Gravitational Potential Energy



Gravity is all around us and acts on everything that has mass. Now gravity is a FORCE and to act against that force you must do WORK.

..cycling up hill, walking up stairs, getting out of bed (unless you're on the top bunk!), lifting a bag off the floor...life is one long struggle against gravity!

So if you are doing work, you must be using energy. Consider moving, say a book, from sitting on the floor to sitting on a shelf. In lifting the book you are expending energy to overcome the gravitational attraction (see the Fact File on gravity) of the book and the earth. Since energy cannot be created or destroyed (only turned from one form to another), the energy you have used in lifting the book must have gone somewhere.....

In raising the book you gave given it gravitational potential energy. It's gravitational, because the work you had to do was against gravity. It's energy because you had to do work. It's potential because although the book is now at rest it still must have the energy you gave it; this energy is available to do work and so the book on the shelf has the potential to do work (for example if it falls off and smashes something on the floor).

So if the book has a mass m and is raised a height h against a gravitational acceleration g then

Gravitational potential energy = ( force ) x distance

= ( mg ) x h

=mgh




Astronomers used to think that stars were powered by the gravitational collapse of gas under its own weight. As gas falls towards the centre of a star it is moving from larger heights to lower heights in a gravitational field (this is exactly the same as a book falling from a shelf to the floor).

The gas at large heights has a large amount of gravitational potential energy which is available to do work. As the gas falls inwards it speeds up - it's converting gravitational potential energy to kinetic energy. The gas becomes turbulent and the kinetic energy is converted into heat energy.

The temperatures reached by this process are hot enough to ignite fusion. But it is this process of fusion, rather than gravitational collapse, that powers stars.


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