Why a man clearing a bar in a high jump experiences weightlessness?
F=ma even at the point of inflection.
The jumper is in free fall. All parts of his body are accelerated at an identical rate so that he does not experience any net force. Yes, (ignoring air resistance), he is weightless in the same sense that an astronaut in orbit about the earth is weightless.
There are multiple meanings of the word "weight" which are subtly different. The one that I prefer to is that weight is the apparent downward inertial force that must be countered in order to support a body, motionless, in a particular frame of reference. The jumper's "weight" then depends on which frame of reference we adopt.
We can adopt a freely falling frame of reference in which the object is at rest, weightless.
We can adopt a frame of reference tied to the surface of the earth in which the jumper weighs the normal amount.
We can adopt a frame of reference tied to the center of the earth in which the jumper weighs a little bit more than what a spring scale would read due to the fact that the earth is rotating.
The man experiences "weightlessness" not only as he clears the bar, but throughout the entire jump after he loses contact with the ground. All bodies in free fall are regarded as experiencing weightlessness. Ironically, within the framework of classical physics, bodies in free fall experiencing "weightlessness" have a single force acting on them, and it is their weight; go figure. Within the framework of general relativity, the picture is much more clear cut, because, in free fall, the force acting on the body is zero as it moves through curved spacetime.
Thanks everyone! :D
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