Muscular system

Question 1:

Fill in the blanks with the right word from the brackets.
(a) The process of digestion starts from the ............. . (stomach / mouth)
(b) Eyelids have ............. muscles (voluntary / involuntary)
(c) ............ is not a function of the muscular system. (production of blood cells / Performing movements)
(d) Muscles of the heart are .............. (ordinary muscles / cardiac muscles)
(e) Pushing forward the food that has been chewed is the function of the ............. . (stomach / oesophagus).

ANSWER:

(a) The process of digestion starts from the mouth.
(b) Eyelids have voluntary muscles.
(c) Production of blood cells is not a function of the muscular system. 
(d) Muscles of the heart are cardiac muscles.
(e) Pushing forward the food that has been chewed is the function of the oesophagus.​
 

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Question 2:

Find a match for me.

Group 'A'	Group 'B'
(1) Cardiac muscles	(a) always function
(2) Are brought about by muscles	(b) we never feel tired.
(3) Pepsin	(c) uncontrolled and painful contraction of muscles.
(4) Cramps	(d) chewing movements of jaws.
(5) Skeletal muscles	(e) enzyme of the gastric juice.

ANSWER:

​

Group 'A'	Group 'B'
(1) Cardiac muscles	(b) we never feel tired.
(2) Are brought about by muscles	(d) chewing movements of jaws.
(3) Pepsin	(e) enzyme of the gastric juice.
(4) Cramps	(c) uncontrolled and painful contraction of muscles.
(5) Skeletal muscles	(a) always function

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Question 3:

Who is telling a lie ?

Organ	Statement
1. Tongue	My taste-buds can tell only a sweet taste.
2. Liver	I am the largest gland in the body.
3. Large intestine	I am 7.5 meter long.
4. Appendix	Digestion is impossible without me.
5. Lung	I play an important role in excretion.

ANSWER:

The tongue, large intestine, appendix and lungs are lying.

1. Tongue can taste salt, sweet and bitter tastes.
2. Large intestine is 1.5 meter long.
3. Appendix is not required for digestion.
4. Kidneys play an important role in excretion.

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Question 4:

Give reasons.
(1) Food becomes acidic in the stomach.
(2) Cardiac muscles are said to be involuntary muscles.
(3) Intoxicating substances should not be consumed.
(4) Your muscles should be strong and efficient.

ANSWER:

a. Food becomes acidic in stomach because of the presence of hydrochloric acid in the stomach. As the food reaches the stomach, it gets mixed with the hydrochloric acid present there. The hydrochloric acid plays the following roles in digestion:

1. It initiates protein digestion by activating the conversion of pepsinogen to pepsin.

2. It provides acidic medium for the activity of pepsin and other enzymes present in gastric juice.

b. Cardiac muscles are said to be involuntary because they are not controlled by our will. These muscles are found in heart and are the only muscles which work throughout the life without getting tired or fatigue.

c. Intoxicating substances should not be consumed because they harm the body directly or indirectly. Substances such as alcohol cause damage to the nervous system and the digestive system as well. Liver is the main organ which is affected by alcohol. Tobacco is one of the leading causes of oral cancer. Smoking has a direct effect on the respiratory system of an individual and can lead to lung cancer as well.

d. Our muscles should be strong and efficient because they carry out various vital functions in our body. From helping us to breath, digest food to helping us in walking and lifting weights, muscles play an important role. If our muscles are not strong enough we would get tired easily and feel fatigue. It is required to keep them in a healthy state by regular exercising and taking proper diet. 

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Question 5:

Answer the following.
(a) How many types of muscles are there? Which are those types?
(b) What causes the problem of acidity? What is its effect on the body?
(c) Name the different types of teeth. What is the function of each type?

ANSWER:

a. There are 3 types of muscles:

• Skeletal muscle fibre: They are found attached to the skeletal bones and are voluntary. Skeletal muscle fibres are striated and are bundled together in a parallel manner by a sheath of tough connective tissues.

• Smooth muscle fibres: They are present in the walls of internal organs such as blood vessels, stomach, etc., and are involuntary. Smooth muscle fibres are fusiform (taper at both ends) and non- striated. They are held together by cell junctions and are bundled together in a sheath of connective tissues.

