In classical mechanics, which is concerned with discovering and applying the physical laws of motion, certain measurements are needed. These include length, area, volume, mass, force, and speed. Length can be measured with a ruler, tape measure, caliper, or surveyor's wheel, among others. Area is usually obtained from measurements of length, but can also be measured by tracing a planimeter around the perimeter of the shape. Volume is measured in hollow vessels such as graduated cylinders or measuring cups. Mass is usually inferred from weight, which is the force occurring from the earth's gravity accelerating a body. Force is commonly measured with a spring scale, and speed is most familiarly measured with a speedometer, as in a car.
Time is measured by watches, clocks and calendars. In the past, hourglasses, sundials and the study of the stars were used to keep track of time. Time behaves unlike mechanical properties in that it is impossible to set time to a certain value; that is, we cannot use a time machine to set the time to 1500 the way we use a measuring cup to produce 1/3 cups of flour.
Other quantities we often measure are magnetism, electrical properties, and heat. For electrical properties, there are specially designed multimeters that can measure many different quantities of interest. Heat is measured by various kinds of thermometer, the most familiar of which is a mercury thermometer, which contains in a glass tube a calibrated amount of the metal mercury, which is liquid at room temperature and expands to make the column of mercury longer in heat.
Many different units of common physical quantities have been used throughout history. Units are necessary so that one person can correctly interpret a measurement another has made, and measuring instruments must be correctly calibrated to give readings in the appropriate units. Historically, units of length may often have been based upon the length of someone's body part, and there are some who believe the English system's "foot" comes from this origin. Today, most countries use the metric system for measurement, first adopted in France in 1799. Only Burma, Liberia, and the United States do not use the metric system.