when we have an orbit with the total energy of less than zero,is it gradually gonna collide the centre?

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Question by 28meter: when we have an orbit with the total energy of less than zero,is it gradually gonna collide the centre?
It's written:
If E < 0, motion is bound, and must remain
within a finite region of the (orbital) plane.
but It seems a bit odd to me...
first...
How does it have a negative energy...!!!
how is it possible...?
and in second,
its not reasonable that it would have a non zero energy and stay in the same orbit...hmm???


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