• Cardiac muscle fibres: This is the only type of muscle present in the heart. They are contractile in nature and are involuntary. Plasma membranes of cardiac cells are fused together by cell junctions, and hence, the cells stick together. Communication junctions present as intercalated discs facilitate the contraction of cardiac cells as a unit.

b. Stomach contains special cells called oxyntic or parietal cells which secrete hydrochloric acid in stomach. It plays two important roles like:

1. It initiates protein digestion by activating the conversion of pepsinogen to pepsin.

2. It provides acidic medium for the activity of pepsin and other enzymes present in gastric juice.

However, if the level of this HCl exceeds its normal levels, it results in acidity/acid reflux. Higher levels of HCl can result in the following effects on the body:

1. burning sensation in the stomach
2. burning sensation in the throat and heart
3. difficulty in swallowing
4. regurgitation
5. restlessness
6. belching
7. nausea
8. prolonged sour taste in the mouth
9. indigestion

Exposure to higher levels of HCl for longer durations can also result in the development of peptic ulcers/ulcers in the stomach. 


c. There are 4 types of teeth which are found in humans:

1. Incisors are the teeth present at the front portion of the mouth. There are four incisors in each jaw. They are used for biting and cutting food.
2. Canines are located next to the incisors. There are two canines in each jaw. They are used for tearing and piercing food.
3. Premolars lie next to the canines. There are four premolars in each jaw. They are used for chewing and grinding food.
4. Molars lie next to the premolars at the end of the jaw. There are six molars in each jaw. They are also used for chewing and grinding food.

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Question 6:

Sketch and label a diagram of the digestive system and describe it in your own words.

ANSWER:

Major constituent organs of the human digestive system are: buccal cavity, oesophagus, stomach, small intestine, large intestine, rectum and anus.

Buccal cavity − It includes the teeth, saliva and tongue. The teeth break down the food. Digestion of food begins in the mouth. The tongue helps in the chewing and swallowing of food.

Oesophagus − The food passes from the mouth, down the oesophagus, into the stomach, as a result of the movement of the walls of the oesophagus.

Stomach − It mixes the food received from the oesophagus with digestive juices.

Small intestine − The food from the stomach moves into the small intestine, which receives intestinal juices from two glands − liver and pancreas. These juices help in the further digestion of food.

Large intestine − Water is absorbed in the large intestine.

Rectum and anus − Undigested food particles are thrown out with the help of the rectum and anus.

Std. 7.Sound

Question 1:

Fill in the blanks .

(a) Sound is generated by the rhythmic ............. of an object.

(b) The frequency of sound is measured in ............... .

(c) If ............. of sound is decreased , its loudness also decreases.

(d) A medium is necessary for .................... of sound.

ANSWER:

(a) Sound is generated by the rhythmic vibration of an object.

(b) The frequency of sound is measured in Hertz (Hz).

(c) If amplitude of sound is decreased, its loudness also decreases.

(d) A medium is necessary for propagation of sound.

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Question 2:

Match the pairs

Group 'A'                       Group 'B'

(a) Flute                         (1) Frequency less than 20 Hz

(b) Frequency                (2) Frequency more than 20000 Hz

(c) Sound level              (3) Vibrations in the air 

(d) Ultrasonic sound     (4) Measured in Hz

(e) Infrasonic sound      (5) Decibel

ANSWER:

Group 'A'	Group 'B'
(a) Flute	(3) Vibrations in the air
(b) Frequency	(4) Measured in Hz
(c) Sound level	(5) Decibel
(d) Ultrasonic sound	(2) Frequency more than 20000 Hz
(e) Infrasonic sound	(1) Frequency less than 20 Hz

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Question 3:

Give scientific reasons.

(a)  In earlier times, people used to listen for the arrival of a distant train by putting their ear to the rail.

(b) The sounds generated by a tabla and a sitar are different.

(c) If you were both on the moon , your friend will not be able to hear you call.

(d) We can hear the movement of a mosquito's wings but we cannot hear the movement of our hands.

