[Physics] Why does it take a projectile as long to get to its apex as it does to hit the ground

newtonian-mechanicsprojectilesymmetrytime-reversal-symmetry

I was once asked the following question by a student I was tutoring; and I was stumped by it:

When one throws a stone why does it take the same amount of time for a stone to rise to its peak and then down to the ground?

  • One could say that this is an experimental observation; after one could envisage, hypothetically, where this is not the case.

  • One could say that the curve that the stone describes is a parabola; and the two halves are symmetric around the perpendicular line through its apex. But surely the description of the motion of a projectile as a parabola was the outcome of observation; and even if it moves along a parabola, it may (putting observation aside) move along it with its descent speed different from its ascent; or varying; and this, in part, leads to the observation or is justified by the Newtons description of time – it flows equably everywhere.

  • It's because of the nature of the force. It's independent of the motion of the stone.

I prefer the last explanation – but is it true? And is this the best explanation?

Best Answer

I would say that it is a result of time reversal symmetry. If you consider the projectile at the apex of its trajectory then all that changes under time reversal is the direction of the horizontal component of motion. This means that the trajectory of the particle to get to that point and its trajectory after that point should be identical apart from a mirror inversion.

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