Relative Gravity: How Size Affects Force of Gravity

[DB]

What is the force that pushes your body down on a bigger planet making you weigh more? I know it's gravity, but how does more size produce more gravity exactly? And when people say you would weigh more or less on a different planet, they mean more or less relative to the Earth right?

 

[selfAdjoint]

The current effective theory of gravity is general relativity, so the explanation is in terms of that. GR says that the curvature of spacetime geometry near a point is determined by the energy and momentum near that point. Zero curvature would be euclidean geometry and nonzero curvature is some kinds of noneuclidean geometry. "Straight lines" in a noneuclidean geometry are curves. So your heavy body is trying to move on a straight line but because the geometry is different it moves on a curved path in spacetime. A curved path in spacetime embodies an acceleration. So that's why.

And yes, the "heavier" or "lighter" on different planets is relative to Earth.

 

[Phobos]

And when people say you would weigh more or less on a different planet, they mean more or less relative to the Earth right?

Right. Your mass is essentially constant, so as you move yourself (your mass) to different gravitational fields (e.g., different planets), the weight you measure (which is mass x gravity) differs accordingly.

 

[DB]

Thanks guys.