Vedantu’s Friction Concept Explanation
This is the stopping force that opposes the relative movement or the tendency of relative motion of two solid surfaces, fluid layers, and material elements when they slide each other, known as friction. It acts tangentially along with the contact. Friction always exists in pairs.
On microscopic observation, we find friction occurs due to irregularity in surfaces.
Types of Friction
Friction may be classified into two main types as follows:
Internal Friction
It is the force that restricts motion between the elements constituting a solid material at the time of deformation.
External Friction
It happens when two bodies that are in contact with each other have the tendency to move, or they are in actual relative motion.
There are many types of friction like Dry friction, Fluid friction, Lubricated friction, Skin friction etc.
Fluid friction If the friction happens, in between layers of any viscous fluids that are moving with each other, the phenomenon is called fluid friction.
Lubricated friction occurs when any lubricant fluid present in between two solid objects.
Skin friction occurs when a force restricts the movement of fluid across any body’s surface.
Dry Friction
If a force restricts the relative lateral movement of two solid surfaces which are in contact with each other, the phenomenon is called dry friction or external friction, it may be further classified into static friction, limiting friction and kinetic friction.
Static Friction
It happens in between nonmoving solid surfaces when the tendency of relative motion occurs in the bodies which are in contact and the body is at rest. It is represented by ‘fs’. Static friction never exists on its own. It opposes the impending motion that means it always takes place under the applied force. If there is no applied force, there is no static- friction. Its value is variable and self-adjusting. It has no formula as its value is variable. The maximum value of static friction is known as Limiting friction, represented as ‘flimiting’. The value of static force may be described as below:
0 ≤ fs ≤ flimiting
Flimiting = μsN (μs is the coefficient of static friction or limiting friction and N is the normal reaction exerted by the surface)
Laws of Static Friction:
The magnitude of the max. The Force of the static friction is independent of the area of contact between two surfaces.
The magnitude of maximum force is always comparative to the normal force; therefore if the normal force increases, the maximum external force that the object can endure without moving, also increases.
Limiting Friction
It is the maximum force of static friction, i.e. achieved when a body just starts moving over another body’s surface when in contact with each other. The following equation determines its value:
Flimiting = μsN (μs is the coefficient of static friction or limiting friction and N is the normal reaction exerted by the surface)
Laws of Limiting friction:
There are the following four laws for limiting friction:
The force of Limiting friction between two objects is directly proportional to the normal reaction:
F ∝ N
Therefore if a body is heavy, the normal reaction, i.e. mg increases, so force increases and more force is needed to move the heavy body in comparison to a lighter body.
The direction of limiting friction is always opposite to the direction in which one object is just about to move over another object.
Limiting friction’s force is independent of the area in between two bodies in contact.
Limiting friction’s force between any two objects in contact depends on the nature of their material or the roughness and smoothness they possess.
Kinetic Friction
It happens in between moving surfaces or when motion is there. When we increase the applied force slightly more than limiting friction, the actual motion starts. The force of friction at this point is known as ‘Kinetic friction’. Its value is constant and does not depend on the relative motion and area of contact. Fk denotes it. Kinetic friction fk is directly proportional to N (normal reaction).
Fk ∝ N
Fk = μkN (μk = constant and known as the coefficient of kinetic friction, depends on the surfaces which are in contact).
If we plot a graph between the friction and applied force, we find that at the stage of rest, the static function is equal to the applied force as static friction is self-adjustable. Therefore the graph will be a straight line and linear. Just after the stage of limiting friction when the body is just about to start, the value of friction always decreases and then becomes constant, during the motion as shown in the following figure:
(Image will be uploaded soon)
Laws of Kinetic Friction:
There are following four laws of kinetic friction:
The kinetic friction always opposes the relative motion and has a constant value. Its value depends on the nature of two surfaces of the objects in contact.
The value of kinetic friction fk does not depend on the area of contact; therefore, the normal reaction is the same throughout.
The kinetic friction is not entirely dependent on velocity.
The value of kinetic friction is directly proportional to the normal reaction between both the surfaces in contact.
FAQs on Friction
1. What is friction in Physics?
In physics, friction is defined as a resistive force that opposes the relative motion or the tendency of relative motion between two surfaces in contact. It always acts tangentially to the surface. This force arises primarily due to the microscopic irregularities present on the surfaces that interlock with each other when they come into contact.
2. What are the main types of friction?
Friction can be broadly classified into four main types based on the state of motion and the nature of the surfaces in contact:
- Static Friction (f_s): The force that prevents an object from starting to move when an external force is applied. It is self-adjusting and acts on stationary objects.
