Imagine a rectangular area defined using imaginary boundaries in 2D space. It is not moving. Now, imagine a much smaller rectangle entering the bigger rectangle from one of its sides. This smaller rectangle has some finite mass and uniform velocity.Therefore, there is some change in the net momentum of the bigger rectangle as mass enters it and by Newton's 2nd law there must be some force.
Is application of 2nd law in such a manner legitimate? If so then where does the force act and if not, why not? Because after all force is defined as rate of change of momentum, is it not?
Any help would be greatly appreciated.