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Question 1:

What are the effects that a force can have on an object?

Answer:

A force can have following effects on an object:

  • It can bring a stationary object into motion and thus, bring about a change in its previous position of rest. For example, kicking a ball which is at rest moves it away from its earlier position.
  • Speed of a moving object can be changed by force. For example, a goalkeeper stops the ball by applying force.
  • Force can bring about a change in the direction of motion of a moving object. For example, when a batsman hits a ball with his bat, the ball's direction alters.
  • Force can also change the size and shape of the object. Example, when a sponge is squeezed, it shrinks in size, and its shape changes too.

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Question 2:

How can you electroplate an iron nail with copper? Explain with the help of a diagram.

Answer:



To electroplate copper on an iron nail, copper sulphate solution is taken in a beaker. A wide copper strip and the iron nail are connected with a battery and dipped into the beaker.
Copper sulphate when dissolved into water forms positively charged copper ions and negatively charged sulphate ions.
When an electric current is passed through the solution, the copper ions (cations) move towards the cathode (iron nail) and get deposited over it. At the anode, the copper strip loses electrons to form copper ions in the solution.

Page No 258:

Question 1:

Which friction is smaller−sliding friction or rolling friction?

Answer:

Rolling friction is smaller than sliding friction. A rolling friction is the force of friction between two surfaces, that takes place when one surface rolls over the other, whereas sliding friction is the force of friction between two surfaces that occurs when one surface is dragged over the other.

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Question 2:

To increase loudness of sound by four times, by how much should the amplitude of vibration be changed?

Answer:

To increase loudness of sound by four times, the amplitude of vibration needs to be doubled.

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Question 3:

Sound above which frequency is called ultrasonic?

Answer:

Sound that is above the frequency of 20 KHz (or 20,000 Hz) is called ultrasonic.

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Question 4:

Which of these are sensitive to colour−rods or cones?

Answer:

Cones are sensitive to colour.

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Question 5:

How long does light from the sun take to reach earth?

Answer:

It takes approximately 8 minutes and 20 seconds for light from the sun to reach earth.

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Question 1:

If you wear pointed heel in the left leg and flat shoe on the right leg, which leg would exert greater pressure on the ground? Why?

Answer:

The pointed heel on the left leg will exert more pressure on the ground. This is because force exerted per unit area will be more with the heel as compared to the flat shoe, as the area on which the force is exerted is smaller in the former. Smaller the area over which force is applied, greater the pressure.

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Question 2:

Why does polishing a surface reduce friction?

Answer:

Polishing a rough surface makes it smooth, and hence it reduces friction.

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Question 3:

What is noise pollution?

Answer:

Excess of sound with high amplitude in our surroundings, which could be harmful for us, is termed as noise pollution. It might cause hearing loss or even psychological disorders.

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Question 4:

State the rule regarding attraction and repulsion of charges.

Answer:

Like charges repel each other, whereas opposite charges attract each other. This means that two negative charges would repel each other, and likewise, two positive charges would also repel one another. But a negative charge and positive charge would tend to attract each other.

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Question 5:

What do you mean by the 'chemical effect' of electric current?

Answer:

Electric current can bring about chemical changes. This is known as the chemical effect of electric current. For example, electrolysis of water can break water into hydrogen and oxygen, thus bringing about a chemical change.

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Question 6:

What is the function of a periscope in a submarine?

Answer:

With the aid of a periscope, the sailor in a submarine can see objects on the surface of the sea and also the enemy's ships from inside the submarine.

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Question 7:

State the first law of reflection.

Answer:

The first law of reflection says that angle of incidence is equal to angle of reflection, i.e.,
i = r.

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Question 8:

Why is light dispersed by a glass prism?

Answer:

Light is dispersed by a glass prism because it bends the light by different angles.​ Therefore, light gets dispersed into its different coloured components. 

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Question 9:

Why does the Pole Star appear to be stationary in the sky?

Answer:

The Pole Star appears to be stationary in the sky because it lies exactly on the axis of rotation of the earth.

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Question 10:

Name four methods by which germs in water can be killed  at home.

Answer:

Germs in water can be killed at home by boiling, ultra violet rays, reverse osmosis and chlorination.

Page No 258:

Question 1:

What is the difference between contact and non-contact forces? Explain with the help of one example of each force.

Answer:

Contact force Non-contact force
Contact force is exerted by one object on another only when the objects are in contact with each other. Non-contact forces act on objects all the time.
For example: Frictional force or muscular force. For example: Gravitational force.

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Question 2:

Is friction between two surfaces dependent on how hard they press against each other? Why?

Answer:

Yes, friction between two surfaces is dependent on how hard they press against each other. Friction is a contact force that arises due to contact between two surfaces. 

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Question 3:

Why is electroplating of one metal on another done? Give one example for each.

