Very few terms are used in physics with greater frequency and assurance than mass, and few are more difficult to define. Mass is often confused with weight. This is a mistake not helped since the unit of measurement for both mass and weight is the gram. The MASS of an object is the quantity of matter that the object contains. The WEIGHT of the object is equal to the gravitational force with which the object is attracted to the earth. FORCE is what makes an object start to move, speed up, slow down, or keep moving against resistance. Force may be either a push or a pull. You exert a force when you push against a truck, whether you move the truck or only try to move it. You also exert a force when you pull on a heavy piano, whether you move the piano or only try to move it. Forces can produce or prevent motion.

A tendency to prevent motion is the frictional resistance offered by an object. This frictional resistance is called frictional force. While it can never cause an object to move, it can check or stop motion. Frictional force wastes power, creates heat, and causes wear. Although frictional force cannot be entirely eliminated, it can be reduced with lubricants.

INERTIA is the property that causes objects at rest to remain at rest and objects in motion to remain in motion until acted upon by an outside force. An example of inertia is one body that has twice as much mass as another body of the same material offering twice as much force in opposition to the same acceleration rate.

Inertia in a body depends on its motion. The physical principles of mass and inertia are involved in the design and operation of the heavy machinery that is to be placed into motion, such as an engine's flywheel and various gears that are at work in the ship's engineering plant. The great mass of the flywheel tends to keep it rotating once it has been set in motion. The high inertia of the flywheel keeps it from responding to small fluctuations in speed and thus helps keep the engine running smoothly.

ENERGY

Can you define energy? Although everyone has a general idea of the meaning of energy, a good definition is hard to find. Most commonly, perhaps, energy is defined as the capacity for doing work. This is not a very complete definition. Energy can produce other effects which cannot possibly be considered work. For example, heat can flow from one object to another without doing work; yet heat is a form of energy, and the process of heat transfer is a process that produces an effect. A better definition of energy, therefore, states that energy is the capacity for producing an effect.

Energy exists in many forms. For convenience, we usually classify energy according to the size and nature of the bodies or particles with which it is associated. Thus we say that MECHANICAL ENERGY is the energy associated with large bodies or objects-usually, things that are big enough to see. THERMAL ENERGY is energy associated with molecules. CHEMICAL ENERGY is energy that arises from the forces that bind the atoms together in a molecule. Chemical energy is demonstrated whenever combustion or any other chemical reaction takes place. Electrical energy (light, X rays, and radio waves) is associated with particles that are even smaller than atoms.

Mechanical energy, thermal energy, and chemical energy must also be classified as being either stored energy or energy in transition.

STORED ENERGY can be thought of as energy that is actually contained in or stored in a substance or system. There are two kinds of stored energy: (1) potential energy and (2) kinetic energy. When energy is stored in a substance or system because of the relative POSITIONS of two or more objects or particles, we call it potential energy. When energy is stored in a substance or system because of the relative VELOCITIES of two or more objects or particles, we call it kinetic energy.

Mechanical energy in transition is called work. Thermal energy in transition is called heat. In the next section we will discuss mechanical and thermal energy and energy transformations.

If you do not completely understand this classification, come back to it from time to time as you read the following sections on mechanical energy and thermal energy. The examples and discussion given in the following sections will probably help you understand this classification.