Momentum

Where would you be if you ran out of momentum? Most likely you’d be stuck wherever you happen to be when you ran out of it. Though maybe you happened to be facing uphill at that moment when momentum was lost, and rather than simply being stuck you actually start going backwards, back from where you came from - at least that is I suppose if you were somehow on wheels, in a car, whatever. Maybe if there were no wheels you’d be like I said simply stuck, and there’d be little in the way of movement in any direction. That might only be for a while however. Maybe you just need refuelling - it’s not that you lost all belief in where you were going, it’s just that the energy going out wasn’t being replaced by energy coming in. 

If you did lose all belief, that is of course a more serious matter - though it mightn’t even to add be a bad thing. If you were going say in the wrong direction it’s good to lose belief in it, and having lost this belief momentum in its direction thankfully is lost also. It’s a different matter if you were going in the right direction. Then like I said it’s hopefully just a matter of refuelling, and away you can go again.