The unit of Stefan's constant \sigma is

Stefan's law is E=\sigma (T^4) \Rightarrow \sigma =\dfrac{E}{T^4} where, E=\dfrac{Energy}{Area \times Time}=\dfrac{Watt}{m^2} \sigma =\dfrac{Watt-m^2}{k^4}=Watt-m^{-2}k^{-4}

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>>Class 11
>>Physics
>>Units and Measurement
>>Fundamental and Derived Units
>> $Standard Units of Measurement$

Standard Units of Measurement

Q: What do we call the gravitational force between the earth and an object?

The gravitation force between the earth and object is called weight.It is also equal to the product of acceleration due to gravity and mass of the object. Weight = m \times g ,where, m= mass of the object, g = acceleration due to gravity = 9.8 m/s^{2}

Q: A motorboat, whose speed in 15 km/hr in still water goes 30 km downstream and comes back in a total of 4 hours 30 minutes. The speed of the stream (in km/hr) is:

Let the speed of the stream be x km/hr Speed downstream =(15+x)km/hr Speed upstream =(15-x)km/hr Distance travelled downstream =30km Distance travelled upstream =30km Time upstream =\dfrac {\text{Distance upstream}}{\text{Speed upstream}} Time upstream =\dfrac{30}{15-x} Time downstream =\dfrac {\text{Distance downstream}}{\text{Speed downstream}} Time downstream =\dfrac{30}{15-x} Total time taken 4.5hrs 4.5=\dfrac{30}{15-x}+\dfrac{30}{15+x} 4.5=\dfrac{30(15+x)+30(15-x)}{225-x^{2}} 4.5=\dfrac{900}{225-x^{2}} 225-x^{2}=200 x^{2}=25 x=5km/hr Hence the correct option is (B).

Q: Why do we need standard unit measurement?

We need standard unit for measurement to make our judgement more reliable and accurate. For proper dealing, measurement should be same for everybody. Thus there should be uniformity in measurement. For the sake of uniformity we need a common set of units of measurement, which are called standard units. Nowadays Si units in science and technology almost accepted universally.

Q: What do you mean by fundamental & derived quantities? Give the derived unit of Force.

Fundamental quantity: quantities which are independent on other physical quantity. ex: length, mass, time, current, amount of substance, luminous intensity, thermodynamic temperature, Derived quantity: quantities which depend on fundamental quantities. ex: Area, volume, density, speed, acceleration, force, velocity etcForce is a derived unit if the mass has been declared a fundamental unit. It is the force which imparts to the unit of mass the unit of acceleration. The derived unit of force is kg.m/s^2

Q: Write the differences between mass and weight of an object.

MassWeight1. The mass of an object is the quantity of matter contained in it.1. The weight of an object is the force with which it is attracted towards the centre of the earth.2. SI unit of mass is kilogram (kg).2. SI unit of mass is kilogram (kg).3. The mass of an object is constant.3. The weight of an object is not constant. It changes with the change in acceleration due to gravity.4. The mass of an object can never be zero.4. The weight of an object can be zero.

Q: What are the difference between the mass of an object and its weight?

Mass is the actual amount of material contained in a body and is measured is kg, gm, etc. Whereas weight is the force exerted by the gravity on that object mg, The mass is independent of everything but weight is dependent on the gravitational force of different bodies.

Q: Which of the following is not the fundamental quantity,

We know that the fundamental quantities are mass, length and time. velocity = \dfrac{displacement}{time} hence velocity is derived quantity

Q: Which of the following is a fundamental unit?

Fundamental quantities are those quantities which do not depend on other quantities for their measurement. Current is a fundamental quantity and does not depend on anything else. Ampere is the unit of current. It is a fundamental unit. All other units in the options are of dependent quantities.

Q: Which one of the following is not a derived unit?

Among the given units kilogram is the fundamental or base unit whereas others are the derived units.

Physics

definition

SI unit of length and identify its sub units

The S.I. unit of length is meter (m).
The 17th General Conference on Weights and Measures (CGPM), 1983 has defined the meter as:
"The meter is the length of the path traveled by light in vacuum during a time interval of

\frac{1}{2 9 9 7 9 2 4 5 8}

of a second."
Sub units of meter: (1) centimeter (cm), (2) millimeter (mm), (3) micrometer (

μ m

) or micron, (4) nanometer (nm)

definition

Basics of standard unit of measurement

In ancient India length was measured using hand, muthi, fist etc, but measurement from it is not accurate, hence French created a standard unit of measurement called the metric system.
Metric System is further classified to SI system (Standard International System) and CGS (centimeter, gram and second system).
Foreg : In SI system,unit of length is metre denoted by "