Units And Measurement - Class 11 Medical Physics - Extra Questions

Calculate the number of light years in a petametre. (where 1 peta = $$10^{15}m$$)1.

$$1 Light year=9.46\times { 10 }^{ 15 }m\\$$
$$1 petameter={ 10 }^{ 15 }m$$

$$\therefore$$Number of light year in a petameter
$$=\dfrac { { 10 }^{ 15 } }{ 9.46\times { 10 }^{ 15 } } \\ =\dfrac { 1 }{ 9.46 } =0.1$$


The first overtone of an open organ pipe beats with the first overtone of a closed organ pipe with a beat frequency of $$2.2$$ Hz. The fundamental frequency of the closed organ pipe is $$110$$ Hz. Find the lengths of the pipes.

Given,
$$v=330m/s$$

$$\dfrac{2v}{2L_1}-\dfrac{3v}{4L_2}=2.2$$

$$\dfrac{3v}{4L_2}=110$$

$$L_2=\dfrac{9}{4}=2.25m$$

$$\dfrac{v}{L_1}=\dfrac{3v}{9}+2.2=112.2$$

$$L_1=\dfrac{330}{112.2}=2.94m$$


Give reasons for the following:
Measurements are very important for life.

We need to measure the quantities of most of the things around us to be able to take correct decisions regarding their utility. To find the exactness of an unknown quantity, we need its measurement.


Arrange the following lengths in descending order of their magnitudes: Light year, parsec end Astronomical unit

Parsec > light year > Astronomical unit


Name the three fundamental quantities.

The three fundamental quantities are:
   $$\cdot$$ Length
   $$\cdot$$ Mass
   $$\cdot$$ Time


Name the three systems of unit and state the various fundamental units in them.

The three systems of unit and 
the corresponding fundamental units are:

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What do you understand by the term 'measurment' ?

Determining the exact value of an unknown quantity by comparing it with a known fixed quantity is called as measurement.


How do you measure the distance of a far off star by spectroscopic method?

There is no direct method currently available to measure the distance to stars farther than 400 light years from Earth, so astronomers instead use brightness measurements. It turns out that a star's color spectrum is a good indication of its actual brightness. The relationship between color and brightness was proven using the several thousand stars close enough to earth to have their distances measured directly. Astronomers can therefore look at a distant star and determine its color spectrum. From the color, they can determine the star's actual brightness. By knowing the actual brightness and comparing it to the apparent brightness seen from Earth (that is, by looking at how dim the star has become once its light reaches Earth), they can determine the distance to the star.


Mention two advantages of the metric system.

  1. It is a world wide standard.
  2. It is a decimal system of weights and measures, therefore it is easy ti convert between units.
  3. It is easy to use.


A SONAR (sound navigation and ranging) uses ultrasonic waves to detect and locate objects under water. In a submarine equipped with a SONAR the time delay between generation of a probe wave and the reception of its echo after reflection from an enemy submarine is found to be $$77.0$$ $$s$$. What is the distance of the enemy submarine? (Speed of sound in water = $$1450$$ $$ms^{-1}$$).

Let the distance between the ship and the enemy submarine be$$ S.$$
Speed of sound in water  $$=1450 m/s$$
Time lag between transmission and reception of Sonar waves $$=77 s$$
In this time lag, sound waves travel a distance which is twice the distance between the ship and the submarine $$( 2S ).$$

Time taken for the sound to reach the submarine  $$=\dfrac{1}{2} \times  77 = 38.5 s $$

Distance between the ship and the submarine, $$S= 1450 \times  38.5 = 55825 m = 55.8 km$$


What do you understand by Mass?

Mass is the quantity of matter in a body regardless of its volume or any forces acting on it.


A LASER is a source of very intense, monochromatic, and unidirectional beam oflight. These properties of a laser light can be exploited to measure long distances.The distance of the Moon from the Earth has been already determined very preciselyusing a laser as a source of light. A laser light beamed at the Moon takes 2.56 s to return after reflection at the Moons surface. How much is the radius of the lunar orbit around the Earth ?

Time taken by the laser beam to return to Earth after reflection from the Moon $$=2.56\ s$$  
Speed of light $$=3 \times 10^8 m/s$$ 
Time taken by the laser beam to reach Moon $$= 1/2 \times  2.56 = 1.28\ s$$  
Radius of the lunar orbit $$=$$ Distance between the Earth and the Moon 
                                          $$= 1.28 \times 3 \times 10^8 \\ = 3.84 \times 10^8 m \\ = 3.84 \times 10^5 km$$ 


Which unit will you use to find the length of your notebook and why ?

Centimetre. Because a notebook has a length for which metre would be too large a unit to measure and millimetre would be too small.


Describe in steps how would you measure the length of a pencil using a metre rule. Draw a diagram if necessary.

To measure the length of a pencil using a metre rule, place metre rule with its marking close to the object. Let PQ be a pencil.
The end P of the pencil coincides with the zero mark on the ruler. The end Q of the pencil is ready by keeping the eye at the position 'B' vertically above the end Q. So the length of pencil is $$4.3 \,cm.$$
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What is measurement ? How is a measurement expressed ?

Measurement is a comparison of an unknown quantity with a known fixed quantity of the same kind.
The value obtained on measuring a quantity is called its magnitude. The magnitude of a quantity is expressed as numbers in its unit.


Explain the meaning of the term 'least count of an instrument' by taking a suitable example.

The least count of an instrument is the smallest measurement that can be taken with that instrument. 
For example. 
(i)A sundial can have a least count of 1 hour and a vernier calliper can have a least count of 0.01 CM. 
(ii) if there are 10 divisions between 0 and 5s mark of a stop watch, the least count of that stop watch is 0.5s.


What are the fundamental quantities?

The quantities, which are independent of other physical quantities are called fundamental quantities.


Class 11 Medical Physics Extra Questions