# How can the momentum in one direction flow into another?

In the Stress-Energy tensor of General Relativity, the momentum density in one direction can flow into another, which is an object's shear stress. Still, how can the momentum travelling in one direction flow into another?

• because space-time is curved... Imagine a scenario in fewer dimensions Sep 1, 2017 at 14:35
• Just imagine I am running by you while you are standing still on a skateboard. While I am approaching you, I push you or pull you a little bit. What will happen to you? You will move toward me, right? You get some momentum. This amount of momentum is the flow of the momentum. This flow of momentum passes from me to you, in the direction orthogonal to my momentum. Although I am running ahead, the force exerted on you transfers the momentum, and thus the flow. Sep 3, 2017 at 4:12

You push the table. Initially the box stays in place, sliding over the ice since there is no friction to push it along. But when the box meets the wood, suddenly it is dragged along with the table thanks to friction. In effect, what you have done is transfer some $x$-momentum (i.e., horizontal) in the $y$ direction (i.e., vertically).