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why is orbital energy always positive but near the surface of earth you have negative energy?

Physics gravitation question


  1. Actually, in the conventional language, a body in orbit has negative energy.
    But regardless, the value of the classical energy is reference-frame dependent. It depends entirely on where you choose the origin of the coordinate system to be.

  2. When we use formulas assuming that gravity is uniform, which are applicable near Earth’s surface, we define our datum of zero GPE in every problem we do. Often the datum is the floor or ground, at which GPE is defined to be zero. Focus on the phrase “we define our datum of zero GPE”.
    In the case of GPE as per Newton’s law of gravitation, we define GPE to equal zero when objects are infinitely separated. We choose the infinite point as the datum, because it makes the calculus of deriving the formula easier. Seldom do we revise the datum of zero GPE such that it corresponds to a point within the actual orbit, because it makes the mathematics of deriving the formula more difficult. So orbital energy, which is GPE + KE for a body in orbit is always less than zero if the body is to remain in orbit. If the GPE + KE of a body in orbital motion is greater than zero, than it implies that the body has reached escape velocity, and will escape the orbit.

  3. Near the surface of earth the gravitational pull is more and so it easily pulls the satellite from its orbit to the ground.So its orbital energy becomes -ve


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