Does weight determine the gravity of a planet?

Question

Is it weight or density that determines the gravitational pull of a planet? Because Earth is more dense than, say, Neptune, yet Neptune has 20 or more moons, so... whats the deal?

Answer 1

It is mass that determines gravity. But how strongly you feel that gravity is determined by your distance from the center of the planet, so in that way density is indirectly involved.

Consider this: Earth has a mass of 5.972e24 kg and a radius of 6,378 km while Neptune has a mass of 1.0247e26 kg and radius of 24,766 km. The larger mass means that Neptune's gravity is much stronger than Earth's, if you measure both at the same distance from the centers of each planet. However, at a distance of 6,378 km from the center of the Neptune, you are deep inside the planet whereas at that distance from the center of the Earth, you are on the surface. So you can never get close enough to Neptune's center of mass to feel its stronger gravity the way you can with Earth, and it is due to Neptune's lower density. In fact, with a REALLY high density you get a REALLY small radius and can get so close to the center of mass that escape velocity gets higher than the speed of light, which is a black hole. Earth would be a black hole if its radius was only 100 meters. But even in that case, its gravity at 6,378 km from the center would still be what it is now on the surface, because the gravity at any fixed distance is due to the mass only.

Mathematically, it is expressed this way: The strength of gravity is GM/R^2, where M is mass, R is distance and G is the universal gravitational constant.

Answer 2

mass determines gravity. nothing has weight until gravity pulls on it. so if the earth is more dense than neptune, but neptune has more mass anyway, neptune will have a stronger gravitational pull.

however, a planet's gravitational pull does not determine the number of moons it has. this is just the number of bodies that got caught in its orbit. Neptune is on the edge of the solar system, so it is more likely to run into stuff and pull it in. stuff that gets to earth has to make it past a lot of other planets and the asteroid belt to get caught in our gravitational pull, plus not get sucked in toward the sun. it is much harder for earth to 'catch' a moon.

Answer 3

One thing to remember is that Earth and Neptune are fundamentally different in their structure, in that Neptune, Jupiter, Saturn, and Uranus are what are known as Gas Giants, and they are made up almost entirely of Gas, with possibly a solid core at their centre. Mercury, Venus, Earth, Mars and Pluto are planets(sorry, Pluto is not considered a planet anymore) that have solidified into rock (think of the Gas Giants as stars that failed to ignite). Although the solid planets are more dense, they are much smaller than the Gas Giants, and have much smaller masses than the larger Gas Giants, which dictates the gravity of the planet, not it's relative density.

Answer 4

It is the overall mass that determines the gravitational force a planet exerts. The density is the amount of mass per unit volume. Weight is an accelerated force between two masses which is how it is calculated.

Answer 5

Hi. Mass determines the gravity, density and closeness to the center determine orbital speed and escape velocity. Earth could have many more moons if it could capture them. Just count the satellites.

Answer 6

Every planetary body, including the Earth, is surrounded by its own gravitational field, which exerts an attractive force on any object. This field is proportional to the body's mass and varies inversely with the square of distance from the body.

Answer 7

Gravity is just a theory, and Newton and Einstein theories of gravity sort of conflict with each other and the laws of physics, such as conservation of energy. They really havn't any concrete proof other than "something" is holding things together,s ome suggest invisible energy or "dark matter" is the glue that holds structure in the universe.

Answer 8

Mass.If you squeezed earth down to one mile in diameter its mass would stay the same its surface gravity would increase but its effect on its surroundings would stay the same.