Chapter 6 – Measurement of Physical Quantities
1. Write answers to the following questions in your own words.
(a) Why is the weight of the same object different on different planets?
Ans: The Gravitational force that acts on the mass is called its weight. The gravitational force of the earth that attracts an object towards its center is called the weight of the object. It is also called the weight of the earth. But on different planets, the gravitational force may be of different magnitudes. Therefore, the weight of the same object is different on different planets.
(b) What precautions will you take to make accurate measurements in day-to-day affairs?
Ans:
(i) Appropriate devices or instruments should be used while taking any measurements.
(ii) The device should be used in accordance with scientific principles.
(iii) It is necessary to standardize the weights and measures, which must carry the stamp of standardization.
(iv) The balance should be stable with the pointer upright.
(v) The weight must be made of a proper metal.
(vi) The underside of the pan of the balance should be checked for any tempering.
(c) What is the difference between mass and weight?
Ans:
| Mass | Weight |
|---|---|
(i) The amount of matter present in a substance is called its mass. | (i) The gravitational force that acts on its mass is called its weight. |
(ii) Mass is a scalar quantity. | (ii) Weight is a vector quantity. |
(iii) Mass does not change from place to place. | (iii) Weight changes from place to place. |
2. Who is my companion?
| Group ‘A’ | Group ‘B’ |
|---|---|
(1) Velocity | (a) litre |
(2) Area | (b) kilogram |
(3) Volume | (c) metre/second |
(4) Mass | (d) kilogram/cubic metre |
(5) Density | (e) square metre |
Ans:
| Group ‘A’ | Group ‘B’ |
|---|---|
(1) Velocity | (c) metre/second |
(2) Area | (e) square metre |
(3) Volume | (a) litre |
(4) Mass | (b) kilogram |
(5) Density | (d) kilogram/cubic metre |
3. Explain giving examples.
(a) Scalar quantity
Ans: A quantity that is expressed by its magnitude alone is called a scalar quantity.
For example: Only magnitude, i.e., a value with the unit, is used to express quantities such as length, breadth, area, mass, time, work, temperature, density, etc.
(b) Vector quantity
Ans: The quantity that is expressed completely only when magnitude and direction are both given is called the vector quantity.
For example: The aeroplane travelling at a speed of 500 km/h to the North and another aeroplane travelling at 500 km/h to the East are different from each other. Here, speed is the same, but the direction of velocity is different.
4. Explain, giving examples, the error that occur while making measurements.
Ans:
(1) Not using appropriate devices:
The shopkeeper and the hawkers may not use standardized weights; instead, they may use stone pieces or some other materials, which will result in inaccurate measurements.
For Example: Sometimes while buying gasoline or diesel, the indicators on the machines may not work properly.
(2) Not using the device properly:
While weighing on balance, the jerk given causes the pointer to fluctuate and give an improper reading.
For Example: While measuring the cloth from the bundle, the shopkeeper may not measure the cloth with accuracy. So we should pay attention to the measurements while buying things.
5. Give reasons.
(a) It is not proper to measure quantities by using body parts as units.
Ans: It is not proper to measure quantities by using body parts as units because length of the body parts changes from person to person, so accurate measurement can not be found with the body parts as units.
(b) It is necessary to get the weights and measures standardized at regular intervals.
Ans: Due to constant use, the weights and the weighing apparatus may lose their standardization. Using such devices may result in malpractice. Therefore, It is necessary to get the weights and measures standardized at regular intervals.
6. Explain the need for accurate measurement and the devices to be used for that.
Ans: The need for accurate measurement is dependent on the following factors:
(i) The measurement of anything should be absolutely accurate in our daily lives. If measurements are not done properly, they may have long-lasting negative effects.
(ii) The measurement of precious substances is extremely important. Also, the substances used in very small quantities have to be measured meticulously and accurately. e.g., precious metals like gold and silver should be accurately measured.
(iii) Proper instruments are now available due to advancements in science and technology. Devices that measure very small magnitudes of quantities like distance, mass, time, and temperature are now available. These devices and instruments should be used with precision.
(iv) Digital thermometers are not used for the accurate measurement of body temperature.
(v) Sports competitions like the Olympics need extremely accurate clocks for measuring distances and time.