Measurement of length, mass and time
The rules of physics are described as mathematical relationships among physical quantities. The standard of measurement is called the unit of that physical quantity. For example: the unit of length is meter which has a precise definition.
The physical quantities can be classified as follows:
1. base quantities
2. Derived quantities
The physical quantities that are independent of each other are called base quantities or fundamental quantities. Other quantities which can be expressed in terms of the base quantities are called the derived quantities.
System of Units (to mesaure the length, mass and time):
1. FPS System: This system defines the unit of length as foot, unit of mass as pound and the unit of time as second.
2. CGS System: This system defines the units of length as centimetre, unit of mass as gram and unit of time as second.
3. MKS System: This system defines the unit of length as meters, unit of mass as kilogram and unit of time as second.
4. SI System: This system is commonly used in verious measurements throughout the world. The system is based on seven basic units and two supplementary units.
Basic Units
| Unit | Precise definition | Symbol for unit |
|---|---|---|
| Metre | One metre is the distance travelled by light in the vacuum during a time interval of (1/299792458) seconds. | m |
| Kilogram | It is the mass of a platinum-iridium cylinder kept at the National Bureau of weights and measurements, Paris. | kg |
| Second | The second is the time taken by the light of a specified wavelength emitted by a cesium-133 atom to execute 9192631770 vibrations. | s |
They have three quantities in physics or mechanics:- Length , Mass and Time.
Length
A ruler is a tool used in, for example, geometry , technical drawing, to measure lengths or distances or to draw straight lines.
The ruler is the tool used to rule simple straight lines and the calibrated tools used for determining length is called a measurement, however common usage calls both tools rulers and the special name straightedge is used for an unmarked rule.
The use of the word measure, in the sense of a measuring tools, only survives in the phrase tape measure, a tool that can be used to measure but cannot be used to draw straight lines.
Example
1 kilometer = 1000 meter
1 meter = 100 centimeter
Time
Time is denoted by numbers or names or periods like hours , weeks ,days ,years .Time is an abstract measurement of elemental changes over a non spatial continuum. It is an irreparable, series of occurrences within this non spatial continuum.
It is also used to denote an interval between two relative points on this continuum. we just see watch and arrow indicate hours, minutes and second .
Mass
Mass refers to the intrinsic property of all material objects to resist changes in their momentum. Weight, refers to the downward force produced when a mass is in a gravitational field.
The Imperial units of mass include the ounce, pound, tons. In mass the metric unit is called gram and kilograms and both are the unit of mass .
One device for measuring weight or mass is called a weighing scale . A spring scale is used to measure force but it is not mass, a balance compares weight, both require a gravitational field to operate.
Some of the most accurate tools for measuring weight or mass are based on load cells with a digital read-out, but require a gravitational field to functional.