We’ve been using the term mass without a clear definition. Our everyday concept is that mass is the amount of matter an object contains. Now we see that a more precise way of defining an object’s mass is in terms of its acceleration in response to a force. FIGURE 5.17 shows that an object with twice the amount of matter accelerates only half as much in response to the same force. The more matter an object has, the more it resists accelerating in response to a force. You’re familiar with this idea: Your car is much harder to push than your bicycle. The tendency of an object to resist a change in its velocity (i.e., to resist acceleration) is called inertia. Consequently, the mass used in Equation 5.3, a measure of an object’s resistance to changing its motion, is called inertial mass. We’ll meet a different concept of mass, gravitational mass, when we study Newton’s law of gravity in Chapter 13.