Frictional Force Questions

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Very Short Answer Questions [1 Mark Questions]

Ques. The maximum value of static friction when the body is on the verge of starting motion is known as

1. Limiting friction
2. The angle of repose
3. Static friction
4. Impending motion

Ans. The correct answer is a. Limiting friction

Explanation: Limiting friction may be determined by multiplying the static frictional coefficient by the normal reaction force of the body.

Ques. _____ is known as the motion that would take place under the applied force if friction were absent.

1. Constrained motion
2. Linear motion
3. Unconstrained motion
4. Impending motion

Ans. The correct answer is d. Impending motion

Explanation: Impending motion is defined as the motion that would occur under the given force if friction did not exist. Constrained motion is defined as motion that cannot progress randomly in any way. Unconstrained motion is defined as motion with no restrictions. Linear motion is defined as one-dimensional motion along a straight line.

Ques. Limiting friction of a body depends on

1. The volume of the smaller body on a larger surface
2. The periphery of the contact surfaces
3. Nature of surfaces
4. Area of contact of surfaces

Ans. The correct answer is c. Nature of surfaces

Explanation: Because limiting friction is determined only by the coefficient of friction of the surfaces, it depends only on the nature of the surface, i.e., smoothness or roughness.

Ques. The value of the frictional force for a body is always constant when it is static and does not vary with applied force.

1. True
2. False

Ans. The correct answer is b. False

Explanation: Friction is a self-adjusting force that is determined by the normal reaction, which changes according to the applied force. Under static conditions, the maximum magnitude of the force of friction is limited to the product of the coefficient of friction and the weight of an object.

Ques. The static coefficient of friction is greater than the kinetic coefficient of friction.

1. True
2. False

Ans. The correct answer is a. True

Explanation: Because static frictional force is greater than kinetic frictional force, dividing both values by constant normal reaction force provides a higher value of static coefficient of friction.

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Short Answer Questions [2 Marks Questions]

Ques. What is meant by the frictional force?

Ans. The frictional force is the force produced when two surfaces come into contact and slide against each other. These forces are primarily affected by the surface roughness and the amount of force required when they are combined. The volume of frictional force is affected by the angle and position of the object.

Ques. What causes friction?

Ans. Friction is caused by irregularities on the two surfaces that come into contact. As a result, as one object moves over another, the surface irregularities become interconnected, causing friction. The more roughness, the more irregularities, and the greater will be the friction.

Ques. Why is friction a necessary evil?

Ans. In many cases, friction is necessary for example, kinetic friction that dissipates power is required for quickly stopping relative motion. Brakes on machines and vehicles are based on this. Similarly, we are able to walk because of friction. A car cannot move on a very slippery road.

As friction has so many advantages and disadvantages, we may say that friction is a necessary evil.

Ques. Define the coefficient of friction.

Ans. The friction coefficient is defined as the ratio of the frictional resistive force to the perpendicular force pushing the objects together. It is given by the formula

µ = F/N

Where

• F is the force of friction
• N is the normal reaction force
• µ is the coefficient of friction

Also Read:

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Long Answer Questions [3 Marks Questions]

Ques. What is the Difference Between Static Friction and Kinetic Friction?

Ans. The differences between static friction and kinetic friction are

Ques. What are the factors that affect friction?

Ans. The following are the factors that affect friction

• Nature of the medium of contact between the bodies: It is found that friction is caused by the interlocking of irregularities on the two surfaces in contact. A rough surface has a larger number of irregularities, so the force of friction is greater. A smooth surface has a small number of irregularities, so the force of friction is less.
• Normal reaction: As the normal reaction increases, the interlocking between two surfaces in contact increases as they press harder against each other, and hence friction increases.
• Area of contact: The force of friction is independent of the area of contact.

Ques. What are the different types of friction?

Ans. Frictional forces are classified as

• Dry Friction: The force that competes with one solid surface sliding across another solid surface is known as dry friction. Dry friction always opposes the sliding surfaces and can have the effect of opposing motion or producing motion in bodies.
• Fluid Friction: Fluid friction occurs when two fluid layers move in opposing directions. This intrinsic resistance to the flow is referred to as viscosity.

