Mass and Inertia

What is Inertia?

Inertia is nature's tendency to fight (resist) any change in movement.

Newton's Law of Interia says the following:

1. A body in motion fights to stay in motion.

2. A body at rest fights to stay at rest (because of inertia) unless acted on by a net external force.

3. Nature hates change.

Inertia is the reason why objects that are not moving or are moving at a constant speed along a straight line have a zero acceleration

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Points to remember:

1. Mass is the quantitative measure of interia (not weight)!

2. Inertia is a concept, not an equation.

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On Inertia

What is Inertia?

Inertia comes from the Latin word, iners, meaning idle, sluggish.It is the resistance of any physical object to any change in its state of motion, including changes to its speed and direction. It is the tendency of objects to keep moving in a straight line at constant velocity.

Newton's Definition of Inertia

Isaac Newton defined inertia as his first law in his Philosophiæ Naturalis Principia Mathematica, which states:

The vis insita, or innate force of matter, is a power of resisting by which every body, as much as in it lies, endeavours to preserve its present state, whether it be of rest or of moving uniformly forward in a straight line.

What is Mass Actually ?

The mass of an object is a fundamental property of the object; a numerical measure of its inertia; a fundamental measure of the amount of matter in the object. Definitions of mass often seem circular because it is such a fundamental quantity that it is impossible to define in terms of something else.

While the mass is normally considered to be an unchanging property of an object, at speeds approaching the speed of light one must consider the increase in the relativistic mass.

The mass of an object is informally defined as the amount of "stuff" the object has, which is often called matter. The mass of an object determines the body's resistance to being accelerated by a force. (This is mathematically expressed using Newton's second law, $F=ma.$ As $m$, or mass, increases, the acceleration on the body is decreased given a constant force $F$.)

Mass is a different measure than weight. Weight is simply the force on an object due to a gravitational pull, and is directly proportional to mass.