Science textbook solutions for Class 7 Science Chapter 6 Measurement of Physical Quantities

Important points to remember :

1. Quantities such as mass, weight, distance, speed, temperature, volume are called physical quantities. 
2. An international  system of units based on seven fundamental units, called the System International (SI),  is  currently used  all over the world.  It is also  called the  metric system. 
3. Scalar quantity A quantity  that  can be completely expressed by its magnitude alone is called a scalar quantity. 
4. The quantity that is  expressed completely  only when magnitude and direction  are both given is called a vector  quantity. 
5. Our body  has weight because of the gravitational force of the  earth.  
6. The gravitational  force of the moon  being less, our weight will  turn  out to  be less there. Our mass, however, is the same at both the places.
7. There  are many physical quantities  but a majority  of them are related  to each  other.  For example,  you have learnt  that the  quantity  ‘speed’ is the  ratio  of the quantities  ‘distance’ and ‘time’.

Question 1: Write answers to the following questions in your own words.
a) Why is the weight of the same object different on different planets?
Answer : Your mass doesn’t change on other planets, but the gravity and size of those planets affect how much you weigh. So, your weight depends upon the gravity of the planet and your weight keeps changing based on planet gravity.

The amount of gravity depends on the size of the planet. A small planet has less gravity and the larger planet has more gravity. If you stand on more massive planets like Jupiter you will weigh more and if you stand on less massive planets like Mars you will weigh less.

b) What precautions will you take to make accurate measurements in day-to-day affairs?
Answer : The precaution, that is to be taken to make accurate measurement in day to day affairs is to : ‘Think hundreds of times, before uttering a word. ‘ This can help you a lot in making an accurate measurement in any affair, as, most of the quarrels occur when we speak before thinking ’bout the matter.

c) What is the difference between mass and weight?
Answer : The terms “mass” and “weight” are used interchangeably in ordinary conversation, but the two words don’t mean the same thing. The difference between mass and weight is that mass is the amount of matter in a material, while weight is a measure of how the force of gravity acts upon that mass.

1. Mass is a property of matter. The mass of an object is the same everywhere. Weight depends on the effect of gravity. Weight increases or decreases with higher or lower gravity.
2. Mass can never be zero. Weight can be zero if no gravity acts upon an object, as in space.
3. Mass does not change according to location. Weight varies according to location.
4. Mass is a scalar quantity. It has magnitude. Weight is a vector quantity. It has magnitude and is directed toward the center of the Earth or other gravity well.
5. Mass may be measured using an ordinary balance. Weight is measured using a spring balance.
6. Mass usually is measured in grams and kilograms. Weight often is measured in newtons, a unit of force.

Question 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

Answer :
1) Velocity – c) metre/second
2) Area – e) square metre
3) Volume – a) litre
4) Mass – b) kilogram
5) Density – d) kilogram/cubic metre

Question 3: Explain giving examples.
a) Scalar quantity
Answer : A quantity which has only magnitude and no direction is known as scalar quantity. Distance, work, speed, time, energy, etc. are all scalar quantities. These quantities can be expressed completely by their magnitude alone.

b) Vector quantity
Answer : A quantity which has magnitude as well as direction is known as vector quantity. Force, velocity, displacement, etc. are all vector quantities. These quantities are expressed completely only when their directions are defined along with their magnitude.

Question 4: Explain, giving examples, the errors that occur while making measurements.
Answer : Measurement Error
Definition: The measurement error is defined as the difference between the true or actual value and the measured value. The true value is the average of the infinite number of measurements, and the measured value is the precise value.

Types of Errors in Measurement
The error may arise from the different source and are usually classified into the following types. These types are
1.Gross Errors
2. Systematic Errors
3. Random Errors

1. Gross Errors
The gross error occurs because of the human mistakes. For examples consider the person using the instruments takes the wrong reading, or they can record the incorrect data. Such type of error comes under the gross error. The gross error can only be avoided by taking the reading carefully.

2. Systematic Errors
The systematic errors are mainly classified into three categories.
1. Instrumental Errors
2. Environmental Errors
3. Observational Errors

3. Random Errors
The error which is caused by the sudden change in the atmospheric condition, such type of error is called random error. These types of error remain even after the removal of the systematic error. Hence such type of error is also called residual error.

Question 5: Give reasons.
(a) It is not proper to measure quantities by using body parts as units.
Answer: It is not proper to measure quantities by using body parts as units because these are not reliable and unique. These units of measurement vary from person to person.

b) It is necessary to get the weights and measures standardized at regular intervals.
Answer : Because of difference in physical conditions from place to place, the reference chosen as units for weights and measures varies. So, it becomes necessary to get the weights and measures standardized at regular intervals.

Question 6: Explain the need for accurate measurement and the devices to be used for that.
Answer : In ancient times people use the foot, the footstep, the handspan or the arm to measure length. But these units of measurement were unreliable because they vary from person to person. Measurement is required for a variety of things in our daily life. If you want to get a dress stitched then you need to know the length of the cloth that would be required to make it. Similarly when you have fever you measure the temperature of your body by a thermometer. To make a cake the ingredients have to be used in the right proportion. All these measurements must be made accurately using the standard units. There was a need for units of measurement which could be uniformly used by everyone units are called standard units.

In our daily life we use various types of measuring devices. the type of device to be used depends on the type of work and accuracy that is needed.

# Use your Brain power 
1. Why would the weight of an object  be maximum at the poles and minimum  at the equator ?
Answer :  Since, the equator is more far from centre of earth as compared to that of distance of poles from center of earth. Hence, the more the distance will be the less will be the weight measured. This is the main reason reason why the weight of an object is maximum at poles as compared to that on equator

2. Why is the weight of an object at a high altitude less than its weight at the sea-level ?
Answer : Since the Earth’s attraction decreases as the square of the distance to its center, the weight of an object is slightly smaller at high altitude (e.g., at the top of a mountain) than at sea level, or at the equator than at the poles (because the Earth is slightly bulging). Objects in orbit travel with very high speed.

# Find out 
1. What is an atomic  clock ? Where  is it kept ?
Answer : Atomic clocks are used to coordinate systems that require extreme precision, such as Global Positioning System ( GPS ) navigation and the Internet. A group of atomic clocks located in a number of places throughout the world is used in conjunction to establish Coordinated Universal Time ( UTC ). The master atomic clock ensemble at the U.S. Naval Observatory in Washington, D.C., which provides the time standard for the U.S. Department of Defense. The rack mounted units in the background are Microsemi (formerly HP) 5071A caesium beam clocks.

2. How is the velocity of light used for determining the standard metre ?
Answer : Velocity of the ;light in air or Vacuum = 3 × 10⁸ m/s. For the convenience, We used to take the value as 3 × 10⁸ m/s. Although, For determine the definition of the meter, correct speed of the light is taken. 1 meter is defined as the distance travelled by the light in 1/299792000 s.

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