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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.

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closed as unclear what you're asking by John Rennie, AccidentalFourierTransform, CuriousOne, ACuriousMind, user36790 Apr 20 '16 at 2:41

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

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    $\begingroup$ your experiment is not clear. How does mass "enter it" . Force and momentum changes are vectors, it needs a direction and a point of impact or a surface of impact. A whole side of the moving one hitting a side of the large at rest one? physicstutorials.org/home/mechanics/dynamics/… $\endgroup$ – anna v Apr 18 '16 at 4:15
  • $\begingroup$ I apologise for not being clear. The smaller rectangle moves in +x direction and the sides of both rectangles are parallel to x and y axes. The imaginary rectangle is to the 'right' of the smaller rectangle. Due to +x velocity component the smaller rectangle enters the bigger rectangle through on of its sides but has not reached the opposite side. $\endgroup$ – Mr. Hypotenuse Apr 18 '16 at 4:23
  • $\begingroup$ I draw this analogy from the idea of fluid entering a control volume but in this case the medium is not continuous. $\endgroup$ – Mr. Hypotenuse Apr 18 '16 at 4:25
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    $\begingroup$ Space doesn't have imaginary boundaries. You can't put a nail into space and declare it a physical object. Mass and momentum are properties of bodies. Mass is independent of observers and momentum depends on them. $\endgroup$ – CuriousOne Apr 18 '16 at 4:48
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    $\begingroup$ By entering of a mass into another, if you mean collapse of two masses into a single one, this problem is to be dealt with conservation of momentum. The conservation of momentum problems like this are well consistent with Newton's law. $\endgroup$ – UKH Apr 18 '16 at 6:23