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CBSE Questions for Class 8 Physics Friction Quiz 5 - MCQExams.com
CBSE
Class 8 Physics
Friction
Quiz 5
Friction is not required in
which of the following cases?
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Man running on the ground.
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Friction between parts of a machine.
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Writing with a pen.
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Monkey climbing a tree.
Explanation
A)
If there is no friction between man and ground then man will slip/ fall.
B)
If there is no friction between parts of machine then there is no loss of energy and work efficiency increases.
C)
If friction is not present between pen and copy then we cannot write properly.
D)
Friction is required for a monkey to climb on tree otherwise it will slip down.
Hence Option B
Frictional force is nearly negligible on :
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Rough surfaces
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Semi-smooth surfaces
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Smooth surfaces
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None of these
Explanation
On smooth surfaces due to lack of uneveness their is no resistance to motion of the body hence their is no frictional force.
The depressions and elevations present on the surface of objects causes friction.
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True
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False
Explanation
Yes the given statement is true.
Friction is a force that resists the motion between two objects in contact with each other. When an object is not smooth i.e it has depressions and elevations present on its surface and when we try to move one rough object over another, the elevations gets stucked in the depression of the object which leads to frictional force as more energy will be needed to slide the two objects over another.
Which of the following activity would not be possible in the absence of friciton?
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holding a glass
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writing on a paper
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sitting on a chair
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all of the above
Friction does not depend on the nature of the surface in contact.
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True
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False
Explanation
The given statement is false.
Friction force depends on the nature of the surface in contact. If the surface area is rough then the frictional co-efficient of the surface will also be larger thus will need more force to slide increasing friction. On the other hand if the surface area is smooth the co-efficient of friction between two surfaces will be less. Thus friction depends on the nature of the surface in contact.
Driving a car on a wet road is difficult because water
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makes it difficult to turn the car.
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decreases the friction between the tyres and the road.
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increases the friction between the tyres.
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makes friction between the road and tyres zero.
Explanation
We know that greater the friction more is the ease of driving. Water decreases the friction between the tyres of a vehicle and the road hence, driving a car on a wet road is difficult.
If rough surface is polished beyond a certain limit than the magnitude of frictional force will
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Nothing can be said.
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sometime increases and sometime decreases.
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increase.
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decrease.
Explanation
Friction decreases with smoothness only to a certain degree. When the surfaces are further polished, the Vander Waal forces and electrostatic forces between the atoms of the surfaces increases, this happens because the surfaces come closer when polished.
From following statements choose the correct combination.
A.
Grease is used to reduce friction in bicycle chain
B.
Ball bearings are used to reduce friction in a fan
C.
Oil can be used to increase friction in a car engine.
D.
Talc can be used to reduce friction when an object is sliding on a surface.
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A, B, D
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A, C, D
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B, C, D
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A, B, C
Explanation
Friction is reduced by using oil, grease, ball bearings and talc.
Because by the use of this things, the surface becomes smooth and therefore it helps in reducing the force of friction.
The application of decreasing friction includes :
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Ball bearing used in moving parts of a machine
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The streamlined shape of an aircraft
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The layer of air between the hull of a hovecraft.
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All of the above
Explanation
Friction opposes the relative motion between two surfaces in contact. Friction is sometimes undesirable.
Friction can be reduced by:
i.
using ball bearings in moving parts of the machine.
ii.
giving suitable shapes to bodies moving in fluids. For example, the streamlined shape of an aircraft
iii. using
air cushion between the moving parts. For example, the layer of air between the hull of a hovercraft.
The friction of the surface of a wooden bench
can be increased by;
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rubbing it with sand paper.
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using lubricants like oil and grease.
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using powder lubricants.
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grinding or polishing the surface.
Explanation
When the surface of the wooden bench rubbed with sand paper then more roughness will appear on the surface hence it increase the friction of the surface of wooden bench.
What would happen to a moving object if there would be no friction?
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The object will stop
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The object will keep moving
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The object will change speed
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The object will change direciton
Explanation
An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.
Friction oppose relative motion. In its absence, a moving object will not stop.
Answer (B)
the object will keep moving
Friction on a dry marble floor is ----- than that on a wet marble floor.
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More
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Less
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Same
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None of these
Boats and aeroplanes are ................. so as to reduce fluid friction.fill in the blank.
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large
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broader
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streamlined
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bigger
Explanation
Boats and aeroplanes are streamlined so as to reduce fluid friction.
Aeroplanes, boats, fishes and birds which move through fluids have bodies of special shape, called streamlined shape, so as to reduce the friction due to fluid and avoid energy loss.
For example,if the boat isn't streamlined, then it also has to push the water out of its way, increasing the drag force.
Air resistance is also a form of friction.
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True
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False
Explanation
Air resistance is a special type of frictional force that acts upon objects as they travel through the air. The force of air resistance is often observed to oppose the motion of an object. This force will frequently be neglected due to its negligible magnitude (and due to the fact that it is mathematically difficult to predict its value). It is most noticeable for objects that travel at high speeds (e.g., a skydiver or a downhill skier) or for objects with large surface areas.
Hence, the statement is true.
Unevenness in the surfaces of the moving body and the surface on which it moves causes:
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gravitational force
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frictional force
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centripetal force
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centrifugal force
Any substance which is used to reduce the roughness of a surface is called a:
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Lubricant
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Lubrant
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Lubrication
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None of these
Explanation
A lubricant is a substance that helps to reduce friction between surfaces in mutual contact, which ultimately reduces the heat generated when the surfaces move. It may also have the function of transmitting forces, transporting foreign particles, or heating or cooling the surfaces.
