Help with understanding gravity and why some things float?

Main Question or Discussion Point

My understanding of Newtonian gravity is that every massive object creates a force vector field that exerts a force on every other massive object. I also understand that the Newtonian gravity model creates the same acceleration on another massive object regardless of its mass.

My understanding of relativistic gravity is much more limited although I do believe it is a curvature of the space-time by massive objects. Objects apear to be 'attracted' to the massive object when really they are travelling along the curved 3d surface of 4d spacetime.

So my question is, how do both of these models of gravity how certain things which are lighter than the medium they are in float. For example, a cork floats in water and helium floats in air. Should the earth not still produce a force on these objects and pull them to the earth?

Thanks.

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rock.freak667
Homework Helper
The Earth is still exerting a force on them equal to mg. The reason they float is due to upthrust.

But what is it that is applying the upthrust? If it is the equal-opposite force of the medium acting on it, then don't things that are more massive still float?

rock.freak667
Homework Helper
But what is it that is applying the upthrust?
The cork in the water is displacing the volume of water, that is causing the upthrust.

I don't think we are understanding your question, for sure you're not asking "why things float" classically.

I am asking why things float with respect to both classic and relativistic gravity. I know it may seem like a stupid question, but I really don't understand.

Cleonis
Gold Member
My understanding of relativistic gravity is much more limited although I do believe it is a curvature of the space-time by massive objects. Objects apear to be 'attracted' to the massive object when really they are travelling along the curved 3d surface of 4d spacetime.
If I understand you correctly:
You have examined descriptions of relativistic gravity that discussed only the case of free fall motion. (I presume you know that orbiting motion is a form of free fall motion.)

In your mind you are focused on the following aspect of the descriptions you read: for orbiting motion there is no newtonian force.

Your question is: what about the non-orbiting case? Is there a force in the non-orbiting case?

Let's say an object is positioned on thin ice. (I'm using the visual imagery of thin ice to emphasize that the surface of a planet provides upward force to prevent you from moving down.)
If you are standing on that ice the newtonian force that you experience is exerted by the ice. This force is exerted at the contact surface. If the ice is strong enough it will support you.

In newtonian terms a gravitational force is assumed, acting upon you as a whole, pulling you towards the center of gravitational attraction.
In terms of relativistic physics gravitation is described as a tendency to move towards the center of gravitation. Whether you are orbiting or being supported by a surface, gravitation manifests itself as a tendency to move towards the center of gravitation.

Your physical sensation arises from the surface, pushing upwards, preventing you from moving down.

tiny-tim
Homework Helper
hi mitcho!
… how do both of these models of gravity how certain things which are lighter than the medium they are in float. For example, a cork floats in water and helium floats in air. Should the earth not still produce a force on these objects and pull them to the earth?
yes the gravity is the same …

the difference is in the pressure …

the molecules in the fluid medium are moving, and they bounce off the solid …

the molecules underneath are, on average, moving faster than those on top, so their momentum changes more, ie they exert more force, which provides a net upthrust

Redbelly98
Staff Emeritus
Homework Helper
Or put another way:

If an object is completely submerged in water, the force due to water pressure is greater at the bottom of the object than at the top. This is a simple consequence of water pressure increasing with depth, and provides an upward force on the object. If that upward force is stronger than the object's weight, it will rise to the surface (i.e., float).

Put another way....

The water and the cork both want to fall downward.
The water and the Cork cannot occupy the same volume.
The water is heavier then the cork, therefore the water falls downward and displaces the cork forcing it upward.

Gravity creates pressure difference in fluids.

rcgldr
Homework Helper
Gravity creates pressure difference in fluids.
This is the key. Gravity exerts a downwards force onto a fluid, with one of the results being that pressure increases with depth (due to the weight per unit area of the fluid above a point at any depth), and pressure results in an increasing force per unit area in all directions, including upwards. At any point in the fluid, the gravity per unit area is equal and opposing to the pressure at that depth, and the upward and downward forces at any point within the fluid are equal and opposing.

However in the case of a submerged "buoyant" object with less density than the the fluid that surrounds the object, gravity pulls downwards less on that object than it does the surrounding fluid per unit volume, and the upwards force per unit area due to pressure from the surrounding fluid, which is a function of depth, is greater than the downwards force per unit area of the "buoyant" object, so the object is pushed upwards, with it's upper portion moved above the top surface of the fluid, until the downwards gravity force per unit area of the buoyant object is equal and opposing to the upwards component of force per unit area due to pressure from the fluid related to the depth of the fluid.

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So my question is, how do both of these models of gravity how certain things which are lighter than the medium they are in float. For example, a cork floats in water and helium floats in air. Should the earth not still produce a force on these objects and pull them to the earth?
Take a cork and some water with equal volume to the the cork. Place the water on one side of a weighing scale and the cork on the other side. The force of gravity on the water is greater because it has more mass for a equal volume so the water is pulled down with more force than the cork so the water side moves down and in so doing so the cork side moves up. The cork moves upwards despite the fact gravity is pulling the cork downwards! For a cork freely floating in water, there is no obvious lever or pulley system, but the water and the cork compete for the same space when the cork is underwater, and the water wins displacing the cork upwards. All the other explanations given by earlier posters are correct. This is just another way of looking at it.