What is the $SI$ unit of force?

In which direction do the following forces act when an object is in motion?
$(i)$ Frictional force
$(ii)$ Gravitational force
$(iii)$ Centripetal force

The qualitative definition of force is given by

$A$ book of mass $2 \, kg$ is sliding with a constant velocity of $4 \, m s^{-1}$ on a frictionless horizontal table. What is the force required to keep the object moving with the same velocity (in $, N$)?

What type of force is acting in the cases given above?

State whether the following statement is true or false:
The first law of motion is also known as Galileo's law of inertia.