Question

Why the velocity of object at rest is considered to be uniform object

Answer 1

As, the object is at rest ,it cannot have any Velocity . So, it's Velocity is 0.

Answer 2

While you are applying the force, the object will move accelerated. At the moment when you stop applying the force, the object will keep moving uniform with its current velocity.

By Newtons 2nd Law: F=dpdt⇒dp=FdtF=dpdt⇒dp=Fdt, where dp is the change of momentum. Momentum, by definion, equals mass times velocity (p=mvp=mv). So as long as you have force, you will have the changing of momentum, and if the mass is constant, the change of velocity.