Learning Check 8.2
Describe the three different standards that have been used in the U.S. since 1893 to define the meter.
Originally, the meter was defined as the length of a particular metal bar. In 1960, the standard was changed to be a certain number of wavelengths of light emitted by the element krypton. In 1983, this standard was changed again to the present definition of the meter. This definition is that a meter is the distance light travels in a vacuum in 1/299,792,458 of a second.
Explain what a base unit is in the SI system.
The SI system has seven fundamental units of measure. These seven units of measure are called the base units. All other units of measure in the SI system are derived from these seven base units.
Explain what a derived unit is, and give some examples of derived units.
A derived unit is a unit of measure based upon a combination of two or more of the seven base units in the SI system. Examples include the joule for measuring energy, the newton for measuring force, and the cubic meter for measuring volume.
The three most commonly used base units are the meter, the kilogram, and the second. For each of these three units, make up two examples of your own that help illustrate their size.
The examples used in the text are that the meter is slightly longer than a yard, and that a kilogram is the amount of mass that a six-volt lantern battery has. Students can make up whatever illustrations they like to answer this question as long as they are reasonably close examples.
Why is it important to define base units of measure in terms of a natural standard, instead of an artifact?
When a man-made object is used as a primary standard, all other measurement references must be compared to it. The primary standard has to be kept under carefully controlled conditions. To make a copy of the primary standard, people must travel around the world with measurement materials under very controlled conditions. When a natural standard is used, the reference standard can be created anywhere people need it to be created for calibrating measurement instruments. This gives everyone equal access to the measurement standards.
