Static vs Sliding Friction
 

When there is relative motion or an attempt between two surfaces in contact, forces are created opposing the movement. Generally these forces are known as friction. Friction occurs among solid surfaces, fluid surfaces, and among fluid /solid surfaces. The friction inside a fluid is known as viscosity. Discussion of this article mainly focuses on the friction forces acting on solid surfaces.

On Macroscopic scale, the origin of the frictional forces is attributed to the irregular surfaces of the bodies. When small surface irregularities such as crevices and protrusions on the surface are subjected to relative motion, they obstruct each other’s motion to create reaction forces. There are laws that explain the behavior of frictional forces.

1. When two surfaces are in contact and in relative motion or in an attempt to do so, at the point of contact, the frictional force on the body is opposite in direction to the motion of the body.

2. If the frictional forces on the bodies are just sufficient to keep the bodies in equilibrium then the frictional forces are called limiting friction, and the magnitude of the friction can be found considering the equilibrium.

3. The ratio of the limiting friction to the Normal reaction between two surfaces depends on the substances which the surfaces are composed of and the nature of the surfaces, not on the magnitude of the Normal reaction. The ratio is known as the Coefficient of friction.

4. The magnitude of the limiting friction is independent of the contact area of the two surfaces.

5. When in motion, the friction force is opposing the direction of motion and independent of velocity. The ratio between the friction force and the normal reaction between the surfaces remains constant and slightly less than that of the limiting friction case.

Microscopically, the origin of the frictional forces is attributed to the repulsive forces between the electromagnetic fields of the molecules.

What is Static Friction?

When the body is in static (stationary) state, frictional forces acting on the body is known as static frictional forces. In this case, the vector sum of the external forces acting on the body is equal to the magnitude of the frictional forces but opposite in direction; hence the body remains in equilibrium. The frictional forces increase proportional to the resultant external force acting on the body until it reaches a limit and starts to move. The maximum static friction is the limiting friction.

Friction is independent of the contact area of the two surfaces and depends on the material and the nature of the body. Once the resultant external force exceeds the limiting friction the body starts to move.

What is Sliding (Dynamic) Friction?

When the body is in motion, the frictional forces acting on the body is known as the dynamic frictional forces. The dynamic frictional force is independent of the velocity and acceleration. The ratio between the frictional force and the normal force between the surfaces also remains a constant but slightly less than the ratio for the limiting friction.

What is the difference between Static Friction and Sliding (Dynamic) Friction?

• Coefficient of static friction is slightly higher than the coefficient of dynamic friction

• Static friction varies proportional to the external forces, while the sliding (dynamic) frictional forces remain constant, independent of velocity and the acceleration (and the resultant external force).

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