Answer:

Electroplating is done either to make the metal look attractive, or to protect it from corrosion. For example, gold is electroplated on cheap metal jewellery to make it look attractive, and zinc is coated over iron when making bridges in order to protect it from rusting.

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Question 4:

Calculate the number of images seen when two plane mirrors are placed inclined at an angle of 10° to each other.

Answer:

The number of images formed by the two mirrors at angle of 10° can be calculated by formula:

nθ-1 = number of images
In this case n = 360/10 - 1 = 35
Therefore, the number of images seen is 35.

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Question 5:

What can be the consequences of excess greenhouse gases in the earth's atmosphere?

Answer:

Excess greenhouse gases in the earth's atmosphere can lead to global warming that will further increase earth's temperature. This would result in the melting of ice in polar regions, which would subsequently lead to a rise in the sea level. This will lead to flooding of coastal areas with sea water. It is believed that global warming could also alter climate patterns.



Page No 259:

Question 3:

Explain why a swimming pool full of water appears less deep than it actually is.

Answer:

A swimming pool full of water appears to be less deep than it actually is. This is because an optically denser medium (i.e., water) when seen from an optically rarer medium (i.e., air), appears less deep than it actually is. When a ray of light passes from air to water it bends towards the normal, but when the ray passes from water to air, it bends away from the normal. Thus, emergent rays come from virtual image which is above the bottom of pool and therefore, it appears to be less deep.

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Question 4:

What are the uses of artificial satellites?

Answer:

The uses of artificial satellites are:

  • They aid in television and radio transmission.
  • They enable telecommunication.
  • They help in the study and forecast of weather by taking pictures of clouds from outer space and sending them to earth.
  • They help locate mineral ores and enable the assessment of yield in agriculture by taking photographs from the sky. This is called remote sensing.
  • They help in the collection of information about space and increase our knowledge of the universe.

Page No 259:

Question 1:

Tyres of heavy vehicles are broad so that
(a) friction with the ground increases.
(b) pressure on the ground increases.
(c) friction with the ground decreases.
(d) pressure on the ground decreases.

Answer:

Tyres of heavy vehicles are broader than lighter ones, so that they can uniformly distribute their huge weight and exert less pressure on the road.

Hence, the correct answer is option D.

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Question 2:

The pressure exerted by a liquid
(a) does not change with depth.
(b) decreases with depth.
(c) increases with depth.
(d) is different in different directions at the same depth.

Answer:

(c) Increases with depth
In liquids (for example water bodies), pressure increases with depth.

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Question 3:

Friction can be increased by
(a) using ball bearings between the two surfaces.
(b) making the surfaces rough.
(c) lubricating the surfaces.
(d) using streamlined shapes.

Answer:

Friction can be increased by making the surfaces rough. The rougher a surface, greater the friction. 

Hence, the correct answer is option B.

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Question 4:

Noise is produced by
(a) high frequency vibrations.
(b) low frequency vibrations.
(c) regular vibrations.
(d) irregular vibrations.

Answer:

(d) Irregular vibrations
Irregular vibrations produce noise.

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Question 5:

An earthquake measuring 5 on the Richter scale is more powerful than another of magnitude 4 by
(a) 1.25 times
(b) 100 times
(c) 0.80 times
(d) 10 times

Answer:

Richter scale is not a linear scale. An earthquake that measures 5 on it is 10 times more powerful than an earthquake that measures 4.

Hence, the correct answer is option D.

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Question 6:

Which of these is plated on iron to prevent it from reacting chemically with food?
(a) chromium
(b) tin
(c) silver
(d) gold

Answer:

Iron plates are coated with tin to make food containers. This prevents iron from reacting chemically with the food.

Hence, the correct answer is option B. 

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Question 7:

A plane mirror produces a
(a) virtual inverted image.
(b) virtual erect image.
(c) real erect image.
(d) virtual or real erect image depending on the distance of the object from the mirror.

Answer:

A plane mirror produces a virtual erect image of the same size as of the object.

Hence, the correct answer is option B.

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Question 8:

When light travels from an optically rarer medium to a denser medium,
(a) it bends away from the normal.
(b) it bends towards the normal.
(c) it remains undeviated.
(d) it is laterally displaced.

Answer:

(b) It bends towards the normal
When light travels from an optically rarer medium to a denser medium, it bends towards the normal. This happens due to refraction.

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Question 9:

A group of a few stars that appear to be close together in the sky when viewed from the earth is called
(a) galaxy.
(b) constellation.
(c) solar system.
(d) universe.

Answer:

(b) Constellation
A group of stars that appear to be close together in the sky when viewed from the earth is termed as a constellation.

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Question 10:

Which of these is a greenhouse gas?
(a) Carbon dioxide
(b) Methane
(c) Water vapour
(d) All of these

Answer:

(a) All of these
All of the above gases can absorb the heat radiated by earth. Therefore, all of them are greenhouse gases.



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