2 Magnetic and electric forces are also long-range, but gravity is unique in being both long-range, and always attractive, thus never cancelling out (unlike electromagnetism, where the forces may either attract or repel). Gravity is a long-range force in contrast to the strong and weak forces (Table 1). Therefore every particle in the entire universe actually attracts every other particle. As Newton showed, the attractive gravity between any two masses gets less and less as the distance between them gets greater, but it never quite reaches zero ( see aside‘Designer Gravity’). Note that gravity is by far the weakest of the four, yet it dominates on the scale of large space objects. Gravity is one of four known fundamental forces of nature (Table 1). Newton’s universal Law of Gravity is one of the great science discoveries of all time. Earth’s gravity actually causes the moon to fall about one millimetre away from a straight-line path, each second, as it orbits the earth (Figure 1). He realized that the same force that causes an apple to fall to the ground also holds the moon in its orbit. Isaac Newton asked this question in 1686, and concluded that gravity was an attractive force between all objects. The moon continually falls toward, or around, the earth, as do the planets around the sun. In this distance it deviates about one millimetre from a straight line due to the earth’s gravitational pull (dotted line). During each second the moon travels about one km (about half a mile). An illustration of the moon’s orbit, not drawn to scale. Gravity – the ‘string’ that holds things in orbit.įigure 1.
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