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Distinguishing Between Inertia and Mass

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  • 0:02 What Is Inertia?
  • 3:06 What Is Mass?
  • 5:00 Lesson Summary
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Lesson Transcript
Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

Mass and inertia are both related to the amount of matter an object has, but they aren't exactly the same thing. In this video lesson, you'll see how mass affects an object's inertia, which in turn affects that object's motion.

What Is Inertia?

A long time ago, there lived a really smart guy named Galileo Galilei. Galileo was a great scientist, with interests in physics, math, astronomy, and philosophy. One of his most important contributions to the scientific world was a discovery he made about moving objects.

Earlier, another famous guy, Aristotle, had said that there were two kinds of motion: natural and unnatural. He believed that natural motion occurred without a force - things like the sun, the moon, and other, similar types of objects in the sky moved without anything pushing or pulling them. Unnatural motion then, was motion that required a force - something pushing or pulling the object to make it move.

But Galileo discovered that this was, in fact, not correct! Galileo said that, yes, a force was needed in order to get an object moving, but once that object was moving, it would keep moving even with no force acting on it. Through various experiments, he was able to show that all objects have the tendency to remain as they are - either at rest or in motion. This property is called inertia, and it's not a force, rather it is the property of matter to resist changes in motion.

So, if an object is moving, it resists not moving, and if it's stationary, it resists movement. That's inertia! This was a great discovery indeed. But, a little while later, another great scientist named Isaac Newton came along (the guy under the apple tree) and refined Galileo's idea of inertia. Galileo was concerned with how things moved; Newton was interested in why things moved.

Newton's work gave Galileo's concept of inertia the status of a scientific law, his first law, usually called the law of inertia. This law states that 'every object continues in its state of rest or of uniform speed in a straight line unless acted on by a net force.' In summary, objects at rest stay at rest, and objects in motion stay in motion unless a force changes their state.

Inertia is pretty cool stuff. You know that famous trick of pulling a table cloth out from under dishes on a table? The dishes don't fall off the table because of inertia - they're at rest, so that's how they stay! Satellites in orbit stay in orbit because they are already moving - no force acts on them to stop them from doing so.

There's a caveat here, though. Friction is a force that affects motion. You've experienced friction plenty of times; that burn you feel sliding down a metal slide is friction between your skin and the slide. When you rub your hands together, there's friction between them. Your car stops at a red light because your foot pushes your brake parts together, creating friction. Friction occurs for solids, liquids, and gases, and acts in the direction that is opposite to the direction of motion. The amount of friction depends on the surface of the objects moving against each other, so it's an important factor in the movement (or rest) of objects.

What Is Mass?

Larger objects are generally harder to get moving than smaller objects, right? This is because larger objects tend to be more massive. Mass, which is a measure of inertia, is the amount of matter in an object. The mass of something depends on the amount and type of 'stuff' it's made of. Kick an empty can, and it's easy to get moving because it's not very massive. But fill the can with lead, and it's much harder to kick into motion because lead is very massive.

This also means that the more massive an object is, the more inertia it has. This makes sense because an object made of more 'stuff' will be harder to either get moving or stop moving. Would you rather try to move an elephant or a mouse? How about stopping a speeding semi or a speeding motorcycle? Both the elephant and the semi have more mass, so they have more inertia. It would be much easier to move the mouse and stop the motorcycle because they have less mass, and therefore less inertia.

Be careful, though, because larger objects are not always more massive. Your pillow is clearly larger than your car battery, but the car battery is more massive because it is made of more 'stuff.' That stuff just happens to be packed into a smaller space than the 'stuff' in your pillow.

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