Notify me of follow-up comments by email. Notify me of new posts by email. Derived Quantity A derived quantity is defined based on a combination of base quantities and has a derived unit that is the exponent, product or quotient of these base units. Mini Physics Administrator of Mini Physics. I like this group is very interesting Reply. Solution : The weight of an object on the moon is about one-sixth of its weight on the earth. This is because the value of acceleration due to gravity on the moon is about one-sixth of that on the earth.
The units of all such physicalquantities which can be expressed in therms of the fundamental units of mass, length and time are calledderived units. Hint: Fundamental quantities are mass, time, current, length, temperature, amount of substance and luminous intensity. All other physical quantities are derived quantities and can be made from fundamental quantities. Momentum is the product of mass and velocity. For example, mass is a fundamental quantity because the mass of an object does not change, but the weight does.
The measurement of weight is actually the measurement of the gravitational pull on an object. To understand this, think of the moon. A man may weigh lbs.
His mass however, stays the same in either location. Fundamental quantities are units of measurement expressed by designated systems. This is how the radian is defined. We will see many more examples throughout this text.
For now, the point is that every physical quantity can be derived from the seven base quantities in Figure , and the units of every physical quantity can be derived from the seven SI base units. For the most part, we use SI units in this text. Whenever non-SI units are discussed, they are tied to SI units through conversions. The initial chapters in this textbook are concerned with mechanics, fluids, and waves. In these subjects all pertinent physical quantities can be expressed in terms of the base units of length, mass, and time.
Therefore, we now turn to a discussion of these three base units, leaving discussion of the others until they are needed later. The SI unit for time, the second abbreviated s , has a long history.
Cesium atoms can be made to vibrate in a very steady way, and these vibrations can be readily observed and counted. In , the second was redefined as the time required for 9,,, of these vibrations to occur Figure. The fundamental unit of time, the second, is based on such clocks. This image looks down from the top of an atomic fountain nearly 30 feet tall.
The SI unit for length is the meter abbreviated m ; its definition has also changed over time to become more precise. This measurement was improved in by redefining the meter to be the distance between two engraved lines on a platinum—iridium bar now kept near Paris.
By , it had become possible to define the meter even more accurately in terms of the wavelength of light, so it was again redefined as 1,, The length of the meter will change if the speed of light is someday measured with greater accuracy. Distance traveled is speed multiplied by time. The SI unit for mass is the kilogram abbreviated kg ; it is defined to be the mass of a platinum—iridium cylinder kept with the old meter standard at the International Bureau of Weights and Measures near Paris.
Exact replicas of the standard kilogram are also kept at the U. Scientists at NIST are currently investigating two complementary methods of redefining the kilogram see Figure. The determination of all other masses can be traced ultimately to a comparison with the standard mass. There is currently an effort to redefine the SI unit of mass in terms of more fundamental processes by You can explore the history of mass standards and the contenders in the quest to devise a new one at the website of the Physical Measurement Laboratory.
Complementary methods are being investigated for use in an upcoming redefinition of the SI unit of mass. SI units are part of the metric system , which is convenient for scientific and engineering calculations because the units are categorized by factors of Figure lists the metric prefixes and symbols used to denote various factors of 10 in SI units.
Thus, a thousand kilograms 10 3 kg is written as a megagram 1 Mg since. Incidentally, 10 3 kg is also called a metric ton , abbreviated t. This is one of the units outside the SI system considered acceptable for use with SI units. As we see in the next section, metric systems have the advantage that conversions of units involve only powers of There are cm in 1 m, m in 1 km, and so on. In nonmetric systems, such as the English system of units, the relationships are not as simple—there are 12 in.
0コメント