An object of mass 3 kg initially at rest is subjected to a constant force of 9 N for 2 seconds. What will be its final velocity?

The correct option is 6 m/s.

Step 1: Identify the given quantities from the problem:
Mass of the object, $m=3\text{ kg}$
Initial velocity, $u=0\text{ m/s}$ (since the object is initially at rest)
Applied force, $F=9\text{ N}$
Time duration, $t=2\text{ s}$

Step 2: Calculate the acceleration using Newton's Second Law of Motion:
Newton's second law states that force is the product of mass and acceleration ($F=m\times a$).
Rearranging the formula to solve for acceleration ($a$):

$a=\frac{F}{m}$

Substitute the given values into the equation:

$a=\frac{9\text{ N}}{3\text{ kg}}=3{\text{ m/s}}^2$

Step 3: Calculate the final velocity using the first equation of motion:
The first equation of motion relates final velocity ($v$), initial velocity ($u$), acceleration ($a$), and time ($t$):

$v=u+at$

Substitute the known values:

$v=0+(3{\text{ m/s}}^2\times 2\text{ s})$

$v=6\text{ m/s}$

Therefore, the final velocity of the object after 2 seconds is 6 m/s.