ANSWER:

(a) In earlier times, people used to listen for the arrival of a distant train by putting their ear to the rail because they knew that sound travels faster through solids than gases. Thus, by putting their ear to the rails used to give them the idea of the arrival time of a train in advance.

(b) Due to difference in the pitch and timbre, the sound produced by a tabla and a sitar are different.

(c) We know that sound cannot travel through vacuum. Since there is no atmosphere on the moon atmosphere, so we cannot hear sound on the Moon. 

(d) The frequency of sound produced by the movement of mosquito's wings is in the audible range of humans whereas that produced by the movement of our hands falls in the inaudible range. Due to this reason, we can hear the movement of a mosquito's wings but cannot hear the movement of our hands.

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Question 4:

Write answers to the following questions .

(a) How is sound produced ?

(b) What does the intensity of sound depend upon ?

(c) Explain how the frequency of oscillation is related to the length of a pendulum and the amplitude of its oscillation.

(d) Explain the two ways by which the pitch of the sound generated by a stretched string can be changed.

ANSWER:

(a) The back and forth movement of an object produces sound. An object moving back and forth is said to be in vibration. Hence, sound is produced by vibrating objects.

(b) Intensity of sound depends on the amplitude of vibration of the sound. It is proportional to the square of amplitude of vibration of the sound.

(c) Frequency of oscillation of a pendulum decreases with increase in the length of the pendulum and vice-versa. It is independent of the amplitude of oscillation of pendulum provided the amplitude is small.

(d) Frequency or pitch of vibration of a stretched string can be changed by varying the

• tension in the string: If the string of the instrument is under high tension, then the pitch of the note produced on plucking the string will be high i.e. the note will be shriller because frequency of vibration of the string will be high.
• thickness of the string: If the string of the instrument is thin, then the pitch of the note produced on plucking the string will be high as frequency of vibration of the string will be high.

Std. 7 Properties of magnetic field

Question 1:

Write the appropriate term in the blanks .

(a) The alloys called.......... and  ..............  are used for making industrial magnets.

(b) A magnetic field can pass through ............ and  ............... .

(c) The intensity of a magnetic field is indicated by the lines of ................ .

(d) The real test of a magnet is .............. .

ANSWER:

(a) The alloys called alnico and nipermag are used for making industrial magnets.

(b) A magnetic field can pass through cardboard and water.

(c) The intensity of a magnetic field is indicated by the lines of magnetic force passing perpendicularly through a unit area.

(d) The real test of a magnet is repulsion.

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Question 2:

With whom should I pair up?

Group 'A'                            Group 'B'

(a) Compass                       1. The highest magnetic force 

(b) Door of a cupboard      2. Like poles

(c) Repulsion                     3. A magnet

(d) Magnetic pole              4. A magnetic needle 

ANSWER:

Group 'A'	Group 'B'
(a) Compass	4. A magnetic needle
(b) Door of a cupboard	3. A magnet
(c) Repulsion	2. Like poles
(d) Magnetic pole	1. The highest magnetic force

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Question 3:

Write answers to the following questions:

(a) Distinguish between the two methods of making artificial magnets.

(b) Which substances are used for making electromagnets?

(c) Write a note on 'magnetic field'.

(d) Why is a magnetic needle used in a compass?

(e) Explain with the help of a diagram how the intensity and direction of the magnetic field of a bar magnet can be determined.

ANSWER:

(a) The two methods of making artificial magnets are:

Single Touch Method	Double Touch Method
The magnetic strength of the magnet created using this method is low.	The magnetic strength of the magnet created using this method is high as compared to single touch method.
The magnetism of the magnet created using this method lasts for a shorter time.	The magnetism of the magnet created using this method lasts for a longer time as compared to single touch method.

(b) To make an electromagnet we require the following things:

• A long conducting copper wire
• An iron nail on which the copper wire has to be wound
• A switch
• A battery

(c) Magnetic field is defined as the region around a magnet where the magnetic force can be experience by an object. Magnetic field lines are the geometrical representation of magnetic field. Following are the characteristics of magnetic field lines:

• Magnetic field lines emanate from the North Pole and terminate at the South Pole of a magnet (outside the magnet).
• The degree of closeness of magnetic field lines represents the relative strength of the magnet.
• No two field lines can intersect each other. 

