# Energy - Key terms

### CONSERVATION OF ENERGY:

A law of physics which holds that within a system isolated from all other outside factors, the total amount of energy remains the same, though transformations of energy from one form to another take place.

### COSINE:

For an acute (less than 90°) in a right triangle, the cosine (abbreviated cos) is the ratio between the adjacent legand the hypotenuse. Regardless of the size of the triangle, this figure is a constant for any particular angle.

### ENERGY:

The ability of an object (or in some cases a non-object, such as a magnetic force field) to accomplish work.

### FRICTION:

The force that resists motion when the surface of one object comes into contact with the surface of another.

### HORSEPOWER:

The British unit of power, equal to 550 foot-pounds per second.

### HYPOTENUSE:

In a right triangle, the side opposite the right angle.

### JOULE:

The SI measure of work. One joule (1 J) is equal to the work required
to accelerate 1 kilogram of mass by 1 meter per second squared (1 m/s
^{
2
}
) over a distance of 1 meter. Due to the small size of the joule,
however, it is often replaced by the kilowatt-hour, equal to 3.6
million(3.6 · 106) J.

### KINETIC ENERGY:

The energy that an object possesses by virtue of its motion.

### MATTER:

Physical substance that occupies space, has mass, is composed of atoms (or in the case of subatomic particles, is part of an atom), and is convertible into energy.

### MECHANICAL ENERGY:

The sum of potential energy and kinetic energy within a system.

### POTENTIAL ENERGY:

The energy that an object possesses by virtue of its position.

### POWER:

The rate at which work is accomplished over time, a figure rendered mathematically as work divided by time. The SI unit of power is the watt, while the British unit is the foot-pound per second. The latter, because it is small, is usually reckoned in terms of horsepower.

### REST ENERGY:

The energy an object possesses by virtue of its mass.

### RIGHT TRIANGLE:

A triangle that includes a right (90°) angle. The other two angles are, by definition, acute or less than 90°.

### SCALAR:

A quantity that possesses only magnitude, with no specific direction.

### SI:

An abbreviation of the French
*
Système International d'Unités,
*
which means "International System of Units." This is
the term within the scientific community for the entire metric system,
as applied to a wide variety of quantities ranging from length, weight
and volume to work and power, as well as electromagnetic units.

### SYSTEM:

In discussions of energy, the term "system" refers to a closed set of interactions free from interference by outside factors. An example is the baseball dropped from a height to illustrate potential energy and kinetic energy the ball, the space through which it falls, and the ground below together form a system.

### VECTOR:

A quantity that possesses both magnitude and direction.

### WATT:

The metric unit of power, equal to 1 joule per second. Because this is such a small unit, scientists and engineers typically speak in terms of kilowatts, or units of 1,000 watts.

### WORK:

The exertion of force over a given distance. Work is the product of
force and distance, where force and distance are exerted in the same
direction. Hence the actual formula for work is
*
F
*
· cos θ ·
*
s,
*
where
*
F
*
= force,
*
s
*
= distance, and cos θ is equal to the cosine of the angle
θ (the Greek letter theta) between
*
F
*
and
*
s.
*
In the metric or SI system, work is measured by the joule (J), and in
the British system by the foot-pound.