- #1

- 202

- 4

**Time rate of Momentum Change in CV = Time rate of Momentum Change into CV - Time rate of Momentum Change Out of CV + Sum of External Forces**

Why is Sum of External forces in there? I know that Time rate of Momentum Change is dp/dt and represents force, so the equation is pretty much saying

**force inside = force in - force out**

But why are these external forces not already counted for in the Momentum Changes into and out of the CV? What is the intuition behind including the External Forces? Also, why is it a

**+ Sum of External Forces**and not

**-Sum of External Forces**?