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For example, Newton's Law of Gravity, along with his Laws of Motion, explains why the Moon orbits the Earth. The Moon is a natural satellite of the Earth: as it moves "forward" in its orbit, it also falls "down" towards the center of the Earth (due to gravity). This combined motion is balanced in such as way that the Moon stays the same height (more or less) off the surface of the Earth.
Newton's Law of Gravity explains why the Solar System sticks together, why there are tides, and why objects orbit other objects: it is the force of gravity that produce and maintain these results.
Using his Three Laws of Motion, Kepler's Laws of Planetary Motion, and pure reasoning, Newton discovered the Law of Universal Gravitation. Newton's Law of Gravity states that "there exists a universally attractive force between any two objects called gravity. The magnitude of this force is proportional to the product of the two masses, and inversely proportional to the square of the distance between their centers."
Another example of gravity in action are the tides we experience twice daily. The gravitational force that the Moon exerts on the Earth stretches the Earth is such a way that the water (and to a lesser extent, the rocky surface) of the Earth is pulled outward along the line of sight between the Earth's and Moon's centers, both on the near side and far sides of the Earth. For those near large bodies of water, this means that there are two tides, or risings of water level, daily. See the image to the left.
The Sun, although much farther away, exerts a strong gravitational force on the Earth, so the Sun also contributes to Earth's tides. The Sun's tidal force ends up being about half as strong as the Moon's tidal force.
The Sun's and Moon's tidal forces don't always work together constructively. For the two tidal forces to work together constructively, the Moon must be in either the New Moon or Full Moon phase. In these cases, the tides are quite strong (high), and are called spring tides.
When the Moon is either in the First Quarter or Third Quarter phase, the Sun's and Moon's tidal forces primarily work against each other, and the tides, called neap tides, are relatively weak.
Click the image to the left see the tides change from spring to neap.
One last thing about Newton's Law of Gravity and his Laws of Motion: objects of different weight dropped from the same height off the surface of the Earth (or any Solar System body) fall to the ground in the same amount of time. This can only be true when the acceleration due to gravity is a constant. The acceleration due to gravity for Earth is 32 feet per second per second, or 9.8 meters per second per second. On the Moon, it is one sixth that value.
In that regard, the commander of the Apollo 15 mission, David Scott, conducted an experiment on the Moon, where he dropped a feather and a hammer from the same height. The both hit the Moon ground at the same time, as expected.Click here to see a movie of that historic event. Notice the slower descent of the objects, given the weaker gravity.
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