Field lines due to a bar magnet is shown below:

(d) The Earth behaves as a magnet with its magnetic South-Pole coinciding with the geographic North-Pole and magnetic North-Pole coinciding with the geographic South-Pole. Thus, in a compass, a magnetic needle is used to show the geographic north direction by aligning itself in the direction magnetic South-Pole of the Earth.

(e) Take a drawing cardboard and sprinkle some iron filings on it. Notice the position of the iron filings as a whole. Now, take a bar magnet and slowly bring it below the cardboard. You will observe that the iron filings tend to attract towards the magnet as shown below.

To find the direction of the magnetic field around a bar magnet, place a magnetic compass with its South-Pole near one of the poles of the bar magnet. Locate and the point where the North-pole of the needle points. Now, lift the needle and place its South-Pole on the marked point. Now, again see where it North-Pole points. Repeat this process and you will get the direction of the magnetic field as shown below.

To find the intensity of magnetic field around a region of a bar magnet, we need to calculate the number of lines crossing through unit area of that region. For example, if we have to calculate the intensity of magnetic field at points A and B, then draw unit areas around A and B first and then calculate the number of lines passing through that area. By doing so, we see that intensity of field at A is greater than that at B.

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Question 4:

Give detailed information about how the merchants of olden times used a magnet while travelling.

ANSWER:

In the past, it was known that a piece of magnetite (stones having magnetic properties), when hung freely, points in the North-South direction. Thus, these stones were used by merchants of olden times to find the direction while travelling through unknown regions. These stones are also called loadstones (leading stones).

Heat (Std. 8)

Question 1:

A. Whom should I Pair with?

Group A	Group B
a. Temperature of a healthy human body	296 K
b. Boiling point of water	98.6∘F
c. Room temperature	0∘C
d. Freezing point of water	212∘F

B. Who is telling the truth?
a. The temperature of a substance is measured in joules.
b.Heat flows from an object at higher temperature to an object at lower temperature.
c. Joule is the unit of heat.
d. Objects contract on heating.
e. Atoms of a solid are free.
f. The average kinetic energy of atoms in a hot objects is less than the average kinetic energy of atoms in a cold objects.

C. You will find if you search.
a. A thermometer is  used to measure............... . 
b. The apparatus used to measure heat is called a..................... .
c. Temperature is the measures of the............................ Kinetic energy of the atoms in a substance.
d. The heat contained in a substannce is the measures of the....................... kinetic energy of atoms in the substance. 

ANSWER:

A. 

Group A	Group B
a. Temperature of a healthy human body	98.6∘F
b. Boiling point of water	212∘F
c. Room temperature	296 K
d. Freezing point of water	0∘C

B. 
a. Sentence a is lying as heat energy is measured in joules.
b. Sentence b is telling the truth as heat energy flows from an object at higher temperature to an object at lower temperature.
c. Sentence c is telling the truth as joule is the unit of heat.
d. Sentence d is lying as objects expand on heating.
e. Sentence e is lying as atoms of a solid are closely packed because of force attraction between them.
f.  Sentence f is lying as the average kinetic energy of atoms in a hot objects is greater than the average kinetic energy of atoms in a cold objects.

C.
a. A thermometer is  used to measure temperature.
b. The apparatus used to measure heat is called a calorimeter.
c. Temperature is the measures of the average kinetic energy of the atoms in a substance.
d. The heat contained in a substance is the measures of the total kinetic energy of atoms in the substance. 

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Question 2:

Nishigandha kept a vessel containing all the ingredients for making tea in a solar cooker. Shivani kept a similar vessel on a stove. Whose tea will be ready first and why?

ANSWER:

Shivani's tea will be prepared first.
In Shivani's case, the intensity of the flame in contact with the vessel is very high due to which the flow of heat will be faster. Thus, the time taken by the tea to reach its boiling point will be less. Hence, tea will be prepared fast.
In case of Nishigandha, the intensity of radiation reaching the vessel is not as high as compared to stove's flame. Thus, the flow of heat will be slower in this case due to which more time will be taken to reach the boiling temperature of the tea. Hence, tea will be prepared at slower rate.

