Motion and Its Causes
Force and Newton’s Laws of Motion
How does gravity act on us? Is it a contact force?
Newton recognized that the gravitational force of Earth acts not only on objects close to Earth, like the famous apple, but also on objects as far away as the moon. But, how exactly does a celestial object such as the sun reach out the approximately 93 million miles and hold Earth in its orbit? An early idea was that it was an “action at a distance” force, for example, that the Sun attracted all objects, like Earth, without anything between the two.
In the middle of the nineteenth century, physicist Michael Faraday (1791–1867) proposed that one magnet creates a field of force around it, and another magnet interacts with the field at its location. Based on the field idea, Earth then creates a gravitational field. The apple and the moon have a force exerted on them, not directly by Earth, but by the gravitational field that exists at the location of the apple and the moon.
What would happen to Earth if the sun suddenly disappeared? How soon would Earth recognize that the sun’s gravitational field was gone? It couldn’t happen instantaneously, because Einstein’s Special Theory of Relativity says that no information can travel faster than the speed of light. So, it would take about eight minutes before Earth would both experience the lack of sunlight and the lack of gravitational force.