Exercise Solution of Physics Class 9 (Chapter 1)

UNIT No. 1

Physical Quantities and Measurement

Exercise

1.1         Encircle the correct answer from the given choices.

        i.            The number of base units in SI are:

(a)        3                      (b)        6                      (c)        7                      (d)       9

      ii.            Which one of the following units is not a derived unit?

(a) pascal                     (b) kilogramme            (c) newton                   (d) watt

    iii.            Amount of a substance in terms of numbers is measured in:

(a) gram                      (b) kilogramme            (c) newton                   (d) mole

    iv.            An interval of 200u s is equivalent to

(a) 0.2 s                       (b) 0.02 s                     (c) 2x10^-1 s                  (d) 2x10^-4 s

      v.            Which one of the following is the smallest quantity?

(a) 0.01 g                     (b) 2 mg                      (c) 100 u g                   (d) 5000 ng

    vi.            Which instrument is most suitable to measure the internal diameter of a test tube?

(a) metre rule               (b) Vernier Callipers   (c) measuring tap        (d) screw gauge

  vii.            A student claimed the diameter of a wire as 1.032 cm using Vernier Callipers. Up to what extent do you agree with it?

(a) 1 cm                       (b) 1.0 cm                    (c) 1.03 cm                  (d) 1.032 cm

viii.            A measuring cylinder is used to measure:

(a) mass                       (b) area                        (c) volume                   (d) level of a liquid

    ix.            A student noted the thickness of a glass sheet using a screw gauge. On the main scale, it reads 3 divisions while 8^th division on the circular scale coincides with index line. Its thickness is:

(a) 3.8 cm                    (b) 3.08 mm                (c) 3.8 mm                  (d) 3.08 m

      x.            Significant figures in a expression are:

(a) all the digits

(b) all the accurately known digits

(c) all the accurately known digits and the first doubtful digit

(d) all the accurately known and all the doubtful digits

Answers of 1.1 (MCQs)

i.	(c)	ii.	(b)
iii.	(d)	iv.	(d)
v.	(d)	vi.	(b)
vii.	(c)	viii.	(c)
ix.	(b)	x.	(c)

 

1.2         What is the difference between base quantities and derived quantities? Give three examples in each case.

Ans:

Sr.#	Base Quantities	Derived Quantities
	Base quantities are the quantities on the basis of which other quantities are expressed.	The quantities that are expressed in terms of base quantities are called derived quantities.
	Base quantities are seven (07) in numbers.	Derived quantities are more than seven (07) in numbers.
	For Example: length, mass, time	For Example: area, volume, speed

 

1.3         Pick out the base units in the following:

joule, newton, kilogramme, hertz, mole, ampere, metre, kelvin, coulomb and watt.

Ans:

Base Units: kilogramme, mole, ampere, meter, kelvin

1.4         Find the base quantities involved in each of the following derived quantities:

(a) speed                       (b) volume                  (c) force                      (d) work

1.5         Estimate your age in seconds.

1.6         What role SI units have played in the development of science?

Ans:

i.                    With the developments in the field of science and technology, the need of commonly acceptable system of units was seriously felt all over the world particularly to exchange scientific and technical information. The SI system fulfils this need.

ii.                  SI unit is in use all over the world.

iii.                They are easy to use because multiples and sub multiples of different units are obtained simply by multiplying or dividing with 10 or some power of 10.

1.7         What is meant by vernier constant?

Ans:

The smallest reading that can be measured by a vernier calliper is called the vernier constant or least count of vernier callipers. Its value is 0.1 mm or 0.01 cm.

OR

It is difference between one small division on main scale and one vernier scale division.

Vernier Constant = (One small division on main scale) – (One vernier scale division)

= 1 mm – 0.9 mm

= 0.1 mm

1.8         What do you understand by the zero error of a measuring instrument?

Ans:

Zero error is basically a systematic error which exist in measuring instrument. It is a type of error in which an instrument gives a reading when the true reading at that time is zero.

For Example: In vernier callipers, when no object is placed between the jaws of vernier callipers, close the jaws of vernier callipers gently, if the zero of main scale does not coincide with the zero of vernier scale then error is called zero error.

1.9         Why is the use of zero error necessary in a measuring instrument?

Ans:

The use of zero error is very necessary in the measuring instruments, if zero error exists in an instrument it shows measurement more or less than actual measurement. i.e. incorrect measurement. The measurement can be corrected by zero correction.

1.10     What is a stopwatch? What is the least count of a mechanical stopwatch you have used in the laboratories?

Ans:

A stopwatch is used to measure the time interval of an event. The mechanical stopwatch that we used in laboratories has a least count of 0.1 second.

1.11     Why do we need to measure extremely small interval of times?

Ans: We need to measure extremely small intervals of times

i.                    To calculate instantaneous time rate of change of a variable.

ii.                  For greater accuracy in results.

iii.                In physics many events occur for very short interval of time, to study that events.

1.12     What is meant by significant figures of a measurement?

Ans:

“In any measurement all the accurately known digits and the first doubtful digit in are called significant figures.”

For example, a student measures the length of a book as 18 cm using a measuring tape. The numbers of significant figures in his/her measured value are two. The left digit 1 is the accurately known digit. While the digit 8 is the doubtful digit for which the student may not be sure.

1.13     How is precision related to the significant figures in a measured quantity?

Ans:

Significant figures reflect the precision of a measured value of a physical quantity. More significant figure means greater precision.