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Science questions in short. [3 marks ]

Friction · Gujarat Board (GSEB) STD 8 (GSEB - English Medium). 10 questions.

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questions in short. [3 marks ]

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10 questions · timed · auto-graded

Question 13 Marks
We have two identical metal sheets. One of them is rubbed with sand paper and the other with ordinary paper. The one rubbed with sand paper shines more than the other. Give reason.
Answer
While rubbing with sand paper, more frictional force is produced between the layers of metal sheet and sand paper which causes more force on dust particles and they are removed easily, so it will shine more. But in case of ordinary paper, the force of friction is not sufficient to remove all the dust, so it will shine less in this case.
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Question 23 Marks
The frictional force between two moving surfaces which are in contact with each other is called kinetic friction or sliding friction.
Example If a boy apply some pushing force on the box, the box will start sliding on the surface and the friction between the surfaces is called sliding friction.
The force required to keep a body in motion is less than the force required to start the motion, so we can say that sliding friction is less than the static friction.
Image
(i) Explain why the sliding friction is always greater than rolling friction?
(ii) Mention two different examples for sliding friction.
Answer
self
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Question 33 Marks
In Physics classroom, a teacher told to his students that when a cricket player want to catch a ball then first he rub the hands. Thus, there is less chance of slipping the ball from hands.
(a) Explain the reason of not slipping of ball from hand.
(b) If the player makes his hands oily instead of rubbing them, would he be able to catch the ball?
(c) Mention the type of teaching skills of teacher is being represented here.
Answer
a. The player rub his hand due to which the palms become warm which absorb the moisture from the surface and make the surface rough. So, due to roughness of hands, the ball may get a better grip between the hands and will not slip.
b. The player makes the palm surface oily. The number of irregularities decreases and the surface becomes smooth. So, the ball slips off easily from the hand.
c. As, teacher wants to teach the students through easy example. So, this act of teaching shows the experience and ideality of teacher.
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Question 43 Marks

Sonakshi investigated how a pencil cell travels on a ramp when the table is covered with different surfaces (wood, cotton cloth and silk cloth). She noted the time taken by the pencil cell to travel on each surface three times. Her results are mentioned below.

Surface

Time (s)

Time (s)

Time (s)

Wooden

5.01

4.99

5.21

Cotton cloth

7.02

7.25

7.20

Silk cloth

4.20

4.20

4.25

Q.1. Name the two factors on which the magnitude of a frictional force depends.
Q.2. From the chart given above, state which travelled fastest and find the average time it took.
Q.3. Explain with one example how friction can be both a positive and a negative part in our everyday life?

Answer
self
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Question 83 Marks
Explain with the help of an example that heat is produced by friction.
Answer
As friction opposes the relative motion between the two surfaces, it produces heat. So, when we rub our hands together vigorously for few minutes, we feel hotness. Similarly when we strike a matchstick against the rough surface, it catches fire.
In machineries also, heat is produced due to friction as the machine parts rub against each other.
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Question 93 Marks
Few days back, Shradha, a student of class VIIIth was driving the car. She went to a crop field. The level of road were just above the level of ploughed field. As she wanted to stop the slow moving car, the brakes failed to work at that time. Suddenly, she turned the car towards the ploughed field. After sometime, the car came to rest.
(a) Shradha turned the car towards the ploughed field. Explain why?
(b) Explain how the car came to rest after covering some distance in the ploughed field?
(c) Mention the values that Shradha exhibits here.
Answer
a. She turned her car towards the ploughed field to protect herself from an accident.
b. The car moved from road to the ploughed field due to which the force of friction between the tyre and the ground surface increases suddenly. Because the large amount of irregularities reduces the movement of tyre. Thus, the car came to a rest.
c. The values displayed by Shradha are intelligence and awareness.
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Question 103 Marks
Give examples to show that friction is both a friend and a foe.
Answer
Friction as a friend
i. It allows us to walk comfortably.
ii. It allows us to minimise the speed or to stop the moving objects.
iii. It allows us to write on a board with chalk.
Friction as a foe 
i. It causes wear and tear in objects.
ii. It causes damage to the parts of machines.
iii. It does not allow the free movement of objects.
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questions in short. [3 marks ] - Science STD 8 Questions - Vidyadip