Gravity

April 28, 1999

Gravity and the electric force have a similar form to their equation because both are forces due to interactions and both radiate from a center in all directions. Because masses are positive, the gravitational force is attractive. The electric force is a very strong force compared to gravity. 

Gravity waves come from: pulsating stars, irregularly rotating stars, collapsing stars, exploding stars, or interacting clustered stars.

The gravitations of the Moon and Sun move the sea and land on the Earth. When the Moon is overhead, or on the opposite side of the Earth, the continents rise up to 6 inches and the oceans rise an average of several feet {high tide}. When the Moon is to the right or the left, the continents and oceans are at their low height {low tide}. When the Moon and Sun are both overhead, or both on the opposite side of the Earth, the tide is extra high {spring tide}, like at a new moon or a full moon. When the Moon is overhead, or on the opposite side of the Earth, and the Sun is to the right or the left, high tides are lower {neap tide}, like at a first quarter moon or a third quarter moon. The motion of tides in shallow waters makes heat from friction, which takes energy from the Earth's rotational energy, and so slows the Earth's rotation down, making each day slightly longer and also increasing the Earth-Moon distance slightly each day. The gravity of the Earth, and the elasticity of the land and sea, are the forces opposed to the tides.

The balance point for all the mass of an object {center of mass or center of gravity} can be found by holding the object at 3 points, each in a different direction from its center, and allowing it to swing freely at each point; then draw a line straight down from each of those points, to where the 3 lines meet. Spinning the object also shows the center of mass, because the point the object spins around is the center of mass. For example, you cannot pick up a chair when you are standing against a wall, because the chair and you have a center of mass beyond your toes and so make you tip over. 

Gravity is a force, so the weight of the molecules on the top of a fluid presses on the molecules below, so the deeper molecules have more pressure on them: P = density*depth*(acceleration of gravity}. The pressure does not depend on area, only depth. A force pushes up on an object in a fluid because the lower surface has more pressure than the top surface due to gravity {buoyancy}; the weight of the fluid pushed up around the object {displacement} equals the upward force on the object {Archimedes principle}, since the fluid tries to return to where it was. This buoyant force is in the opposite direction from gravity. The buoyancy equals the difference between the pressure on the bottom of the object and on the top of the object. The displaced fluid and the object both want to occupy the same place, so they are in equilibrium when both forces pushing down are equal. If the center of mass of the floating object is not along the line of the buoyant force, the object rotates around the center of mass. Submarines, fish, boats, balloons, and ice cubes demonstrate buoyancy. 

The ratio of the density of an object to the density of water {specific gravity} is 1 for water, and is measured by a hygrometer.