To keep machines running smoothly and to avoid wear and tear of machines, the best and the most commonly used lubricant is:
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powder
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sand
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oil
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water
Explanation
Oil is the best and the most commonly used lubricant used for keeping machines running smoothly and to avoiding wear and tear of machines as it reduces the friction more effectively.
Because oil being a fluid goes in between all microscopic gaps of machines ,and sticks to it and makes the surfaces smooth hence friction and wear and tear due to friction is reduced.
Graphite powder is used in machines to
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Enhance friction
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Enhance profit
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Reduce friction
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Reduce efficiency
Explanation
Graphite powder is used in some machines to reduce friction because graphite powder consist of fine particles and these fine particles stick to surfaces and make them smooth so friction among surfaces is reduced.
A car will skid more on a wet road than on a dry road and the car tyres there is more:
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friction
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lubricant
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gravity
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heat
Friction between any two objects is due to
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attraction between them
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repulsion between them
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some adhesive forces between them
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irregularities on the surfaces
Explanation
Friction between any two object is due to the irregularities on the surfaces. More the irregular surface more is the friction.
Frictional force increases when surfaces in contact are made very smooth, because of
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Molecular force
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Decrease in surface area
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Decrease in irregularities
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Increase in area
Explanation
When the contact area is made smooth then the molecular forces between surfaces increases this gives rise to frction.
Hence (A) is the correct ans.
Match the following
List I
List II
1
Streamlined body structure
a
Less no. of irregularities
2
Pressing two objects harder against each other
b
Lubricant
3
Grease
c
Increase friction
4
Smooth surface
d
Decrease friction
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1-d, 2-c, 3-b, 4-a
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1-d, 2-c, 3-a, 4-b
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1-a, 2-d, 3-b, 4-c
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1-a, 2-b, 3-c, 4-d
Explanation
1) A streamlined body structure helps in decreasing friction as it helps to move through fluids easily.
2)When two objects are pressed harder against each other, their irregularities lock tightly and increase the friction.
3)Grease acts as a lubricant that reduces friction by forming a layer between two surfaces in contact, thus preventing them from directly rubbing against one another.
4) A smooth surface has fewer irregularities and hence offers less friction.
Alida runs her toy car on dry marble floor, wet marble floor, newspaper and towel spread on the floor. The force of friction acting on the car on different surfaces in increasing order will be
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Wet marble floor, dry marble floor, newspaper and towel.
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Newspaper, towel, dry marble floor, wet marble floor.
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Towel, newspaper, dry marble floor, wet marble floor.
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Wet marble floor, dry marble floor, towel, newspaper.
Explanation
The nature of the surfaces in contacts decides the friction force between them. The friction force will be more if the surfaces in contact are rougher.
Among the following the $$wet\ marble\ floor$$ is least rough therefore least friction force. Then the increasing order of roughness is $$dry\ marble\ floor, news\ paper$$ and most rough is $$towel$$. Therefore the friction force will also be in same order.
After an aeroplane completes a voyage, its muzzle feels warmer than the middle and back portion of the main body. This is because:
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the muzzle tears apart the air layers and suffers more air friction
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the muzzle suffers less air friction
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the muzzle suffers zero air friction
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none of these
Explanation
Aeroplanes are given streamline shapes. Due to this special shape, the muzzle tears apart the air layers faced while the plane is in motion. So, the muzzle encounters more air friction and becomes warmer.
A ball is rolled on four surfaces:
a) marble floor,
b) on sand,
c) on cloth,
d) on wood.
The force of friction acting on the ball on these different surfaces in descending order will be:
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b > c > d > a
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c > b > d > a
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d > b > a > c
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a > b > c > d
Explanation
Friction is caused due to the irregularities in the surfaces in contact. Larger the irregularities, i.e., rougher the surface, more is the friction. The surface of sand is the roughest of all, followed by cloth, wood and marble floor. Thus, maximum friction will be observed in sand followed by cloth, wood and marble floor.
If we drop a cup of water on a plate and a cup of oil on another plate, water is seen to spread faster along diameter. This is an example related to:
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fluid friction
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rolling friction
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sliding friction
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static friction
Explanation
This example relates to fluid friction. Speed of spreading of a volume of liquid depends on the liquid friction.Friction offered by water is less as its density is less than oil.Fluid friction depends on density of liquid.
So oil being more dense have more friction and it moves very slowly due to more friction.
What measures are taken to reduce fluid friction in aeroplanes?
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Optimum speed is maintained with respect to the air
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Metals are choosen in a way to make the aeroplane lighter with smoother surface
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Streamlined body inspired by birds
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All of the above
Explanation
It is very important to minimize the fluid friction for airplanes because more friction will produce more heat, which may harm the plane. Thus, various measures are taken to reduce friction. An optimum speed is maintained and the body structure of the plane is more streamlined so that it can cut through the air with minimum friction. Also, such metals are chosen to make the body of the airplane, which are light, strong, and able to form a smooth surface. This smooth surface ensures lesser friction.
Why are efforts made to reduce friction?
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Energy is lost in overcoming friction.
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Overcoming friction is a time consuming process.
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Both A and B
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None of the above
Explanation
A lot of the applied energy is lost in overcoming friction so as to put a body in motion. Hence efforts are made to reduce friction.
When two surfaces are coated with lubricant, then they will:
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slide upon each other
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stick to each other
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roll upon each other
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jump on each other
Explanation
Lubrication is required to reduce the friction between the two surafces. If two surfaces are coated with lubricant then friction will be reduced so they can slide over each other if one is pushed on the other. It is friction which prevents relative motion between two surfaces.
Aeroplanes are streamlined to reduce friction.
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True
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False
Explanation
Fluid friction can be minimized by giving suitable shapes to bodies moving in fluids. Airplanes are streamlined to reduce friction.
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