Potential Energy and Total Energy Negative

1. Feb 24, 2009

Jordash

1. The problem statement, all variables and given/known data

Explain why it is possible for potential energy and total energy to be either positive or negative, but kinetic energy is always positive

2. Relevant equations

3. The attempt at a solution

I'm thinking that Potential Energy and Total Energy can be opposite and take away from each other, so they would be negative to subtract, but not sure how to think this one through.

2. Feb 24, 2009

Delphi51

Potential energy is measured with respect to a reference point.
For example, in PE = mgh, the PE depends on the h and where it is measured from. If you take ground level to be h = 0, then you have negative PE as soon as you go down below the surface.

The potential energy of an electron in an atom and of a satellite in orbit is measured with a reference point of infinity, so these energies are always negative. Believe it or not, infinite distance is the simplest reference level, because it is the only level that is common to all atoms or planets.

3. Feb 24, 2009

Jordash

Ok, that makes sense, what does it mean when it says Total Energy? I think it means

E=KE+PE
E=1/2mv^2+mgh

So Total Energy can be negative because Potential Energy can be negative, is that right?

4. Feb 24, 2009

Sonolum

Also, look at your equation for kinetic energy: (0.5)*(m)*(v^2) - can any of these variables be negative?

5. Feb 24, 2009

Jordash

well velocity could be I guess but it's square so it will always be positive.

6. Feb 25, 2009

Sonolum

Exactly!! So then, can Kinetic energy EVER be negative?

Now look at the equation for potential - what can be negative there, and can it make the whole value negative?

7. Feb 25, 2009

Jordash

no never, thanks for the help!

8. Feb 25, 2009

Sonolum

No problem! Physics Rules!!