Once we get the rock moving though, momentum begins to take over. Assuming the boulder is round, after a very short time, we no longer have to push at all, we can simply walk beside it and let momentum do it’s work.
Starting something is not easy. It takes a huge amount of effort. So much in fact, that we often quit before we start because even the thought of it seems impossible. If we can push through the start though, momentum will soon take over. And momentum is a powerful thing.