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Question 3:

Write brief answers.
a. Describe a clinical thermometer. How does it differ from the thermometer used in laboratory?

b. What is the difference between heat and temperature ? what are their units.

c. Explain the construction of a calorimeter. Draw the necessary figure.

d. Explain why rails have gaps at specific distances.

e. Explain with the help of formulae the expansion coefficients of liquid and gas. 

ANSWER:

a. Clinical thermometer is used in homes. It has a glass tube with a bulb at one end and the other end closed. This bulb and some part of the tube is filled with thermometric liquid, such as mercury or alcohol. The rest of the volume of the tube has vacuum in it. There is a kink near the end of the glass tube which prevents the falling back of the thermometric liquid once it is removed from the hot body under observation.
It is basically used to measure the temperature of humans. A clinical thermometer has a temperature range of only 35 ºC to 42 °C. 


b.

Clinical thermometer	Laboratory thermometer
This thermometer is used in homes. It is basically used to measure the temperature of human body.	This thermometer is used to measure the temperature of all things, except the human body.
A clinical thermometer has a temperature range of only 35 ºC to 42 °C.	A laboratory thermometer has a temperature range of -10 ºC to 110 ºC.
It has kink.	It does not have kink.

b. 

Heat	Temperature
It is a form of energy which causes in us the sensation of hotness or coldness.	It is measure of the degree of hotness or coldness of an object.
It is measured in joules (J).	It is measure in kelvin (K), Celsius (oC) and Fahrenheit (oF)

c. Calorimeter

A device used for heat measurement is called a calorimeter.

Construction of a Calorimeter

• It consists of a metallic vessel and stirrers. They are made of copper or aluminium.

• The vessel is then kept inside a wooden jacket which contains heat-insulating materials.

• The outer wooden jacket acts as a heat shield, and reduces the heat loss from the inner vessel.

• The outer jacket has an opening through which a mercury thermometer is inserted into the calorimeter.

​

The Universe

Question 1:

Name these –
(a) Birth place of stars
(b) Biggest planet in the solar system
(c) The galaxy which is our neighbour.
(d) Brightest planet in the solar system
(e) Planet with largest number of satellites
(f) Planets without a single satellite
(g) Planet with a rotation different from other planets.
(h) A celestial body that carries a tail along

ANSWER:

(a) Birth place of stars: Nebula
(b) Biggest planet in the solar system: Jupiter
(c) The galaxy which is our neighbour: Andromeda
(d) Brightest planet in the solar system: Venus
(e) Planet with largest number of satellites: Jupiter
(f) Planets without a single satellite: Mercury and Venus
(g) Planet with a rotation different from other planets: Venus and Uranus
(h) A celestial body that carries a tail along: Comets

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Question 2:

Fill in the blanks.
(a) The group of galaxies of which our Milky Way is a part is called ............. .
(b) Comets are made of ............. .
(c) The planet ............. appears as if it is rolling along its orbit.
(d) ............. is a stormy planet.
(e) The Pole Star is the best example of a ............. type of star.

ANSWER:

(a) The group of galaxies of which our Milky Way is a part is called 'Local Group'.
(b) Comets are made of ice and dust particles.
(c) The planet Uranus appears as if it is rolling along its orbit.
(d) Jupiter is a stormy planet.
(e) The Pole Star is the best example of a variable type of star.

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Question 3:

Say if the statements given below are right or wrong. Rewrite the statements after correcting them.
(a) Venus is the planet closest to the sun.
(b) Mercury is called a stormy planet.
(c) Jupiter is the biggest planet.

ANSWER:

(a) No, the statement is incorrect. Mercury is the planet closest to the Sun.

(b) No, the statement is incorrect. Jupiter is called a stormy planet.

(c) Yes, Jupiter is the biggest planet.

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Question 4:

Answer the following.
(a) What is a special characteristic of the planet Mars ?
(b) What are the types of galaxies ?
(c) Which celestial bodies does a galaxy include ?
(d) Name the different types of stars.
(e) What are the types of comets and on what basis are they classified ?
(f) What is the difference between meteors and meteorites ?
(g) What are the characteristics of the planet Neptune ?

