Our present ideas about the motion of bodies date back toGalileo and Newton. Before them people believed Aristotle, whosaid that the natural state of a body was to be at rest andthat it moved only if driven by a force or impulse. It followedthat a heavy body should fall faster than a light one, because itwould have a greater pull toward the earth.
The Aristotelian tradition also held that one could work outall the laws that govern the universe by pure thought: it wasnot necessary to check by observation. So no one until Galileobothered to see whether bodies of different weight did in factfall at different speeds. It is said that Galileo demonstrated thatAristotle’s belief was false by dropping weights from the leaningtower of Pisa. The story is almost certainly untrue, but Galileodid do something equivalent: he rolled balls of different weightsdown a smooth slope. The situation is similar to that of heavybodies falling vertically², but it is easier to observe because theSpeeds are smaller. Galileo’s measurements indicated that eachbody increased its speed at the same rate, no matter what itsweight. For example, if you let go of a ball on a slope thatdrops by one meter for every ten meters you go along, theball will be traveling down the slope at a speed of about onemeter per second after one second, two meters per secondafter two seconds, and so on, however heavy the ball. Ofcourse a lead weight would fall faster than a feather, but thatis only because a feather is slowed down by air resistance. Ifone drops two bodies that don’t have much air resistance, suchas two different lead weights, they fall at the same rate. On themoon, where there is no air to slow things down, the astronautDavid R. Scott performed the feather and lead weightexperiment and found that indeed they did hit the ground atthe same time.
Galileo’s measurements were used by Newton as the basis ofhis laws of motion. In Galileo’s experiments, as a body rolleddown the slope it was always acted on by the same force (itsweight), and the effect was to make it constantly speed up.
This showed that the real effect of a force is always to changethe speed of a body, rather than just to set it moving, as waspreviously thought. It also meant that when-ever a body is notacted on by any force, it will keep on moving in a straight lineat the same speed. This idea was first stated explicitly⁴ inNewton’s Principia Mathematica, published in 1687, and isknown as Newton’s first law. What happens to a body when aforce does act on it is given by Newton’s second law. Thisstates that the body will accelerate, or change its speed, at arate that is proportional to the force. (For example, theacceleration is twice as great if the force is twice as great.) Theacceleration is also smaller the greater the mass (or quantity ofmatter) of the body. (The same force

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