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Old May 29th 2008, 06:47 PM   #1
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Conservation of energy

As we all know that the Earth always has a gravitational field and we are all inside the field. Suppose an object falls normal onto the Earth surface, work is done on the object,i.e. the object gains energy. At the same time, would the Earth loses energy?

If yes, I have a problem. It seems that the earth can have infinite work done on objects as the gravitational field is spontaneously occuring which is only an attractive force acting on objects. Am I right?

When an object is lifted upwards, my chemical energy is changed to potential energy of object, does this process involves energy transfer of the Earth?
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Old May 29th 2008, 07:17 PM   #2
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Originally Posted by werehk View Post
As we all know that the Earth always has a gravitational field and we are all inside the field. Suppose an object falls normal onto the Earth surface, work is done on the object,i.e. the object gains energy. At the same time, would the Earth loses energy?

If yes, I have a problem. It seems that the earth can have infinite work done on objects as the gravitational field is spontaneously occuring which is only an attractive force acting on objects. Am I right?

When an object is lifted upwards, my chemical energy is changed to potential energy of object, does this process involves energy transfer of the Earth?
Think of it this way. There is a force on an object acting in the same direction as the displacement. Therefore work is done.

The falling object has a force on it: it's weight. It is falling, thus work is being done on it.

The Earth, in turn, has a force on it: the third law pair to the weight of the object. So the Earth too is moving and it's moving upward to intercept the falling object. Work is being done on the Earth as well.

The source of all this energy? It's not the gravitational field, it's whatever lifted the object above the Earth's surface. The gain in gravitational potential energy is the energy that does the work on the system.

As to the chemical energy part. One definition of work is that we are transforming one form of energy into another. So when you push hard against that wall that doesn't move, yes, you are doing work! Even though there is no displacement of the wall, chemical energy in your muscles is being transformed into kinetic energy: your muscles are trembling with the effort of futilely trying to move the wall. Your core temperature goes up, you start sweating, etc. This change in chemical energy to kinetic energy is another source for mechanical work to come from. (Which is a good thing, else we'd never be able to move around!)

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