Concepts in Physics

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Matter can be identified as anything that occupies space, has mass, and possess inertia (Weisstein 2009). Inertia relates to one of Newton’s first law of motion, which states that in order ofr an object to obtain motion or momentum, a force has to act upon it (Zimmerman-Jones 2009). That is, an object that is in motion tends to stay in motion unless a force stops it, and an object at rest will stay at rest unless another force causes it to move. This resistance to motion, which must be overcome by force, is inertia, and is one of the fundamental properties of matter. Light, for example, is not considered to be matter in many experiments and situations because it doesn’t appear to have inertia, or mass, necessarily. At other times and with other measurements, light does have both of these qualities. This is why light can be discussed as both a wave, which has no mass and is not matter, and a particle, which is matter (Nave 2005).

Mass can be somewhat confusing as it is so fundamental to matter that it can be difficult to perceive independently. Mass is not weight; but rather weight can be said to be the mass of an object times the force of gravity working on the object (Nave 2005). In a situation with absolutely no gravity, matter would sill occupy space, have mass, and have inertia, but it wouldn’t weigh anything. Even without weight, it would still have certain dimensions and a resistance to changing its state of motion (or lack thereof). Perhaps another definition of matter, then, could be “something with volume and a resistance to changes in motion.”

Kinetic Theory

The kinetic theory relates to the way gas molecules behave in relation to the density of molecules within a given space — or pressure — and temperature. Based on these factors, the velocity of a gas particle — which by definition floats freely through an area until being deflected by a boundary or another gas particle — is changed, and this change can be predicted with a mathematical formula ( 2009; Weisstein 2009). Two or more gasses mixed in one container will eventually reach equilibrium of both temperature and velocity, meaning that a smaller particle must move faster than a larger particle, as velocity is a function of both speed and mass ( 2009). Because of the relationship of volume to a particle’s velocity, and the relationship of that velocity to temperature, the more confined a gas is the hotter it becomes. This is how pressure cookers are able to generate so much cooking power compared to mere fire.

Kinetic Energy

Kinetic energy is very basically defined as the energy of motion; it refers to the work that must be done to move an object with a given mass at a certain velocity (Weisstein 2009). Breaking this down further, we can understand the concept of work as a force applied in a specific direction — a force applied that creates motion or momentum (Nave 2005). Energy can be understood as the potential or capacity for doing work — in order to do x amount of work, there must be x amount of energy expended (Nave 2009). Kinetic energy, then, is the energy in a system that is expended through work, causing a change in velocity to one or more bodies of mass. An object is said to have kinetic energy when it is in motion; the amount of work required to move the object from resting to its current velocity is the amount of kinetic energy it possesses (Physics Classroom 2009).

Kinetic energy, then, is not the same as potential energy. A large boulder perched on the edge of a cliff has a lot of potential energy; a slight shift in its weight due to any number of factors and it could come tumbling down through the air achieving a huge velocity in free fall because of its size. While it is still at the top of the cliff, however, it has no kinetic energy because it is not in motion ( 20090. It has greater and greater kinetic energy every moment it is falling, because the work that gravity is doing continues to cause the boulder to accelerate, and thus gain velocity. Assuming that the cliff isn’t so tall that the boulder reaches its terminal velocity, it will have its greatest kinetic energy right before it hits the ground. The ground will stop the boulder, but the energy will be transferred to the ground — the boulder will do work that causes pieces of earth and turf to fly out in all directions, each clod with its own kinetic energy.

Charles’ Law

Charles’ Law, simply put, is that gases expand when they are heated. In reality the working of this is more complicated, and a more complete definition of Charles’ Law is that at a constant pressure, the volume of a gas will increase by the same factor as its temperature on the absolute temperature scale (Physics Classroom 2009). What makes this discovery even more amazing is the fact that the same constant applies to all gasses; though different gasses might have different pressures at the same volume and temperature, the factor by which they increase is the same constant, 2.7315 ( One can imagine different ways to manipulate the various characteristics of a gas sample using this law; it helps to explain how a change in pressure can create a temperature change.

