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SLIDING FRICTION

Sliding

Sliding is defined as the frictional motion of either a body in motion on a stationary surface or two bodies in relative motion with each other. It can refer to the movement where the surfaces are in direct contact with each other or have a layer of lubrication between them.

Sliding Friction

- Sliding friction is the resistance experienced by two bodies in a relative sliding motion. As is the case with any friction, sliding friction can also produce wear and tear and generally tend to have damaging effects on the bodies in motion.
- Sliding friction is also called kinetic friction. The magnitude of sliding friction is always lesser than that of static friction, and this is the reason why moving a body already in motion is much easier than moving a body that is in a position of equilibrium (rest).
- But sliding friction is usually much greater than the frictional force involved in a rolling motion. Although sliding friction does not depend on the surface area or the total area of contact, sliding friction majorly depends on two factors, namely the nature of the bodies in motion and the weight of the bodies in motion. Sliding friction produces much more sound and heat as by-products when compared with rolling friction. Sliding friction is given by the formula
Fk = μk. N

where
Fk is the force of sliding friction
μk is the coefficient of sliding friction
N is the force acting at an angle perpendicular to the plane of motion (Normal force)

Factors affecting Sliding Friction

  • The texture of the bodies in motion
  • Weight of the objects
  • The initial speed of the objects
  • Presence or absence of a lubricating substance like water, oil etc.
  • Shape or deformities in the objects (if any)
  • Surface adhesion offered by the surfaces in contact

Examples of Sliding Friction

- Let us grasp the concept of sliding friction with the help of an example. Consider a heavy wooden box that has to be moved a certain distance. Because there aren’t any rollers available, the wooden box must be physically pushed from the back.
- Initially, as the wooden box is at rest, the starting force given to the back is not enough to bring about motion, and it is much harder to make it budge even a little. This is because the wooden box experiences static friction, a resistance exerted on the wooden box by the pavement it is on.
- When the amount of force applied increases, the wooden box starts picking up speed and gradually attains a sustainable momentum. This is because the additional force applied on the wooden box is greater than that of the static friction, thus allowing motion. However, it can be noticed that, even after applying the required force needed to bring about motion, the wooden box still experiences a certain resistance to its motion. This is due to the phenomenon of sliding friction.
- This sliding friction force acts in a direction that is perpendicular to the plane of motion. This movement means that the applied force is greater than that of the sliding friction.
- When the applied force is gradually increased, the box is accelerated, and if it is kept constant, the box will have a constant velocity. Conversely, if the force is reduced, the box will gradually come to a halt.

Some other general examples where sliding friction is found are

  • The movement of coins on a carrom board
  • Erasing using a piece of rubber
  • Swiping on mobile devices
  • Tires of a car when it drifts on the road
  • When a matchstick is struck against the side of a matchbox
  • Braking mechanisms
  • Solving a Rubik’s cube

Summary

To better understand sliding friction, let us contrast it against rolling friction.
 

Sliding friction Rolling friction
Sliding friction is the resistance experienced in the sliding motion between two bodies (i.e.) when two bodies are rubbed together. Rolling friction is the kind of friction found in the rolling motion of one body on a surface or two bodies in a relative rolling motion.
Sliding friction is a result of minute, microscopic irregularities on the surface of the objects in motion. Rolling friction is caused by the deformation and recovery of the surfaces in contact.
Factors influencing sliding friction are the texture of surfaces, weight of the bodies in motion etc. Factors influencing rolling friction are the radius of the body in a rolling motion, load on the object etc.
Sliding friction is often much larger in magnitude in comparison with rolling friction. Rolling friction is generally lesser in value in comparison with sliding friction.
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