Friday, 31 July 2020

classical mechanics - Again, why is kinetic energy and velocity independent of position coordinates in Cartesian coordinates



This might be a very simple question. I read one previous post Can the kinetic energy be a function of the position vector?


I know that in Cartesian coordinates, the kinetic energy T=12mv2. And T is not an explicit function of position. So Tx=0, where we suppose x is a coordinate.



But I got confused by one example, we have a ball move vertically from the origin O, with a velocity of V0.


enter image description here


then when the ball reach y1 in the positive y axis, we have mgy=12mv2012mv21

so the velocity at y is v1=12mv20mgy
Does it means that velocity and kinetic energy both are explicit function of position y in this case? I know this is a special case, but the statement that Tx=0 in Cartesian coordinates seems to be quite general. So where have I missed so far? Thanks guys!




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