- Kinetic Friction (f_k): Also known as sliding friction, this is the force that opposes the motion of an object that is already sliding over another surface. Its value is typically constant.
- Rolling Friction: The resistive force that slows down the motion of a rolling object, like a ball or a wheel. It is significantly weaker than sliding friction.
- Fluid Friction: The force that opposes the motion of an object through a fluid (like air or water). This is also commonly known as drag.
3. What is the difference between static and kinetic friction?
The key difference lies in the state of motion of the object. Static friction acts on an object when it is at rest and prevents it from moving. Its magnitude is variable and matches the applied force up to a maximum limit. In contrast, kinetic friction acts on an object that is already in motion. Its magnitude is generally constant and less than the maximum static friction.
4. What are some real-world examples of friction being useful?
Friction is essential for many everyday activities. Some important examples include:
- Walking and Running: Friction between our shoes and the ground provides the grip needed to push off and move forward.
- Vehicle Brakes: The braking system in cars relies on the friction between the brake pads and the wheels to slow down and stop.
- Writing: Friction between a pencil tip and paper allows graphite to be deposited, making writing possible.
- Lighting a Match: Striking a matchstick against a rough surface uses friction to generate enough heat to ignite the chemicals on the match head.
5. How is the force of friction calculated using a formula?
While friction is a complex phenomenon, its magnitude can be estimated using specific formulas for different types:
- The maximum force of static friction (also called limiting friction) is calculated as: fₛ(max) = μₛN, where μₛ is the coefficient of static friction and N is the normal force.
- The force of kinetic friction is calculated as: fₖ = μₖN, where μₖ is the coefficient of kinetic friction and N is the normal force.
Here, the Normal Force (N) is the perpendicular force exerted by the surface on the object.
6. Why is the coefficient of static friction generally greater than the coefficient of kinetic friction?
The coefficient of static friction is greater than that of kinetic friction because of the nature of microscopic contact. When an object is at rest, the microscopic irregularities on both surfaces have sufficient time to settle and form stronger interlocking bonds. A larger force is required to break these initial bonds to start the motion. Once the object is moving, the surfaces are essentially bouncing over one another's high points, preventing deep interlocking and resulting in weaker, transient bonds. This makes it easier to keep the object moving than to start it, hence μₛ > μₖ.
7. Does the area of contact between two surfaces affect the force of friction?
Contrary to common intuition, the force of friction is largely independent of the apparent area of contact between two solid surfaces. The reason is that friction depends on the true area of contact at a microscopic level. If the apparent area is large, the weight of the object is distributed over more space, resulting in less pressure at each contact point. If the area is small, the pressure is higher, causing more deformation. These two effects tend to cancel each other out, keeping the true area of microscopic contact, and therefore the frictional force, relatively constant for a given normal force.
8. How does the nature of surfaces in contact influence friction?
The nature of the surfaces is a primary factor determining the force of friction. This is quantified by the coefficient of friction (μ). Rougher surfaces have more pronounced microscopic hills and valleys, which leads to greater interlocking and a higher coefficient of friction. Conversely, smoother surfaces have fewer and smaller irregularities, resulting in a lower coefficient of friction. For example, the friction between rubber and concrete is much higher than between ice and steel.
9. In what situations is friction a disadvantage, and how can it be reduced?
While useful, friction can also be a disadvantage in many mechanical systems. It causes wear and tear of moving parts, generates unwanted heat, and leads to a loss of energy and efficiency. Friction can be reduced through several methods:
- Lubrication: Applying substances like oil or grease between surfaces creates a thin film that separates them, reducing direct contact and friction.
- Polishing: Making surfaces smoother reduces the microscopic irregularities.
- Using Ball Bearings: These devices replace sliding friction with much weaker rolling friction.
- Streamlining: Designing objects with aerodynamic or hydrodynamic shapes reduces fluid friction (drag) when moving through air or water.
10. What is rolling friction, and why is it significantly less than sliding friction?
Rolling friction is the resistance that occurs when a circular object (like a wheel or ball) rolls on a surface. It is much weaker than sliding (kinetic) friction because the point of contact is constantly changing. Instead of surfaces being dragged across each other, rolling involves a slight deformation of the object and the surface at the point of contact. The energy lost in this continuous deformation and restoration process is far less than the energy required to continuously break the microscopic welds involved in sliding friction, making rolling a much more efficient way to move objects.
