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It is the elementary chapters of Physics that build up your base and understanding of the subject. It is only when you completely master the application of forces on a body that you can move to tackle higher difficulty chapters such as rotational mechanics. Understanding friction often meddles with your concepts for you might just be used to the concept of ideal body situations. 

After you have mastered the common equilibrium problems considering an ideal situation, you will be required to inculcate the concepts of friction into these problems of static or dynamic equilibrium. Problems under this topic will not be completely new if you have already mastered the concepts of equilibrium. But make sure you have strong basics considering equilibrium and the principles of drawing free body diagrams as they play an essentially important role in this field.

Tyres are calibrated to improve friction and provide better grip

Friction on a free body diagram

When surfaces in contact move relative to each other, the friction between the two surfaces converts kinetic energy into thermal energy (that is, it converts work to heat). This property can have dramatic consequences, as illustrated by the use of friction created by rubbing pieces of wood together to start a fire. Another important consequence of many types of friction can be wear, which may lead to performance degradation and/or damage to components.

FDB depicting various forces

Whenever the surfaces of two bodies are in contact, there will be a limited amount of resistance to sliding in between them, which is called friction. The above free body diagram depicts the directions of Normal, Gravitational and Frictional force acting on a body on an inclined plane.

Laws of friction

The laws of friction can be framed as: For a given condition of two dry surfaces in contact,

  • The total friction that can be developed is independent of the magnitude of area in contact.
  • The total friction that can be developed is proportional to the normal force.
  • For low velocities of sliding the total friction that can be developed is practically independent of the velocity, although experiments show that the force, necessary to start sliding is greater than what is required to maintain sliding.

The above laws may be expressed by the simple formula F=μN, where μ is called the coefficient of friction. If F is taken as the force necessary to start sliding, μs is called the coefficient of static friction. If F is taken as somewhat a smaller force to maintain the sliding then μk is called the coefficient of kinetic friction. For elementary understanding, μ is the property of the surface and varies from surface to surface.

Difference between Static and Kinetic friction

The main difference between static and kinetic frictions is that static friction acts while the surfaces are at rest while kinetic friction acts when there is relative motion between the surfaces.

While Static Friction increases with Applied force, Kinetic Friction remains constant
  • Let a body be kept on a rough surfaces in rest with a considerable normal force acting in between the surface of the floor and the body.
  • You now embark upon to push the body on the rough surface to create a relative motion in between them. It is now that the frictional force starts to act in between the surfaces and you fail to move the body.
  • As you continue to increase the force on the body the magnitude of the frictional force also continues to increase. This frictional force is referred to as static.
  • This increase in frictional force happens due to an increase in the value of the coefficient of frictional force (here coefficient of static friction) as the idea of an increase of the normal force is absurd.
  • Finally when the push you exert is greater than maximum force of static friction, the body begins to move.
  • Once the body begins to slide, you will realize that the force required to keep the body in motion is lesser that the force required to bring it into motion. The frictional force acting on the moving body is the kinetic friction.
  • The coefficient of kinetic friction is slightly lesser than the maximum value of the coefficient of static friction. In most sums however the maximum value of the coefficient of the two are taken to be equal, unless mentioned otherwise.

Approaching problems

Problems on friction require you to find two things: the magnitude of the frictional force and its direction.

Make sure to get the directions right
  • Determining the magnitude becomes easy once you manage to determine its possible direction. You won’t even need to employ any extra efforts most of the time if you are careful enough to note the direction of motion.
  • But with the increase in complexity of problems you will be required to religiously involve a greater amount of time to decide the direction. When you try to analyse a relatively complex problem and draw their free body diagrams, also mark the direction of impending motion.
  • Impending motion is the direction in which a body is supposed to be moving when the forces applied on it are resolved. Once you have decided upon the direction of impending motion you can mark the direction of the frictional force in the diagram.
  • The direction of the frictional force is marked such that it prevents the body from moving in the direction of impending motion. In certain sums you are required to find out the minimum and maximum external force to be applied so that a body remains in equilibrium. In these cases analyzing direction of impending motion is important to get correct results without much calculations and confusion.

Books to refer

Now to score well and understand the concepts, the suggested books are as follows:

  • H.C.Verma Concepts of Physics
  • Arihant Understanding Mechanics DC Pandey
  • Resnick Halliday Krane

Understand your concepts well from Resnick Halliday and HC Verma and then start solving the Subjective Questions. Solve the subjective questions from DC Pandey first then Resnick Halliday and finally HC Verma. In the end when your concepts are crystal clear try solving objective questions from previous years papers and other books.

Apart from this, you should also concentrate on work, power and energy as it is an important chapter too in the course of mechanics. Make your concepts stronger and skills sharper. All the best!

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