Newton's First law of Motion and Galileo's Concept of Inertia

"I have see farther other men, it is because I have stood on the shoulders of giants." - Sir Isaac Newton.

When Isaac Newton was making this famous statement, he was referring to such giants as Tycho Brahe, Johannes Kepler, and most specifically, Galileo Galilei. Galileo Galilei is traditionally credited with being the first scientist to formalize the concept of Inertia.

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 then called Inertia. Inertia is defined as the resistance of an object to a change in its state of motion. Sometimes, Newton's first law of motion is referred as the "law of inertia."

The following diagram will help us understand the concept of Inertia:

law of inertia

From this diagram, Newton's first law of motion state that an object at rest tends to stay at rest, with V=0m/s, and an object in motion tends to stay in motion with same speed and in the same direction unless acted upon by an unbalanced force.

From Newton's statement, we can define two different ideas in it: - one, which predicts the behavior of stationary objects, and, the other which predicts the behavior of moving objects.

To understand the origin of Newton's law of motion, we have to go explore the concept of Inertia developed by Sir Galileo Galilei. Before Sir Galileo developed the concept of Inertia, people believed that a push was necessary to keep something moving. For instance, if we push a chair across a room, it seems that the push is necessary to sustain the velocity of the chair. If we stop pushing, the chair stops moving. But Sir Galileo believed that when the push on the chair is taken away, the chair should continue to move along without any assistance. The friction force is then the force that will prevent the chair from continuing its motion.

One way to illustrate this theory, which states that objects stop moving due a force called force of friction, is by taking an example from Galileo's experiment with marbles. Sir Galileo did set an experiment using a pair of inclined planes facing each other:

Galileo's experiment

After placing a marble at any height on one plane, Galileo 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.

This experiment led Galileo to conclude that the difference between the marble's initial and final heights could be attributed to a force called friction. He then summarized that without that force, force of friction, the marble would reach its original height exactly.

Galileo's experiment

The other side of this experiment is also that if the opposing plane is oriented the horizontal, and eliminating the force of friction, the marble will never reach its original height. It will roll forever. This is defined as an object in motion will always stay in motion.

Later on, Isaac Newton built on Galileo's concept of Inertia to include all objects. All objects resist changes in thir state of motion. This means that they all have inertia. This concept led Isaac Newton to add to our understanding of inertia the idea that an object's tendency to resist changes in its state depends upon its mass.

It became a general rule that Inertia depends then on the mass of the object alone. The more mass an object has, the greater its tendency to resist changes in its state of motion.

In general, we can conclude that Isaac Newton's first law of motion relies on Galileo's thoughts about motion. It declares that a force is not needed to keep an object in motion; the more mass an object has, the more inertia it has-- the more tendency it has to resist changes in its state of motion. We can also conclude that Sir Isaac Newton's first law of motion is the same as Galileo's explanation of inertia.

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