Galileo's work as a scientist and inventor influenced Isaac Newton.

When Newton made that famous statement, he was referring to such giants as Tycho Brahe, Johannes Kepler, and, most specifically, Galileo Galilei.

Newton's first law of motion states that objects tend to keep doing what they are doing. In other words, there is a natural tendency for objects to resist changes in their state of motion. This tendency is called INERTIA.

InertiaThe resistance of an object to a change in its state of motion.

Galileo developed the concept of inertia. Prior to Galileo and Newton's work on motion, it was believed that the natural tendency of objects was to come to rest. Sooner or later, moving objects would stop moving. It followed, then, that some force was necessary to keep an object in motion, as Aristotle and Ptolemy thought.

Flying in the face of 2000 years of popular wisdom, Galileo boldly asserted the exact opposite. He reasoned that moving objects eventually stopped moving due to a force called friction. And he proved it by playing with marbles.

Galileo's Experiment Galileo set up an experiment using a pair of inclined planes facing each other, as shown in this small experiment. You must have a Flash-enabled browser.

After placing a marble at any height on one plane, he observed that, when released, the marble rolled down that plane and up the opposite plane to approximately the same height. If he sanded the planes to be smoother, he noted that the marble rolled up the opposite plane even closer to its original height.

From this empirical data, Galileo concluded that the difference between the marble's initial and final heights could be attributed to a force he called friction. He theorized that without this force, the marble would reach its original height exactly.

It gets better. Galileo also noticed that changing the plane's angle of orientation (i.e., its steepness) did not affect the marble's movement to its original height. If the slope of the opposite plane was reduced, the marble simply rolled farther to reach its original height. See for yourself below:

Another of Galileo's Experiments Another marble experiment by Galileo revealed that changing the angle of orientation did not affect the marble's movement to its original height. What do you think would happen if the opposing plane is not inclined? Change the angle to 0° to find out! You must have a Flash-enabled browser.

Now you have discovered what Galileo did. If the opposing plane is oriented along the horizontal, and eliminating the force of friction, the marble will never reach its original height. It will roll forever. An object in motion stays in motion.

In July 1994, Comet Shoemaker-Levy-9 slammed into the planet Jupiter. The comet was ripped into 21 pieces, all of which continued in the original direction of the comet -- just as Newton's laws predicted they would.

Newton built on Galileo's concept of inertia to include all objects. All objects resist changes in their state of motion — that is, they all have inertia. The question is, how much inertia?

Obviously, some objects resist changes to their states of motion more than others do, as anyone who has ever tried to push an elephant uphill knows.

Newton added to our understanding of inertia the idea that an object's tendency to resist changes in its state of motion depends upon its mass.

In fact, as a quantity, inertia depends on mass alone. The more mass an object has, the greater its tendency to resist changes in its state of motion.

Newton's first law can now be most fully stated as follows:

The tomato's inertia causes it to remain still even when a bullet passes through it.

The shoulders of giants are powerful forces themselves.

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