Friction Test

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Friction Test - 5

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Question 1
1 / -0

A right circular cylinder of mass M and radius r is sitting on top of a circular platform of radius R such that the axes of the 2 are exactly aligned.

The whole arrangement is rotating about the central axis at 2 revolutions/second. Then, the force of friction between the cylinder and the rotating disk is

A
2M N

B
M N

C
2M N

D
0 N

Solution

As there is no relative motion between the surfaces of the cylinder and the disk, the value of frictional force between them is 0 (zero).
Question 2
1 / -0

A recovery truck is moving on the North-South highway from South to North. It is towing a 1500 kg car behind it. In which direction will the force of friction act on the car?

A
North

B
South

C
East

D
West

Solution

As the vehicles are moving from South to North, the relative motion of the tyres of the car with respect to the surface of the road is towards North. Hence, frictional force on the car will act towards South as the force of friction opposes the relative motion between the surfaces in contact.
Question 3
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A west-bound tractor trailer, which is carrying an abnormally heavy load M, is about to approach a curve on the road. So, it has started to decelerate from its initial speed of 30 m/s with a magnitude of 2 m/s². The load M is sitting without slipping on the flatbed trailer as shown. What is the value of the force of friction on the mass M and in which direction does it act?

A
0 N; any direction

B
30M N; west

C
2M N; west

D
15M N; east

Solution

As the load is sitting without slipping, the acceleration of the load = acceleration of the truck. Hence, acceleration of the load = 2 m/s² towards east.
Thus, external force on the load is nothing but the force of friction between the load set and the surface of the flatbed trailer. Hence, the value of this force of friction is 2M N.
Question 4
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Which of the following statements is false?

A
Force of friction depends on the smoothness of the surfaces in contact with each other.

B
Friction is caused by the irregularities on the two surfaces in contact with each other.

C
No force is required to keep an object moving over a surface so as to overcome friction.

D
Force of friction is greater if a rough surface is involved.

Solution

If we want to move an object, we have to apply the force so as to overcome the friction between the object and the surface.
Question 5
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A marble is rolled down an inclined plane onto 4 different surfaces (A, B, C, D) with the number of irregularities per unit area in the decreasing order as A > B > C > D. On which surface will the marble cover the greatest distance?

A
A

B
B

C
C

D
D

Solution

As surface D has the least number of irregularities, it is the smoothest of all the surfaces. Hence, it will offer the least resistance to the marble. Hence, the marble covers the greatest distance on surface D.
Question 6
1 / -0

Consider four different scenarios as shown:

In which of the following cases will the friction between the mass M and the surface it is in contact with be the greatest if it is tried to be moved?

A
(a)

B
(b)

C
(c)

D
(d)

Solution

The force of friction increases if the surface are pressed harder, so the force of friction is the highest in figure (d) where the object pressed with 2F force which is maximum among the given options.
Question 7
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In which of the following scenarios will the friction between the mass M and the surface it is in contact with be the least if it is tried to be moved?

A

B

C

D

Solution

The force of friction decreases if the two surfaces are not pressed harder. Conversely, it decreases if the force of contact between them decreases. This is happening in option (4), which is the answer.
Question 8
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Friction causes wastage of energy in the form of

A
heat

B
light

C
sound

D
electricity

Solution

Due to friction, energy is generally dissipated (wasted) in the form of heat.
Question 9
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Which of the following is an advantage of friction?

A
Friction causes wastage of energy in the form of heat.

B
Friction causes wear and tear of parts that rub against each other.

C
Friction causes a matchstick to light up and produce a fire.

D
Friction causes aeroplanes to burn a lot of fuel.

Solution

Friction causes a matchstick to light up and produce a fire.
Question 10
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Which of the following activities increases friction in the corresponding case?

A
Sprinkling fine powder on a carrom board

B
A motor mechanic using grease between the moving parts of a machine

C
Gymnasts applying coarse substance on their hands

D
A handyman oiling the hinges of a door

Solution

Gymnasts apply some coarse substance on their hands to increase friction for better grip. Hence, option (3) is correct.
Question 11
1 / -0

A force of 50 N is required to make a slider move with uniform speed over the steel rail of a machine. Which of the following cannot be the value of the force required to move the slider with the same speed having oiled the rail?

A
40 N

B
42 N

C
48 N

D
50 N

Solution

Lubrication (oiling of the surface in this case) reduces friction. Hence, the force required has to be less than 50 N. Hence, option (4) is correct.
Question 12
1 / -0

The efficiency of a gear train is 75%. Which of the following can be its efficiency after greasing it?

A
60%

B
70%

C
75%

D
80%

Solution

Greasing reduces the frictional forces at play. This increases efficiency. Hence, the efficiency after greasing should be greater than 75%. Hence, option (4) is correct.
Question 13
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When a body rolls over the surface of another body, the resistance offered to its motion is called

A
static friction

B
sliding friction

C
rolling friction

D
turning friction

Solution

When a body rolls over the surface of another body, the resistance offered to its motion is called rolling friction.
Question 14
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Under identical conditions, which of the following is true for a given body?

A
Rolling friction > Sliding friction > Static friction

B
Static friction > Static friction > Rolling friction

C
Static friction > Sliding friction > Rolling friction

D
Rolling friction > Static friction > Sliding friction

Solution

Static friction is maximum, sliding friction is less than static friction and rolling friction is least among the three.
Question 15
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Under which of the following phases of its flight, will a helicopter experience the maximum drag from air?

A
When it is lifting off

B
When it is moving forward at a high speed

C
When it is hovering on top of a river without moving

D
When it is coming down while landing

Solution

When a helicopter is hovering, its horizontal or vertical motion is zero. Hence, it experiences no drag. A helicopter takes off and lands vertically at not a very high speed. Hence, it experiences little drag during these two phases. Only when it is moving at a high speed, it will experience the highest drag from air.