ANSWER:

(a) The special characteristic of planet Mars is that it is red in colour. This is due to the presence of iron in its soil.

(b) Various types of galaxies which are identified according to their shapes are:

• Spiral galaxy: These are the spiral structures that extend from the centre into the disc. It consists of stars, gas and dust in a bulge at the centre and in the extended spiral arms.
• Elliptical galaxy: These galaxies have nearly ellipsoidal shape which consists of stars, gas and dust.
• Irregular galaxies: These galaxies don’t have a distinct regular shape.
• Barred spiral galaxies: These are spiral galaxies with a central bar-shaped structure composed of stars.

(c) A galaxy includes stars, cluster of stars, nebulae, clouds of gases and dusts, dead stars, newly born stars etc.

(d) The different types of stars in the universe are stated below:
Sun-like Stars: These are particularly similar to the Sun in size but there is a lot of variation in terms of temperature. Examples are Alpha Centurai, Tau Ceti etc.
Red Giants: These stars do not have as hot outer layer as that of the Sun with temperature ranging from 3000 ^oC to 4000 ^oC but they have a very high luminance than the Sun. They are red in color and their diameter is 10 to 100 times that of the Sun.
Super Nova: They are larger and even brighter as compared to the red giant stars and even the Sun. They are also considered as the primary source of heavy elements in the universe.
Binary or Twin Stars: A pair of two stars in which one revolves around the other or they both revolve around a common centre is known as binary or twin stars. 
Variable Stars: These stars have a variable shape and brightness i.e it keeps on changing whenever they expand or contract. Their brightness decreases when they expand and increases when they contract. Example is Pole Star.

(e) Comets are the solar system bodies that are usually spherical in shape and develop tail when they come near the Sun. Comets become visible when they are close to the Sun and the Earth.

Comets can be categorized into two groups depending on how long they take to complete one revolution around the sun. These are:

• Long Period Comets: These comets complete one revolution around the Sun in 200 and more years. Great Comet of 1843 is one of the examples of this type.
• Short Period Comets: These comets complete one revolution around the Sun in years less than 200. One of the examples is Halley's comet that takes 76 years to complete the revolution. It last appeared in 1985.

(f) Meteors: These are rocky pieces originating from the asteroid belt and enter the Earth’s atmosphere as a result of its gravitational pull. On entering the Earth’s atmosphere, they acquire very high speeds. These high speeds of meteors cause friction with the Earth’s atmosphere and thus the meteors heat up and evaporate completely. Meteors are also known as shooting stars.

Meteorites: These are the unburnt part of meteors which fall to the surface of Earth on ocassions when the meteors do not burn completely in the atmosphere itself.

(g) Following are the characteristics of planet Neptune:

• A season on this planet lasts for about 41 years.
• Wind blows with extremely high speed on this planet.

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Question 5:

Match the following

Group A	Group B
(1) Galaxy	(a) From east to west
(2) Comet	(b) 33 satellites
(3) Sun-like star	(c) Spiral
(4) Saturn	(d) Sirius
(5) Venus	(e) Halley

ANSWER:

Group A	Group B
(1) Galaxy	(c) Spiral
(2) Comet	(e) Halley
(3) Sun-like star	(d) Sirius
(4) Saturn	(b) 33 satellites
(5) Venus	(a) From east to west

our skeletal system

Question 1:

Fill in the blanks with the proper word.
(a) The place where two or more bones are connected is called a .............. .
(b)Cells of the epidermis contain a pigment called .............. .
(c) ...................... and ...................... are the two layers of the human skin.
(d)The human skeletal system is divided into ...................... parts.

ANSWER:

(a) The place where two or more bones are connected is called a joint.
(b)Cells of the epidermis contain a pigment called melanin.
(c) Epidermis and dermis are the two layers of the human skin.
(d)The human skeletal system is divided into two parts.

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Question 2:

Match the pairs.

