# Question-1

**Explain the echo method to find the distance of the moon from the Earth.**

**Solution:**

A LASER is a source of very intense, monochromatic and unidirectional beam.

The echo method is used in finding the distance of the moon from the Earth by sending a laser beam towards the moon instead of sound waves, the echo method becomes useful in finding the distance of moon from the earth. If 't' is the total time taken by the laser beam in going towards the moon and back, then the distance of moon from earth's surface is given by:

S= (C × t)/2

Where C = 3×10^{8 }m/s, is the velocity of the laser beam.

# Question-2

**What is gravitational mass of a body? Give a method to measure it?**

**Solution:**

Gravitational mass is the mass of a body, which determines the gravitational pull exerted on it due to the Earth.

In order to measure the gravitational mass of a body, a common balance is used. The body is placed in the left pan, while the standard weights are placed in the right pan. The gravitational force on the body and standard weights are made equal and the mass of the body is determined.

# Question-3

**Write the basic principle of alpha particle scattering method for estimating the size of the nucleus.**

**Solution:**

An α particle carries two units of positive charge. When it moves towards the positively charged nucleus it is constantly replaced by the nucleus. The velocity and therefore kinetic energy of the α particle goes on increasing. At the distance of closest approach, the entire kinetic energy of the α particle goes on decreasing. In turn, the potential energy of the particle goes on increasing. At the distance of closest approach, the entire kinetic energy of α particle is converted into potential energy. The value of r

_{0}gives us the order of the size of nucleus.

# Question-4

**Mention the largest unit of time.**

**Solution:**

Century is the largest unit of time.

1 century = 100 years.

# Question-5

**Justify the SI system is a coherent system of units.**

**Solution:**

In SI system, all the derived units can be obtained by dividing and multiplying the basic and supplementary units and no numerical factors are required. Hence it is said to be a coherent system of units.

# Question-6

**Mention some repetitive phenomena, occurring in the Nature which could serve as standard time. Which one do we select as most suitable for measuring the standard of time?**

**Solution:**

The phenomenon which repeats itself after regular intervals of time is used to measure time. For example, the rotation of Earth about its own axis or around the Sun, heart beat, oscillation of a pendulum etc. The most suitable phenomenon to measure time has been the rotation of Earth about its own axis. However atomic clocks give us better standards of time measurement.

# Question-7

**What is the technique used for measuring large time intervals?**

**Solution:**

Very large intervals of time, e.g. age of rocks and age of fossils are measured by using the theory of this technique is to calculate the time intervals by noting the ratio of the number of radio active atoms which have decayed with the passage of time to the number of undecayed atoms. Carbon dating is used to estimate the age of fossils, While uranium dating is helpful for estimating the age of a rock.

# Question-8

**What is the difference between angstrom unit and A. U.?**

**Solution:**

Both, angstrom unit and A. U. are units of length, but are different in the following ways:

1 angstrom = 10^{-10 }m

It is abbreviated as A whereas A. U. is the abbreviation for astronomical unit.

1A. U. = 1.496 × 10^{11 }m.

# Question-9

**Give some practical units of standard of mass.**

**Solution:**

Some practical units of standard of mass are:

1. Chandra Shekhar Limit (C. S. L.) = 1.4 times mass of the sun

2. Metric ton = 1000g.

# Question-10

**Why did it become necessary to redefine metre on atomic standard?**

**Solution:**

There are a number of objections to the metre bar, as the primary standard of length. Some of the objections are as follows:

(i) It may be lost or destroyed due to fire of bar or any other reason.

(ii) It is not easily accessible and reproducible as the unit of a physical quantity should be.

(iii) The techniques used for producing the prototype copies of the standard metre, involve the use of microscope and engines. These techniques are not of very high accuracy as required for modern scientific work.

# Question-11

**Why has 'metre' been defined in terms of wavelength and ' second' in terms of periods of radiation?**

**Solution:**

(i) Metre has been defined in terms of wavelength because:

(a) Wavelength is accurately defined.

(b) It is not affected by changes in place, time and physical conditions like temperature.

(c) It is easily reproducible in any good laboratory.

(ii) Second has been defined in terms of periods of radiation, because

(a) This period is accurately defined.

(b) This period is not affected by changes of place, time and physical conditions like temperature.

(c) The unit is easily reproducible in any good laboratory.

# Question-12

**In defining the standard of length, we have to specify the temperature at which the measurement should to be made. Are we justified in calling length a fundamental quantity, if another physical quantity (temperature) has to be specified in choosing a standard?**

**Solution:**

The length of an object varies with the temperature but the standard for length, which is now defined in terms of wavelength of light, is not affected by temperature. Therefore, we are justified in calling length a fundamental quantity.

# Question-13

**What is the most accurate clock?**

**Solution:**

A cesium clock is the most accurate one. Two cesium clocks differ only by 1 sec after running for 5000 years.

# Question-14

**Give echo method to find the distance of a hill.**

**Solution:**

To find the distance of a hill, a gun is fired towards the hill and the time interval t between the instant of firing the gun and the instant of hearing the echo of the gun is determined. Clearly, during this time interval sound first travels towards the hill from the place of firing and then back from the hill to the place of firing. If ν be the velocity of sound, and s the distance of hill from the place of firing, then

2s = ν × t or s =

# Question-15

**Give a method to measure the size of atom using Avogadro’s hypothesis.**

**Solution:**

Consider a mono atomic substance having mass m and volume V. If M and N are molecular mass of the substance and Avogadro’s number respectively, the number of atoms of the substance having mass m =

Assuming that each atom is a sphere of radius r, the volume occupied by atoms in the substance

is V′ =

According to Avogadro’s hypothesis, all the atoms in a given specimen of the substance occupy

2/3 rd the volume V of the substance, hence

(where ρ = density of substance)

or r =

Thus, knowing the values of M, N and ρ , r can be calculated.

# Question-16

**Is there any quantitative or qualitative difference between inertial mass and gravitational mass?**

**Solution:**

Quantitatively inertial mass and gravitational masses are equal (m

_{i}= m

_{g}= m say)

Qualitatively, if the mass is acted upon by an external horizontal force, it is called inertial mass and if it is under the effect of gravity the same behaves as gravitational mass.

# Question-17

**Explain this common observation clearly:**

If you look out of the window of a fast moving train, the nearby trees, houses etc. seem to move rapidly in a direction opposite to the train’s motion, but the distant objects (hills tops, I moon, the stars etc) seem to be stationary. (In fact, since you are aware that you are moving these distant objects seem to move with you)

If you look out of the window of a fast moving train, the nearby trees, houses etc. seem to move rapidly in a direction opposite to the train’s motion, but the distant objects (hills tops, I moon, the stars etc) seem to be stationary. (In fact, since you are aware that you are moving these distant objects seem to move with you)

**Solution:**

Due to motion of the train, the line of sight (line joining the object to the eye) of a nearby tree changes its direction rapidly and accordingly they appear to run in opposite direction. On the other hand, the line of sight of a distant star or a cliff does not change its direction owing to its extremely large distance from the eye. Due to this, the distant cliff or the star appears stationary.