector is a vector with a magnitude of one, used to indicate direction only. It is often denoted by a lowercase letter with a "hat" (e.g., $\hat{\mathbf{i}}$).

4. Define a component vector. A component vector is one of two or more vectors whose vector sum is the original vector. It represents the projection of a vector along a specific axis or direction.

5. Differentiate between speed and velocity.

   • Speed is a scalar quantity that measures how fast an object is moving, defined as the distance traveled per unit time.
   • Velocity is a vector quantity that measures both the speed and the direction of an object's motion, defined as the displacement per unit time.

6. Define and write the formula for centripetal force. Centripetal force is the force that acts on an object moving in a circular path and is directed towards the center of the circle, causing the object to continuously change direction. The formula for centripetal force ($F_c$) is:

   $$F_c = \frac{mv^2}{r}$$

   where $m$ is the mass of the object, $v$ is its speed, and $r$ is the radius of the circular path.

7. Define torque with unit. Torque is a twisting force that tends to cause rotation about an axis. It is the rotational equivalent of linear force. The SI unit of torque is the Newton-meter ($\text{N}\cdot\text{m}$).

8. Define centre of gravity. The centre of gravity is the imaginary point in an object where the entire weight of the object appears to act. It is the average location of the weight of an object.

9. Define Elasticity. Elasticity is the property of a material that allows it to return to its original shape and size after being deformed by an external force, once that force is removed.

10. Define stress and its unit. Stress is the internal restoring force per unit cross-sectional area within a material that arises from external forces. It is a measure of the internal forces acting within a deformable body. The SI unit of stress is the Pascal ($\text{Pa}$), which is equivalent to Newtons per square meter ($\text{N/m}^2$).

11. Define mechanical wave and the name of its two types. A mechanical wave is a wave that requires a medium (like air, water, or solids) to propagate. It transmits energy through the oscillation of particles in the medium. The two main types are: Transverse waves and Longitudinal waves.

12. Define intensity of sound with its formula. The intensity of sound is defined as the average rate at which sound energy is transmitted through a unit area perpendicular to the direction of sound propagation. The formula for sound intensity ($I$) is:

    $$I = \frac{P}{A}$$

    where $P$ is the sound power and $A$ is the area through which the sound energy passes. The SI unit is watts per square meter ($\text{W/m}^2$).

13. Write mirror equation. The mirror equation relates the object distance ($d_o$), image distance ($d_i$), and focal length ($f$) of a spherical mirror:

    $$\frac{1}{f} = \frac{1}{d_o} + \frac{1}{d_i}$$

14. Define interference of light. Interference of light is a phenomenon where two or more light waves superpose to form a resultant wave of greater, lower, or the same amplitude. This typically results in a pattern of bright and dark fringes.

15. Define critical angle. The critical angle is the angle of incidence in the denser medium for which the angle of refraction in the rarer medium is $90^\circ$. If the angle of incidence exceeds the critical angle, total internal reflection occurs.

16. Define total internal reflection. Total internal reflection is an optical phenomenon that occurs when a light ray traveling from a denser medium to a rarer medium strikes the interface at an angle of incidence greater than the critical angle, causing all the light to be reflected back into the denser medium.

17. Write main points of Newton corpuscular theory of light. The main points of Newton's corpuscular theory of light are:

    • Light consists of tiny, massless particles called corpuscles.
    • These corpuscles travel in straight lines at very high speeds.
    • Different colors of light are due to corpuscles of different sizes.
    • Reflection occurs when corpuscles bounce off a surface.
    • Refraction occurs when corpuscles are attracted to the denser medium, causing them to speed up and change direction.

18. De-abbreviate the word "LASER" LASER stands for $\boxed{\text{Light Amplification by Stimulated Emission of Radiation}}$.