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Question
why gravitational potential is negative?it is a term which shows the value of work to take an object in a gravitational field.here the displacement is towards the gravitational force.so,work should be positive.but,it is negative.please explain why....

Answer
Hello Shah,

If the work done to an object increases its mechanical energy (kinetic and/or potential energy), the work is considered positive. Positive energy is added to the object's energy.

But if you take a book down from a high shelf and put it on a lower shelf, that decreased the book's gravitational potential energy. In supporting the book's weight as you moved it, you exerted upward force equal to its weight while moving it down. The work formula says you did negative work:
Work = force*displacement*cos180 = -weight*displacement

I hope I understand your question well enough that this is on target.
Steve

Physics

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Steve Johnson

Expertise

I would be delighted to help with questions up through the first year of college Physics. Particularly Electricity, Electronics and Newtonian Mechanics (motion, acceleration etc.). I decline questions on relativity and Atomic Physics. I also could discuss the Space Shuttle and space flight in general.

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I have a BS in Physics and an MS in Electrical Engineering. I am retired now. My professional career was in Electrical Engineering with considerable time spent working with accelerometers, gyroscopes and flight dynamics (Physics related topics) while working on the Space Shuttle. I gave formal classroom lessons to technical co-workers periodically over a several year period.

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BS Physics, North Dakota State University
MS Electrical Engineering, North Dakota State University

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