# Chapter 6: Energy

## 6.2 Inertia

A property closely tied to energy is a property called inertia. This is a physical property that can be used to describe an object.

Back at the beginning of the chapter, we looked at an example of a stationary coffee mug that we knew would remain stationary until something came along and transferred some energy to it. If you transferred energy to the coffee mug, you could set it into motion. That is, the coffee mug’s motion would change: it would go from not moving to moving. Let’s dig deeper into this.

Consider two boats floating in the water. They are the same size and shape, but one is full of cargo and the other is empty. The boat that is full of cargo is difficult to set it in motion, but the empty one can be set into motion fairly easily. And once in motion, the loaded boat will be difficult to stop, while the empty boat will not. You could say that the boat with the cargo has a large resistance to changes in motion.

Resistance to changes in motion is the essence of inertia. An object with greater inertia will resist changes in motion more than an object with less inertia. The fact that we’re only dealing with changes in motion is important. A change in motion is more that just stopping and starting. In general, we can have several cases of changes in motion:

1. An object is at rest and begins moving.
2. An object is moving and is brought to rest.
3. An object is moving and speeding up (or slowing down).
4. An object is moving and the direction of motion is changing.

An object’s inertia quantifies how much the object resists any of these changes!

In chapter 3, we defined mass as the measurement of an object’s resistance to changes in motion. This means that mass measures an object’s inertia. This is such an important concept that I’ll say it again:

Mass is the measurement of an object’s inertia, the object’s resistance to changes in motion.

We’ll study inertia in more detail in this chapter when we look at rotation, and when we discuss Newton’s laws of motion in chapter 9.