How to explain that initial speed is zero?
I was trying to discuss about free-fall with a friend, who is not scientifically literate. The first hurdle came quickly and unexpectedly as I was trying to demonstrate that the speed of free-fall is not constant : I hold a stone in my hand above the ground and let it fall. Since the stone is at rest initially, and visibly moving with a non-zero speed later on, that must mean that the speed varies along its trajectory, and therefore that it is not constant, right? Well, not really... My friend, a sceptic, did not find it obvious that the instantaneous speed is zero initially. I went on and demonstrated another way why the speed wasn't constant, but I found myself unable to explain (or rather demonstrate) the obvious about the initial speed... :( I must be missing something simple.
Hence my question :
How can we demonstrate with simple words that the free-fall stone must be at rest initially, when it is let go? No maths please, only empirical proofs based on everyday observations. Thanks.
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I think I have something : I hold the stone at a given height, h, ready to open my hand and let it fall. In parallel, I ask someone to take another, identical (same mass, same shape...), stone at the ground level and throw it in the air upward along a straight vertical line. Let's call this other stone the ref(erence) stone. 483
But I ask the person one more thing : he should throw the stone with an initial force so that the ref stone trajectory apex, the highest points it reaches, is exactly h. We know that such an initial force exists and I think that no one can argue, even if he knows nothing about dynamics, that the ref stone has zero velocity at his apex. As the ref stone reaches its apex, and the two stones are at the same level (by construction), I decide to open my hand, letting my stone fall freely. We can then notice that both stones reach the ground exactly at the same time. In fact we would also notice that at any given time the two falling stones remain at the same level. Another way to put it is that they fall at the same rate, which must mean that they have exactly the same speed along their downward trajectory, at any given time. In particular, their speed must be identical at the beginning of their fall, as I open my hand. But we know (by construction) that the ref stone speed is precisely zero at its apex. That must mean the same for my stone at the beginning of its fall.
It's long but I think it works, and it does not require any strong prior knowledge of maths or of dynamics laws and concepts. It might not be the best way though, and I was hoping to find an example a bit less convoluted. 771