Further dry friction is classified as

• Static friction: Static friction occurs when individuals try to move a stationary object on a surface without actually causing any relative motion between the body and the surface.
• Kinetic friction: The force of kinetic friction acts between moving surfaces. An object moving over a surface will experience a force in the opposite direction of its movement.
• Rolling friction: Rolling friction occurs when a wheel, ball, or cylinder freely rolls over a surface, as in ball and roller bearings.
• Sliding friction: The resistance caused by the two objects sliding against each other is referred to as sliding friction.

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Very Long Answer Questions [5 Marks Questions]

Ques. A truck moving at 72 km/h carries a steel girder that rests on its wooden floor. What is the minimum time in which the truck can stop without the girder moving forward? The coefficient of static friction between the steel and wood is 0.5.

Ans. Given

• The initial velocity of the truck, u = 72 km/h = 72 x (5/18) m/s = 20 m/s
• The coefficient of friction between the steel girder and wood, µ_s = 0.5

Since the truck finally stops, therefore its final velocity, v = 0

Now the maximum frictional force between the steel girder and the wooden floor is given by

F_max = µ_sR

Where R is the normal reaction force of the steel girder due to the wooden floor of the truck.

If M is the mass of the steel girder, then the normal reaction force is given

R = Mg

Therefore, maximum frictional force, F_max = µ_sMg

Acceleration produced on the steel girder is given by

a_girder = F_max/M

⇒ a_girder = µ_sMg/M = µ_sg

On substituting the values of µ_s and acceleration due to gravity g, we get

a_girder = 0.5 x 9.8 = 4.9 m/s²

Now the truck will also retard will this acceleration to cause no motion in the steel girder.

Thus the minimum time in which the truck can come to rest without the girder moving forward can be calculated by using the equation of motion

v = u + at

⇒ t = (v - u)/a

On substituting the values, we get

t = (0 - 20)/(-4.9)

⇒ t = 4.08 seconds

Ques. A body is rolling on the ground with a velocity of 1 m/s. After traveling a distance of 5 m, it comes to rest. Find the coefficient of friction. Take g = 10 m/s²

Ans. Given

• The initial velocity of the rolling body, u = 1 m/s
• The distance covered by the body, S = 5 m

Since the body finally comes to rest, its final velocity will be zero i.e. v = 0

Let a be the retardation of the body then using the equation of motion, v² = u² + 2aS, we get

a = (v² - u²)/2S

On substituting the values, we get

a = (0 - 1²)/(2 x 5) = - 1/10 m/s² 

Let m be the mass of the body, then the force of friction (F) acts on the body can be calculated by using Newton’s second law of motion i.e.

F = ma = m/10 Newton

Now normal reaction force acting on the body is given by

R = mg = 10m

The coefficient of friction is given by

µ = F/R

On substituting the values, we get

µ = (m/10) x (1/10m) = 1/100

⇒ µ = 0.01

Hence the coefficient of friction between the body and the ground is 0.01.

Ques. A body of mass 10 kg is kept on a horizontal floor with a coefficient of static friction µ_s = 0.5 and a coefficient of kinetic friction µ_k = 0.45 as shown in the figure

Find the acceleration, force of friction, and contact force on the body by the plane when the driving force is

1. 40 N
2. 60 N

Take g = 10 m/s²

Ans. Given

• The mass of the body, m = 10 kg
• The coefficient of static friction, µ_s = 0.5
• The coefficient of kinetic friction, µ_k = 0.45

Normal force acting on the body, N = mg

⇒ N = 10 x 10 = 100 N

Maximum static friction, Fmax = µsN

⇒ F_max = 0.5 x 100

⇒ F_max = 50 N

Case a: When the driving force is 40 N.

• Since the driving force is less than that of the maximum static friction force, the body will not move. Therefore its acceleration will be zero i.e. a = 0
• The force of friction acting on the body is equal to the static friction (Since the body is not moving). Also, this static friction will be equal to the driving force i.e. F = 40 N.
• Contact force (F_C) is the resultant of the force of friction and the normal force. Therefore

F_C = √(F² + N²) = √(40² + 100²) 

⇒ F_C = 107.7 N

Case b: When the driving force is 60 N.

• Since the driving force is more than that of the maximum static friction force, the body will accelerate with acceleration a.
• Kinetic friction, F_k = µ_kN = 0.45 x 100 = 45 N

∴ a = (F - f_K)/m = (60 - 45)/10 = 1.5 m/s²

• Contact force, F_C = √(Fk² + N²) = √(45² + 100²)

⇒ F_C = 10⁹.7 N

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