The Theory of Inertia

For some reason this photo reminded me of when my younger Sister challenged me to learn the meaning of inertia. To this day I have never forgotten. She said, "Inertia means that when an object is in motion it tends to stay in motion until another object acts upon it." Here is the dictionary meaning:

Inertia and Mass

Newton's First Law
Inertia and Mass
State of Motion
Balanced and Unbalanced Forces

Newton's first law of motion states that "An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force." Objects tend to "keep on doing what they're doing." In fact, it is the natural tendency of objects to resist changes in their state of motion. This tendency to resist changes in their state of motion is described as inertia.

Inertia: the resistance an object has to a change in its state of motion.

Newton's conception of inertia stood in direct opposition to more popular conceptions about motion. The dominant thought prior to Newton's day was that it was the natural tendency of objects to come to a rest position. Moving objects, so it was believed, would eventually stop moving; a force was necessary to keep an object moving. But if left to itself, a moving object would eventually come to rest and an object at rest would stay at rest; thus, the idea that dominated people's thinking for nearly 2000 years prior to Newton was that it was the natural tendency of all objects to assume a rest position.
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