Friction Long Answer Type Questions
Question 1. Why should one take short footsteps while walking on ice (or oily surface)?
Answer:
To walk, one exerts a force F obliquely on the ground. The ground also exerts an equal and opposite reaction force R on the man. It is the horizontal component H, of R, which makes walking possible. Frictional force f supplies this horizontal component.
As an icy surface is very smooth, the frictional force is very small. Hence, any attempt to take longer steps can make H greater than the limiting friction. As a result, the person may slip forward.
Question 2. In rainy season sand is sometimes thrown on railway tracks. Why?
Answer:
The force impressed on the wheel is generally greater than the force of rolling friction; this produces the rolling of the wheel on the railway track. On the other hand, the kinetic friction is fairly higher than the impressed force, and there is no sliding of the wheels.
But in the rainy season, on the wet railway track, there is a danger that the kinetic friction may drop below the impressed force, resulting in a sliding of the wheels. To avoid this, sand is sometimes thrown on the railway track to increase the kinetic friction.
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Question 3. Show that the coefficient of friction is a dimensionless quantity.
Answer:
From the definition, the coefficient of friction,
= \(\frac{\text { limiting friction }(f)}{\text { normal force }(R)}\)
Both f and R are forces and have the same dimension MLT-2.
∴ Dimension of μ = \(\frac{\mathrm{MLT}^{-2}}{\mathrm{MLT}^{-2}}=1\)
Hence, μ is a dimensionless quantity.
Question 4. A chair is kept on the floor. When does friction act between them? Where does this force act? Is the magnitude of this force a constant?
Answer:
When the chair is at rest on the floor, frictional force does not act. Friction comes into play when one tries to drag the chair over the floor and it continues to act when the chair is actually in motion.
- The frictional force acts parallel to the surface of contact between the chair and the floor and opposite to the direction of motion or relative motion.
- The magnitude of the force of friction is not constant. As the force on the chair is gradually increased, the force of friction also increases and reaches a limit, called the force of limiting friction.
- On further increase of the applied force, the chair will start moving. Now, the magnitude of the frictional force decreases a little and thereafter remains constant.
Question 5. Can the value of the coefficient of friction be greater than 1?
Answer:
The value of the coefficient of friction (μ) is usually less than 1. But in some special cases, its value can be equal to or even greater than 1. The value of μ between two metal surfaces, cleaned scientifically and kept in a vacuum, may rise up to 10 approximately. Under these conditions, μ ≈ 1.6 between two copper plates.
Question 6. Explain why it is difficult to write on a paper surface that is too smooth or too rough.
Answer:
A force of friction acts at the point of contact of the pen and the paper, against the motion of the pen. For a very smooth paper, the frictional force is low, and the pen slips on the paper’s surface. For a very rough paper, frictional force is quite high, and this makes it difficult to move the pen over the paper. Thus, these surfaces are not suitable for writing.
Question 7. A body of mass m is kept over an object of mass M. The object is at rest on smooth ground. The coefficient of static friction between the two bodies is μ. What is the minimum force that needs to be applied to the object, so that the body will be able to slip over it?
Answer:
Let the force applied on the object horizontally, so that the body is about to slide be F.
Hence, the common acceleration of the system, a = \(\frac{F}{M+m}\)
so the force on the body = ma = \(\frac{Fm}{M+m}\)
When the body is about to slip, this force is balanced by the force of limiting friction acting at the plane of contact.
∴ Frictional force, f = μmg = \(\frac{Fm}{M+m}\)
or, F = μ(m+m)g.
Question 8. The effectiveness of the brake of a car does not depend on the area of contact of the brakeshoe with the rim of the wheel—explain.
Answer:
A car brake is applied to generate a frictional force against the rolling of the wheel, and thereby to decrease the velocity of the car.
Frictional force does not depend on the area of contact but depends on the nature of the surfaces in contact and the normal reaction. The area of contact of the brake has no effect on its functioning.
Question 9. While polishing a substance, if the polishing cloth is pressed hard, a considerable amount of heat is developed. Why?
Answer:
An increase in pressure increases the normal force. Hence, the force of friction also increases. So, a greater amount of work has to be done against friction while polishing, which in turn, generates a greater amount of heat.
Question 10. Will there be any change in the coefficient of friction between the surfaces of two objects, if they are taken to the moon?
Answer:
The coefficient of friction depends on the materials and the smoothness of the contact planes of the two objects. As these factors remain unchanged even when the objects are taken to the moon, the coefficient of friction will also remain the same.
Question 11. When the wheels of a car are bogged down in mud, why cannot the car move forward?
Answer:
The car engine rotates the wheels; and due to friction between the road and the wheels, the car moves forward. When the friction cannot provide a sufficient reaction force, the engine cannot make the car move forward by rotating the wheels; they continue to rotate in the same place without any translation.
Question 12. After a rainfall, one should not drive very fast on a wet asphalt road. Explain.
Answer:
The coefficient of static friction between an asphalt road and the tires plays an important role in controlling the motion of a car. If the value of the coefficient is small, it becomes difficult to accelerate or steer the car.
In some cases, the wheels may start to skid, and the driver may lose control. Such a situation arises while driving on a wet road at high speed, as, the coefficient of static friction between the tyres and the road falls appreciably when this road gets wet.
Question 13. A body of mass m is placed on a platform of mass M (m«M), moving with a velocity v. If the coefficient of friction between the platform and the mass is μ, then for how long will the body continue to slide on the platform, and what distance will it cover during that time?
Answer:
When the platform moves, the body on it tends to slide backward. So the frictional force on the body acts in the direction of motion of the platform. It is given by, f= μmg.
∴ Its acceleration, a = \(\frac{f}{m}\) = μg
When the velocity of the body becomes v, it no longer slides over the platform.
We know, \(v=u+a t\)or, \(t=\frac{v-u}{a}\)
Here, u=0, a = \(\mu g\)
t = \(\frac{\nu}{\mu g}\)
Again, \(v^2=u^2+2 a s\)
or, \(s=\frac{v^2-u^2}{2 a}=\frac{v^2}{2 \mu g}\)
∴ The body will slide for a time of \(\frac{v}{\mu g},\), and in this interval of time, it will cover a distance of \(\frac{v^2}{\mu g},\).
Question 14. How does the accelerator increase the speed of a car?
Answer:
The accelerator, on being pressed, increases the angular velocity of the wheel, and thereby the rolling friction increases. At the point of contact with the ground, the wheel rotates backward so the rolling friction acts in the forward direction. The car accelerates due to the increase of this rolling friction.