# Work energy

1. Feb 26, 2014

### Miraj Kayastha

When earth pulls a mass with gravitational force why does the energy of the earth-mass system decrease?
Isn't work just transfer of energy meaning constant overall energy?

2. Feb 26, 2014

### dauto

Yes, the energy is transferred from Potential energy (which decreases as stated) to Kinetic energy. The two statements are not contradictory

3. Feb 26, 2014

### AZNathanielC

The amount of energy in a closed system stays constant, it's just transferred into different forms. As dauto said, the energy is transferred to kinetic energy, although the total amount of energy stays constant within the system.

4. Feb 26, 2014

### Miraj Kayastha

Then what does negative potential mean?

5. Feb 26, 2014

### Miraj Kayastha

Why will an object lose energy when external force does negative work on it?

6. Feb 27, 2014

### jbriggs444

It means that positive work would be required to move a unit mass from there to a position which is defined to have zero potential. If, for instance, you've defined the zero point to be at ground level then it means that you are in a hole. You would have to expend energy to climb out.

7. Feb 27, 2014

### Drakkith

Staff Emeritus
Here's how I understand it:

From: http://physics.stackexchange.com/questions/65813/conceptually-what-is-negative-work