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7-1 The ideas of work and energy are intimately related, and this relationship is based on a conservation principle 287

created nor destroyed. When quantities are not created or destroyed, the amount of that
quantity does not change, and this gives rise to some of the most powerful, fundamental AP ® Exam Tip
laws in physics: conservation laws. Conservation laws constrain the possible motions of On the AP® Physics 1 exam,
the objects in a system, or the outcome of an interaction or process. A conservation law when you are asked to justify
is a statement that a measurable physical quantity of a system does not change as the a result, you often need to
system evolves over time. This physical quantity can be used to characterize a system. relate your argument to a
We use the word energy almost every day, but most people would still find it fundamental principle such
hard to define. In science, energy is defined as a scalar quantity used to measure the as this conservation law. For
state or motion of an object or a system (we will see some examples to help make example: If asked to compare
this clearer). Energy is always conserved, but not all energy is equally useful. We will two projectiles on the exam,
continue to explore this idea throughout this chapter and the next. So when we say it would not be enough to
we want to “conserve energy” in our everyday lives, we really mean that we want to say that if they each move
not waste the energy that is most useful to us. through the same height and
Because conservation also considers transfer of a quantity, we need to define a sys- start with the same speed, they
tem to know how to apply a conservation law. This is a little tricky, as different sciences, would therefore have the same
and even different fields within a single science, define the types of systems differently. final speed. For a complete
In chemistry, a closed system is a system where no matter can escape, but energy can be answer, you would need to
relate your response to the
exchanged freely through heating. A closed system can be used when conducting chemi- fundamental principle of energy
cal experiments where temperature is not a factor. In nonrelativistic classical mechanics, conservation.
a closed system is a physical system that doesn’t exchange any matter with its surround-
ings and isn’t subject to any external forces (in quantum mechanics you have to add
information to the list of things that cannot be exchanged). A closed system in classi-
cal mechanics would be considered an isolated system in thermodynamics. Closed sys- WATCH OUT !
tems are often used to limit the factors that can affect the results of a specific problem
or experiment. In thermodynamics, a closed system can exchange energy (as heating or Closed system means something
work), but not matter, with its surroundings. An isolated system cannot exchange energy different in mechanics and
or matter with the surroundings, while an open system can exchange both energy and chemistry.
matter. So just to make sure we don’t miss anything because of a previous definition Chemists don’t worry about
you know, we will define closed, isolated systems as those where no energy or matter interactions that can be
is transferred to or from the system and there are no interactions between objects in described by forces in their
the system and objects outside of the system. The definition for this combination is experiments (they have a
consistent, if a bit repetitive in cases, with all of the other definitions that exist. lot more heating), so their
We saw in Chapter 4 that a force is one way to describe the interaction between definition of closed does not
two objects, so another way to define an isolated system is a system for which no exclude such interactions.
forces are exerted on objects inside the system by objects outside the system. We will For mechanics this is a very
see in this chapter how to use interactions that can be described by forces to trans- important thing to exclude,
fer energy, but there are other processes (such as heating) to transfer energy that are so we will add the word
beyond the scope of this book. The total amount of energy in a closed, isolated system isolated to our description of
cannot change, and all interactions and processes in the system are constrained by that systems, emphasizing that for
fact. In contrast, in a system that is open, energy can cross the boundary of the system, such systems no interactions
in which case conservation no longer means the energy in the system is constant! It or matter cross the system
means that the changes in energy in the system are equal to the transfer of energy into boundaries.
or out of the system by interactions with other systems or processes.
We begin our study of conservation laws with the law of conservation of energy,
the most pervasive conservation law across all areas of physics, and all sciences in AP ® Exam Tip
general. Energy is used in nearly every living process—moving, breathing, circulating
blood, digesting food, and absorbing nutrients. Within these processes, there are differ- Although thermodynamics is
ent types of energy, including kinetic, potential, and internal. The law of conservation not part of AP® Physics 1, it is
of energy states that energy can be converted from one type into another but never important to understand that
destroyed. We have defined energy as a scalar quantity used to measure the state or mechanical energy can go into
motion of an object or a system; but what exactly does that mean? It may be easiest to warming up systems. We will
begin to develop our definition of energy by considering one of the ways to transfer it. learn about the type of energy
associated with temperature
To delve into how energy is transferred, we must first explore the concept of work. change in Chapter 8.
Work is another word with many different meanings in everyday life. In the language of
physics, however, work has a very specific definition: Work is the transfer of energy from
one object or system to another through a mechanical process that happens when an exter-
nal force is exerted on an object or a system along its direction of motion as the point of
contact where the force is exerted on the object or system moves. Remember, mechanics is
the area of physics concerned with motion due to interactions (including in the special case

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