Boyle’s Law

A slightly more complex but very much interrelated concept is illustrated by Boyle’s law, which states that if the temperature of a system is kept constant, the pressure and the volume of a gas bear an inversely proportionate relationship, meaning that as one decreases the other increases and vice versa (Weisstein 2009). This can be understood clearly when it is remembered that temperature is related to the amount of motion that the molecules of gas engage in. If this motion is kept constant, then with a larger space to move around in (i.e. An increase in volume) the molecules will bump into each other and walls less (i.e. A drop in pressure).

The Combined Gas Law

The combined gas law takes Charles’ Law and Boyle’s Law about the relationships between temperature, pressure, and volume in a closed system of gas, and creates a mathematical model that can be used to predict changes in any or all of these variables based on changes that occur in one or two of the others. With any two of the variables known at a given moment, the third can be calculated based on the principles of the combined gas law, and predictions can be made regarding two of the variable even if only one of the others is known — that is, a statement of the range and relationship of the other two factors could be made with certainty (Zimmerman-Jones 2009).


It is impossible to come to a full understanding of any of these gas laws without coming to an understanding of pressure. There are several ways to define pressure based on the system being studied and/or the type of physical force that is creating the pressure, but generally pressure is understood as the action of a force being applied to a surface or object, without creating work — therefore there is no kinetic energy (Nave 2005). The more concentrated the force acting on the surface or object is, the higher the pressure. Thus, the more a gas molecule or molecules bumps up against the surface of a container, the greater the pressure inside that container. If the volume is increased and the temperature is kept constant, the molecules will have more room to travel without bumping into the surface of the container — this is the drop in pressure that was witnessed to create the predictions of Boyle’s Law.

Absolute Zero

The concept of absolute zero is the temperature at which molecules have no kinetic energy, and therefore have no (or minimal and non-transferable) motion (Weisstein 2009; Zimmerman-Jones 2009). Absolute zero is only a theoretical concept and cannot actually be achieved, but the consideration of such a state requires a new system of physics to answer certain questions. For instance, in a system where no motion could be transferred, there would be no pressure. If the pressure of a system of gas dropped to zero (which is predicted by Charles’ Law at a temperature of zero), then according to Boyle’s Law the volume should increase hugely, perhaps infinitely. But because the motion of each gas molecule is minimal, this would not actually occur — the gas would not move to occupy a larger volume, and the combined gas law (and its constituent Laws) would fall apart.

Phase Changes

Matter comes in three basic phases (though there are others in more extreme situations): gas, liquid, and solid. Both gases and liquids are considered fluid because the molecules of matter are not rigidly attached to each other, so the terms “fluid” and “liquid” are not interchangeable in chemistry and physics the way they generally are in everyday speech (Physic Classroom 2009). A phase change is what occurs when matter moves from one of these states to the other. The melting of ice into liquid water, for instance, constitutes a phase change, just as the boiling of water into invisible water vapor, or the condensing of gaseous water vapor in the air into liquid water on a cool windowpane or glass of water, are also phase changes.

Phase changes generally require a great deal of energy, especially when considered to the relative to the specific heat of the matter at hand (Nave 2005). The specific heat is the amount of energy that is required to increase the temperature of a given unit of mass by one degree Celsius, but during a phase change most substances (which tend to have constant specific heats within a given phase of matter) require far more energy to be applied, stopping all change in temperature until the phase change is complete, at which point the temperature of the matter will continue to rise under the application of energy (Nave 2005). This is why a mixture of ice water stays as cold as the ice until al of the ice has melted; the system of a glass of ice water maintains an equilibrium of temperature until the phase change of ice into liquid water is completed.


Nave, C. (2005). “Hyperphysics.” Georgia State University Department of Physics and Astronomy. Accessed 29 September 2009. (2009). The Institute of Physics. Accessed 29 September 2009.

Physics Classroom. (2009). Accessed 29 September 2009.

Weisstein, E. (2009). “Eric Weisstein’s World of Physics.” Wolfram Research. Accessed 29 September 2009.

Zimmerman-Jones, a. (2009). “Physics: Basic Concepts.” Accessed 29 September 2009.