9.2 Measurement of Mass 233
located from the center of rotation, the harder it will be to rotate the mass about that axis. A
measure of how hard it is to rotate something with respect to center of rotation is calledmass
moment of inertia. We will discuss this in detail in Section 9.5.
The other mass-related parameter that we will investigate in this chapter is momentum.
Consider two objects with differing masses moving with the same velocity. Which object is
harder to bring to a stop, the one with the small mass or the one with the larger mass? Again,
you already know the answer to this question. The object with the bigger mass is harder to
stop. You see that in the game of football, too. If two players of differing mass are running at
the same speed, it is harder to bring down the bigger player. These observations lead to the
concept of momentum, which we will explain in Section 9.6.
Mass also plays an important role in storing thermal energy. The more massive something
is, the more thermal energy you can store within it. Some materials are better than others at stor-
ing thermal energy. For example, water is better at storing thermal energy than air is. In fact,
the idea of storing thermal energy within a massive medium is fully utilized in the design of pas-
sive solar houses. There are massive brick or concrete floors in the sunrooms. Some people even
place big barrels of water in the sunrooms to absorb the available daily solar radiation and store
the thermal energy in the water to be used overnight. We will discuss heat capacities of mate-
rials in more detail in Chapter 11.9.2 Measurement of Mass
Thekilogramis the unit of mass in SI; it is equal to the mass of the international prototype
of the kilogram. As we explained in Chapter 6, in the British Gravitational system of units, the
unit of mass is slug, and in U.S. Customary system of units, the unit of mass is lbm (1 kg =Copyright 2010 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).圀圀圀⸀夀䄀娀䐀䄀一倀刀䔀匀匀⸀䌀伀䴀圀圀圀⸀夀䄀娀䐀䄀一倀刀䔀匀匀⸀䌀伀䴀