# Energy is the capacity to do work

1. Jun 9, 2005

### chandran

Energy is the capacity to do work. Work is the result when a force moves an
object by a distance. So Energy is the capacity to move an object by a distance by applying a force.

Now kinetic energy is what? and what is potential energy.

Suppose i am sitting on the surface of earth what is the energy i have?
Do i have zero energy? Suppose i remove the surface below where i sit i
will go down. So can i say i have some energy?

2. Jun 9, 2005

### El Hombre Invisible

Energy is isn't just the capacity to do work. It is also the capacity to transfer heat. It can also be mass.

Kinetic energy is the energy you have by lieu of your movement. When you do work to move something, that energy is not lost but is conserved, in this case in the form of kinetic energy. It's the kinetic energy that's moving you - the work done to move something supplies this energy.

Potential energy has differing theories. In EM and GR, potential energy is the energy supplied by an electric or gravitational field, or the curvature of spacetime itself. In QED this energy is instead supplied by particles of energy travelling from one object to another.

Yes, you have a lot of energy. You have mass, therefore energy. You have kinetic energy regardless of whether you are moving, as a whole, through space. It is not necessarily true to say you have potential energy though - that is energy you may potential acquire.

3. Jun 9, 2005

### pmb_phy

See - http://www.geocities.com/physics_world/mech/what_is_energy.htm

The absolute value of energy has little meaning. It is the constancy of energy that is important. You can assign any value you'd like to your energy. But changes in things like potential energy are what matters. I hope the above link helps.

Pete

4. Jun 10, 2005

### tmwong

is that true that there is absolute energy? in my opinion, i think energy is relative as what i used to believe everything is relative except the speed of light.

5. Jun 10, 2005

### krab

It is still the capacity to do work. But you are not necessarily interested in that work. For example, if you are in a moving vehicle, colliding with a tree does lots of (undesired) work on your vehicle. More usually, you use your brakes and the work that is done appears as heat in the brakes.

OTOH, if you are swinging an axe, you really are interested in this work; it is used to split wood, for example.

6. Jun 10, 2005

### pmb_phy

That may sound nice but its flawed. Kinetic energy could also be defined as the capacity to do work. Its also true that potential energy can be defined as the capacity to do work. In fact Classical Dynamics of Particles & Systems - 3rd Ed., Marion and Thornton, Harcourt-Brace-Jovanovich, (1988) gives the following statement
Pete

7. Jun 10, 2005

### krab

Wierd comment. First you say it is flawed, then you repeat what I said.

8. Jun 10, 2005

### pmb_phy

That is not what I said. I was distinguishing energy from kinetic energy. And I was also distinguishing energy from potential energy. Potential energy and kinetic energy are well defined. But "Energy" is not well defined.

Pete