[alancalverd]

The field must have energy.

Not true - indeed meaningless. You need to expend energy to move a particle that interacts with the field.

Not true at all. The gravitational field most certainty does have energy. That's a well-known fact in gravitational physics. See:
http://www.grc.nasa.gov/WWW/k-12/Numbers/Math/Mathematical_Thinking/possible_scalar_terms.htm

Slight problem here. The dimensions of energy are ML²T^-2 but those of gravitational field intensity are LT^-2 and gravitational field potential L²T^-2. No energy is involved until you introduce a mass into the field.

[PmbPhy]

Slight problem here. The dimensions of energy are ML²T^-2 but those of gravitational field intensity are LT^-2 and gravitational field potential L²T^-2. No energy is involved until you introduce a mass into the field.

Hi Alan,

Thank you SO much for catching that and noting it.

[PmbPhy]

By itself, a lone mass produces a static gravitational field that extends to infinity, but it  cannot do work or "produce energy". So there is no "leak" that drains energy or mass out of the massive object.

I want to add to this that this is true because the mass is alone. That's why he put in a "lone" mass.

[evan_au]

The middle is the centre of gravity and therefore within the mass itself.

This is true for a sphere (eg planet or star).

However, there are some exception cases, for example, the letter "C".

If you made a 3D solid "C", the center of gravity is in the middle of the "C", but it is not within the mass itself, because there is a big hole in the middle.

[PmbPhy]

The middle is the centre of gravity and therefore within the mass itself.

This is true for a sphere (eg planet or star).

However, there are some exception cases, for example, the letter "C".

If you made a 3D solid "C", the center of gravity is in the middle of the "C", but it is not within the mass itself, because there is a big hole in the middle.

Dear Evan,

I believe that you have the wrong idea about what the center of mass and center of gravity are. Please see

Center of gravity; http://scienceworld.wolfram.com/physics/CenterofGravity.html

Center of mass: http://scienceworld.wolfram.com/physics/CenterofMass.html

The center of mass of a ring is at the geometric center of the ring where there is no mass. It's also the center of gravity too. Eeek!!

[JohnDuffield]

The author is talking about 2 contributions. One from the mass and one from the field itself.

Interestingly, when you get down to the level of the electron, when you probe it, you don't detect anything in the middle. It's all just field.

[jeffreyH (OP)]

This post made my day.

Excellent!

The middle is the centre of gravity and therefore within the mass itself. If this is a particle then your argument makes no sense.

What is the gravitating body that you're talking about? The center of gravity isn't always the center of mass.

Badly expressed Pete. Point taken.

[jeffreyH (OP)]

The field must have energy.

Not true - indeed meaningless. You need to expend energy to move a particle that interacts with the field.

Not true at all. The gravitational field most certainty does have energy. That's a well-known fact in gravitational physics. See:
http://www.grc.nasa.gov/WWW/k-12/Numbers/Math/Mathematical_Thinking/possible_scalar_terms.htm

Slight problem here. The dimensions of energy are ML²T^-2 but those of gravitational field intensity are LT^-2 and gravitational field potential L²T^-2. No energy is involved until you introduce a mass into the field.

Thanks for this post Alan. A useful page for quick checks is http://www.ebyte.it/library/educards/sidimensions/SiDimensionsByCategory.html