Why do apples fall down?
Apples fall because they are pulled by gravity toward the center of the earth.
Isaac Newton famously conceived of the force of gravity when struck on the head by a falling apple—or so the legend goes. Newton wasn’t the first person to think about gravity, however; anyone can see that things fall down when you drop them. Newton’s breakthrough was to realize that this was a fundamental and unilateral force of nature, affecting everything from the fall of an apple to the orbit of the moon around the earth. In the summer of 1666, Newton was sitting in his garden when he saw an apple fall from a tree and hit the ground. At this time, he was thinking deeply about what kept the moon in its orbit, and it suddenly occurred to him that there must be a relationship between the force that pulled the apple to Earth and the same force that pulled the moon toward the earth. He was even able to work out just how strong that force must be, and how it got weaker as the distance increased between an object and the larger body that exerted the force.
The ultimate answer that Newton came up with is that gravity is a force of attraction between any and all objects, and that its strength is determined by the size of the objects and the distance between them. There is a force of gravity between an apple and another apple, but because they are so small, it is barely detectable. The earth, in comparison, is large, so for objects directly in its field, the force of gravity between them is quite strong—strong enough to make an apple that comes loose from its tree accelerate toward the ground at a rate of about 32 feet per second per second (i.e., every second the apple falls, its speed increases by about 32 f/s). Bigger planets have stronger gravity; on Jupiter, an apple would accelerate toward the ground at about 83 f/s2, and on the surface of the sun an apple would fall at about 900 f/s2.
But as universal as the theory of gravity may be, there’s still been room for improvement and expansion. About 240 years after Newton, Albert Einstein came along and slightly modified our picture of gravity. He posited that the universe consists of a space-time continuum, and that objects cause bulges in the fabric of space-time, much like balls sitting on a rubber sheet. A big ball, like the earth, causes a very deep bulge, so that a smaller ball, like an apple, will roll down the slope into the bulge. The steepness of the bulge is what determines how quickly other balls will fall into it—in other words, how strong the gravitational force is. So Einstein might have said that the reason an apple falls to the ground is that it is simply following the contours of space-time, which are deformed by the mass of the earth.