'A'	'B'
(1) Ball and socket joint	(a) Knee
(2) Hinge joint	(b) Wrist
(3) Gliding joint	(c) Shoulder

ANSWER:

'A'	'B'
(1) Ball and socket joint	(c) Shoulder
(2) Hinge joint	(a) Knee
(3) Gliding joint	(b) Wrist

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Question 3:

Right or wrong? If wrong, write the correct sentence.
(a) Bones are soft.
(b)The human skeleton protects the internal organs.

ANSWER:

(a) Bones are soft- Wrong
Bones are hard and non flexible.

(b)The human skeleton protects the internal organs- Right

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Question 4:

Put a  mark at the proper places.
(a) The system which gives a definite shape to our body.
□$\mathbf{\square }$ Excretory system
□$\mathbf{\square }$ Respiratory system
□$\mathbf{\square }$ Skeletal system
□$\mathbf{\square }$ Circulatory system

(b) The ................ joint is seen in fingers and toes.
□$\mathbf{\square }$ Hinge joint
□$\mathbf{\square }$ Ball and socket joint
□$\mathbf{\square }$ Immovable joint
□$\mathbf{\square }$ Gliding joint

ANSWER:

(a) The system which gives a definite shape to our body.
□$\mathbf{\square }$ Excretory system
□$\mathbf{\square }$ Respiratory system
 Skeletal system
□$\mathbf{\square }$ Circulatory system

(b) The ................ joint is seen in fingers and toes.
 Hinge joint
□$\mathbf{\square }$ Ball and socket joint
□$\mathbf{\square }$ Immovable joint
□$\mathbf{\square }$ Gliding joint

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Question 5:

Answer the following questions in your words.
(a)What are the functions of your skin?
(b)What should you do to keep your bones strong and healthy?
(c)What are the functions of the human skeletal system?
(d)Which are the various reasons due to which our bones might break?
(e) What are the different types of bones? How many types are there?

ANSWER:

 a. The most important functions of the skin are:

• Protection − The skin provides protection to the body from mechanical injury, ultraviolet rays, from disease-causing microbes, and prevents desiccation.

• Temperature regulation − The skin helps in the maintenance of body temperature at a constant.

• The skin contains numerous sense receptors, which help in perceiving the surroundings.

• The skin is also involved in the synthesis of vitamins such as Vitamin D.

• The skin also helps in disposing off the excess water and salt by evaporation.​

Ans b. The following things should be done in order to keep our bones healthy and strong:

• Have calcium rich foods like milk, yoghurt as calcium is an important component of bones.
• We need to take enough vtiamin D as it helps in the absorption of calcium present in foods.
• Some other important nutrients like potassium, vitamin K and magnesium also help the body to absorb and use calcium. 
• Daily exercise is required to keep our bones in a state of good health.

Ans c. The skeletal system includes all of the bones and joints in the body. It performs the following functions:

• gives a definite shape to the body
• provides support and protection for the soft tissues that make up the rest of the body
• provides attachment points for muscles to allow movements at the joints

Ans d. Bones are structurs which are hard and non flexible. The chances of breaking of bones occurs, if the bones are weak. There are various reasons due to which bones may become weak and may break:

• certain diseases can result in weakening of the bones and cause them to break. For example, osteoporosis and arthirits are diseases which result in the weakening of bones
• defeciency of nutrients like calcium and vitamin D 
• certain stress or injury may result into fracturing of bones
• bone loss also occurs with age and thus making them more fragile
• alcohol and smoking also have a negative impact on bones and bone density

Ans e. There are 4 types of bones:

1. Flat bones: They are thin shaped and, in some cases, provide mechanical protection to soft tissues beneath or enclosed by the flat bone.
2. Irregular bones: They have complicated shapes which cannot be classified as 'long', 'short' or 'flat'. Their shapes are due to the functions they fulfill within the body.
3. Long bones: These type of bones are longer than wider and they function to support the weight of the body and facilitate movement
4. Short bones: They are almost cube shaped bones as they are equal in their length and width.

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Question 6:

What will happen if -?
(a)There are no joints in our body.
(b)There is no melanin pigment in our skin.
(c) Instead of 33 vertebrae in our body, we had one single and straight bone.

