Mass & Gravity

[TimbaLanD]

We all know that gravity is a force associated with mass, but what in mass that causes gravity? Is it possible to get negative mass?

[Severian]

It is the energy (or to be more precise the stress-energy tensor) inherent in the mass which causes the gravitational attraction.

 

You cannot have negative mass, but that is a slightly deceptive statement. It is not that negative mass is forbidden but there is a symmetry in the equations so that any time you have a negative mass you can fiddle with your conventions (ie. what we call mass) to make it positive. (In most cases this is simply because the mass dependenc in the equations is m² and m²=(-m)²).

[TimbaLanD]

Ok, staying with gravity and mass. I have been thinking… The earth is rotating on its own axis, which means we are actually spinning around with is. Is the earth subject to any centrifugal force? I am thinking if the earth stops spinning on its own axis, will the gravity we experience be more than what it is now? I am assuming this because as the earth spins we are being pushed away and gravity is preventing us from being thrown away into space(?). Am I making sense here??

[swansont]

If the earth were no rotating, gravity would be unchanged, but the net force on you would be. What you feel as weight is actually the normal force of the earth on you, and is the difference between the gravitational force and the centripetal force. If you were moving fast enough so that F_g = F_c , you would be in orbit (perpetual freefall) and feel weightless. (assuming no other forces; air resistance would tend to slow you down)

[TimbaLanD]

Right, that makes more sense… So we would only be subjected to this centripetal force if we are touching the ground, right? So basically we are experiencing inertia? What effects does this force have on the planet? Also if the earth starts to spin faster and faster on its axis, is it capable of overcoming gravity?

[5614]

What effects does this force have on the planet? Also if the earth starts to spin faster and faster on its axis, is it capable of overcoming gravity?

I don't think it has any effect on the planet itself.

 

If it did spin fast enough the forces on us to to the centripetal force would exceed the force of gravity, and so as Swansont mentioned "you would be in orbit (perpetual freefall) and feel weightless. (assuming no other forces; air resistance would tend to slow you down)".

[swansont]

Centripetal force is a classification/condition, not a separate kind of force (as gravitational and electrostatic are). An object moving in uniform circular motion must have a net force given by the centripetal force equation.

 

At some point, were the planet spinning fast enough, it would break apart. This would probably happen before the orbital condition was met.

[TimbaLanD]

dont we have to be in touch with the ground to feel the centripetal force?

[Ragib]

TimaLand, I was just thinking if you didnt understand swansont comment about "stress-energy tensor inherent in the mass which causes the gravitational attraction. " If you wanted i propose a simpler, but to may knowledge still accurate with General Relativity, here is it. The Fabric of Spacetime, you could imagine it like a long streched peice of rectangular elastic. If you were to put a mass on it, it would "sink", and place another object close enough, it will slide down. This is the gravataional attraction. The mass bends spacetime. When something else wants to pass near the mass, it falls into the curved spacetime, and will get attracted to it.

[s pepperchin]

TimaLand, I was just thinking if you didnt understand swansont comment about "stress-energy tensor inherent in the mass which causes the gravitational attraction. " If you wanted i propose a simpler, but to may knowledge still accurate with General Relativity, here is it. The Fabric of Spacetime, you could imagine it like a long streched peice of rectangular elastic. If you were to put a mass on it, it would "sink", and place another object close enough, it will slide down. This is the gravataional attraction. The mass bends spacetime. When something else wants to pass near the mass, it falls into the curved spacetime, and will get attracted to it.

 

What you said is accurate when it comes to an anology to gravity however this example doesn't have a device to introduce cetripital force. Unless of course you have a frictionless elastic sheet so that giving a small mass an initial velocity would keep it moving.

[swansont]

dont we have to be in touch with the ground to feel the centripetal force?

 

No. The centripetal force will be any force that keeps you moving in a circle. In this case, gravity is present, and you don't need to touch the ground to be subject to gravity. But gravity on earth is stronger than what is needed, so we feel the normal force from the ground as well. The resultant of those two forces is the centripetal force.

[Ragib]

Sorry s pepperchin, just an analogy, not a complete mathematical theory...Imagine the elastic is like space, giving any mass velocity and it will continue at that speed due to inertia. This is of course only until a force acts upon the mass.

[s pepperchin]

Sorry s pepperchin, just an analogy, not a complete mathematical theory...Imagine the elastic is like space, giving any mass velocity and it will continue at that speed due to inertia. This is of course only until a force acts upon the mass.

 

I understand the anology, I do have a B.S. in Physics.

[Ragib]

I understoof that you knew, just elaborating with the bit about inertia..