ANSWER:

a. Joints are the places where 2 bones are joined together. They permit the various kinds of movement which we carry out in order to fulfill our requirements. In absences of these joints our body would become stiff and we would not be able to carry out our daily tasks.

b. Melanin is a pigment which is found in skin and hair as well. It is important for two reasons, firstly it imparts colour to our skin and the tone of our skin depends on the amount of melanin pigment, secondly it protects our skin and the inner parts from ultravoilet rays. If the melaning pigment is absent, all the humans will have the same skin colour and we will become susceptible to the UV rays present in our atmosphere.

c. The 33 vertebrae together form the structure which we know as spine. They permit the movement of our spine so that we can bend it easily and carry out other functions. In other words we can say that they provide flexibilty to the spine. If in place of them, only one single and straight bone will be present then we would loose our flexiblity and remain in fixed positions. We would always stand straight like a pole and would not be able to sit down or perform other activities.

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Question 7:

Draw diagrams.
(a) Types of joints
(b) Structure of the skin

ANSWER:

(a) Types of joints

1. Gliding joint




2. Ball and socket joint


3. Hinge joint




(b.) Structure of skin

Std. 8 Light

Question 1:

Fill in the blanks
i. The perpendicular to the mirror at the point of incidence is called....................... .
ii. The reflection of light from a wooden surface is.................. reflection.
iii. The working of Kaleidoscope is based on the properties of................... .

ANSWER:

i. The perpendicular to the mirror at the point of incidence is callled normal.
ii. The reflection of light from a wooden surface is diffused reflection.
iii. The working of Kaleidoscope is based on the properties of reflection of light.

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Question 2:

Draw a figure describing the following.
The reflecting surface of two mirrors make an angle of 90^∘ with each other. If a ray incident of one mirror has an angle of incidents of 30^∘, draw the ray reflected from the second mirror. What will be its angle of reflection? 

ANSWER:

Let r₂ be the angle of reflection from the second mirror.

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Question 3:

How will you explain the statement 'we cannot see the objects in a dark room?

ANSWER:

Light enables us to see objects around us. We see an object when light falls on it and gets reflected from its surface and enters our eyes. In a dark room, there is no source of light. Thus, no light falls on the surface of objects and hence we do not see them. This is why we cannot see the objects in a dark room.

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Question 4:

Explain the differences between regular and irregular reflection light.

ANSWER:

Regular reflection of light	Irregular reflection of light
This type of reflection occurs on smooth surfaces such as a plane mirror.	This type of reflection occurs on rough surfaces such as a black board.
In this type of reflection, the reflected rays are parallel to each other.	In this type of reflection, the reflected rays are non-parallel to each other.
In this type of reflection, the reflected rays are in one direction.	In this type of reflection, the reflected rays are in different direction.

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Question 6:

Study the following incident.
Swara and Yash were looking in a water filled vessel. They could see their images clearly in the still water. At that instant, Yash threw a stone in the water. Now their images were blurred. Swara could not understand the reason for the blurring of the images. 

Explain the reason for blurring of the images to swara by answering the following questions.

i. Is there a relation between the reflection of light and the blurring of the images.?

ii. Which types of reflection of light can you notice from this?

iii. Are laws of reflection followed in these types of reflection? 

ANSWER:

i. Yes, there is a relation between the reflection of light and blurring of the images.

ii. Initially, the water was still and its surface was smooth. Thus, regular type of reflection was taking place from its surface i.e. all the light rays coming from Swara's body or Yash's body reflected in the same direction on hitting the water surface and reached our eyes. Because of this, we saw a clear image. 
But when Yash threw a stone in the water, the water surface became rough due to which irregular type of reflection of light occurred on its surface i.e all the light rays coming from Swara's body or Yash's body reflected in different directions on hitting the water surface. Thus, only some part of the reflected rays reached our eyes. Because of this, we see a blurred image.

iii. Yes, laws of reflection are followed in both the types of reflection.

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Question 7:

Solve the following examples.
a. If the angle between the plane mirror and the incident ray is 40^∘, what are the angles of incidence and reflection?
b. If the angle between the mirror and reflected ray is 23^∘, what is the angle of incidence of the incidence ray? 

ANSWER:

a